Diesel engine compression has dropped: timing belt and piston wear

How much should the compression actually be if measured on a cold engine and should it differ much from the values ​​measured on a hot engine?

———- Message added at 09:05 ———- Previous message added at 06:36 ———-

I found the answer to my question myself. So I scoured the Internet, maybe it will be useful for someone: Measuring engine compression is carried out both on a cold engine and on a hot one, both with the addition of oil to the cylinders and without oil. On a cold engine, you need to unscrew all the spark plugs; if it is a diesel engine, then unscrew all the injectors. Take a compression test by rotating the crankshaft with the engine starter while fully pressing the accelerator pedal. The compression gauge arrow will show the compression of a given cylinder when the engine is dry, and this is a very important indication when the cylinder walls are not in oil. After this, you need to re-measure the compression but with the addition of 5-10 ml of oil. This indicator will allow us to determine the state of fit of the valves in the seats, thereby we determine the condition of the working part of the cylinder head. If there is a big difference between the readings, then we can confidently say that the valves are in good contact with the seats; if the difference is not significant, then you need to carry out preventive repairs of the cylinder head. Normal compression readings when measured on a “dry” engine for a gasoline engine are from 10 to 12 bar, for a diesel engine from 28 to 32 bar. In this case, the readings should be with the smallest spread across different cylinders. If the reading is below 10 bar - increased wear of the piston group, if above 12 bar - coking of the piston group. Compression can rise with a “wet” measurement by 4-6 bar for gasoline engines and by 18-24 bar for diesel engines. When measuring compression “hot”, it is necessary to dry the internal cavities of the cylinders with a small amount of technical kerosene: add 10-15 ml to the spark plug holes and ventilate in starter mode, and then measure the compression. We will call this diagnostic method “static compression measurement” because measurements are made with the engine not running at 300-400 rpm.

Checking engine compression is one of the methods for diagnosing malfunctions in a car engine, or more precisely in its cylinder-piston group (CPG).

Such a check will not immediately identify the malfunction, but still incorrect pressure readings that differ from the nominal ones will allow us to conclude that something is wrong and a more in-depth check of the operation of the car’s piston group with more specialized equipment is necessary.

The main symptoms of negative processes occurring in the CPG are:

1. Sudden loss of engine power;
2. One of the cylinders has stopped working - the engine “troubles”, increased engine vibration is felt;
3. Increased oil and fuel consumption.

In most cases, replacing the ignition system plugs solves the problem, but if this does not help, then it is worth measuring the compression of all engine cylinders.

The principle of operation of the compression meter

To carry out measurements, the spark plug is unscrewed and a threaded tip is screwed in instead or a non-threaded tip is pressed, it all depends on what model of the device you have.

When the cylinder moves in the engine, air enters the hose under pressure. When the pressure in the cylinder has reached its maximum, this value is recorded by a pressure gauge.

Despite the fact that this device is easy to use and affordable, this does not give reason to think that measuring engine compression is a simple matter.

Any indicators of a compression meter require analysis and a final verdict only from a specialist who knows exactly in what situations it is worth believing the readings of the device and taking action, and in what cases these indicators are purely informative.

In order to find out what compression should be in the engine cylinders of your particular car model, you should refer to the technical documentation for the car and find out the compression ratio.

Having found out the compression ratio, we multiply it by 1.3 and get an approximate indicator of the compression value.

But it should be understood that modern compression meters cannot carry out perfectly accurate measurements and, like other devices, they also have errors (up to approximately 3 atmospheres).

Also, compression level readings depend on factors such as:

1. Quality of fuel and its octane number;
2. Oil viscosity;
3. On the degree of engine warm-up;
4. Battery charge levels.

Therefore, the analysis of the obtained measurements is not based on standard indicators (although they are also partially taken into account), but a comparative analysis of compression readings in all engine cylinders is carried out, but more on that a little later.

Why check compression?

One of the reasons is that you want to understand how long your car’s engine will last without major repairs. Or maybe you noticed the appearance of extraneous sounds, blue smoke from the exhaust pipe, or uneven engine operation at idle.

Also, engine compression is usually measured when choosing a used car, because this indicator says a lot about the condition of the unit.

You can carry out such diagnostics using a pressure gauge. All you have to do is install it in place of the spark plug and check the pressure in each cylinder one by one. After receiving the results, they must be compared with the characteristics specified in the vehicle's operating instructions.

As a rule, the manual for using or repairing a car contains information about what the compression should be in the cylinders. To carry out such a test, no special skills are needed, but you need to know how to check engine compression correctly. You need to not only get certain results, but also understand what these numbers say.

What should the compression be?

To identify the point of maximum wear of engine parts, you need to know the permissible pressure values. It is related to the degree of compression in direct proportion - the more, the higher the pressure.

Today you can find 3 types of engines in operation with different characteristics:

1. Old engines with a low compression ratio - up to 8.5.
2. Modern gasoline engines in which the air-fuel mixture is reduced in volume by up to 9-11 times.
3. Diesel engines capable of compressing the fuel mixture from 16 to 24 times.

The combustion chamber of an engine running on diesel fuel is characterized by a small volume. Therefore, the motor does not require an electric spark, just strong compression.

The amount of compression is influenced by various factors:

• tightness of valve seating;
• cracks in valve seats;
• presence of lubricant in the cylinders;
• ring wear;
• wear of parts of the cylinder-piston group.

Norm and minimum

Through testing, data on the optimal pressure in the cylinders for different types of engines was obtained. When the engine has reached operating temperature and the battery is fully charged, the compression should be:

1. On older engines equipped with carburetors, the lowest value is 1 megapascal. In old units - 10 bar. If the engine is new, the pressure can reach 13 bar.
2. The optimal pressure in a gasoline engine is 1.5 megapascals. The minimum level is 1.1 MPa.
3. For a diesel engine, the normal value is 2.4-3 MPa.

It is important to know! Analysis of parameter measurements allows us to establish a pattern that a compression ratio with a coefficient of 1.5 gives the optimal pressure in the cylinders.

If we more accurately determine the indicated coefficient, then for 4-stroke gasoline engines it is in the range of 1.2-1.3, for diesel engines - 1.7-2.

Signs of poor compression

Identification of the lower compression limit indicates increased engine wear. Friction occurs between the piston rings and the cylinder, which increases the gap between the parts. For this reason, compression is reduced.

The following signs can indicate that the compression is insufficient:

1. There is a lot of smoke coming out of the breather. This is a valve that serves to remove crankcase gases from injection and carburetor engines.
2. Reduced power of the power unit, especially noticeable on small engines.
3. A lot of smoke comes out of the exhaust pipe as the engine oil burns in large quantities.
4. High oil consumption due to wear of oil scraper rings and caps.

If at least one of these signs is noticed, then there is a reason to conduct a full engine diagnostics.
This will allow timely measures to be taken to restore its functionality. If we consider the reasons for the decrease in compression, we can highlight the main ones:

1. Operating a car with a cold engine, thermostat failure.
2. Overheating of the power unit.
3. Use of low quality lubricant.
4. Maintenance and oil changes are not carried out in a timely manner.
5. The service life of the engine has expired.

If the head gasket is burned out or the valves are damaged, then changing the lubricant will not help. In these cases, engine repair will be required with replacement of piston group parts and other elements.

Permissible compression difference in cylinders

If the measured compression differs between cylinders, this complicates matters. The engine will have to be disassembled and overhauled. Not only will it be necessary to replace rings, valves and sealing caps.

Compression less than the minimum value in one cylinder means that there are defects in the piston or cylinder. In this case, all elements of the cylinder-piston group are usually changed, otherwise the difference in compression will remain and the problem will not go away.

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The normal value is considered to be a pressure of 10-12 bar, depending on the car model and engine. But the permissible difference in compression in different cylinders has also been established. For example, in cylinders 2 and 3 this value may be 0.5 bar lower, which is quite acceptable. It depends on the load on the pistons - where there is more, there is more wear.

Advice! If the pressure in different cylinders differs greatly, for example, by 3 or 4 atmospheres, then this is abnormal. The motor will run intermittently. This can happen in several cylinders at once, which brings a major overhaul closer.

How is compression measured?

The main engine parameter, which is indicated in the operating manual, is the compression ratio. It determines how many times the fuel mixture is compressed before combustion. The cylinder volume divided by the piston stroke is the compression ratio. It does not change if the car owner has not tuned the engine, for example, bored the cylinders or installed a different crankshaft.

But this characteristic should not be confused with compression, which means piston pressure. It is determined by rotating the crankshaft with the starter.

Compression decreases over time due to wear of parts. Its units of measurement can be:

1. Bar.
2. Atmosphere (atm).
3. Megapascal (MPa).
4. Kilogram*force per square centimeter (kgf/cm2).

The most modern unit of measurement is the Megapascal, which is equal to 9.9 atmospheres.

How often to check compression

For prevention, diagnostics are carried out when replacing spark plugs. This procedure should be carried out every 25-50 thousand km.

In addition, emergency diagnosis is carried out for the following signs:

1. The engine consumes a lot of oil - more than 200 ml per 1000 km.
2. Blue smoke comes out of the exhaust pipe.
3. The car has become harder to start in winter.
4. Unstable idle, vibration.

The last symptom indicates a failure of the ignition system or several spark plugs. Before measuring compression, these faults must be eliminated. On diesel engines, wear of valves, pistons and cylinders shows the same signs. Cold starts are especially noticeable - diesel fuel cannot ignite.

Choosing a compression meter

A very important point is which engine is installed in your car, gasoline or diesel.

In this case, it is enough to purchase an inexpensive compression gauge with a threaded tip that can be screwed into the spark plug hole. If there is a tip without a thread, then you will have to press it in, but this is not convenient and the readings of the compression meter will not be accurate.

Pay attention to the position of the pressure release valve. In high-quality compression gauges, it is located directly in the tip, which is inserted into the spark plug hole, and not under the pressure gauge itself. This will allow you to get more accurate measurements, without underestimating the readings.

In this case, the situation is much more complicated and it is not recommended to carry out such measurements yourself for several reasons:

1. It is necessary to remove the glow plugs or injectors. Not every driver can do this and not everyone has a special tool to perform such work;
2. You will need a special adapter, which is purchased separately or may already be included in the kit of expensive compression meters.

In this case, it is better for you to contact a car service.

You can purchase an imported compression tester, which is initially equipped with a large number of adapters for various brands of cars, but such a kit is not cheap and is used mainly in car repair shops.

Alternative

A more complex, convenient, but not much more effective device. The main difference is the ability to record the received data on plastic cards or paper at different periods of time and subsequently analyze them.

But if you want to track the increase in pressure over time, it will be difficult with this device. This is the main disadvantage of the compressograph.

They are the most modern instruments for measuring engine compression.

Their operating principle is not based on fixing the pressure in the cylinders, but on measuring the pulsation of the current consumed by the starter.

The strength of the current consumed is directly proportional to the increase in compression in the cylinders and the increasing load on the engine crankshaft.

The main advantage of this device is that in just a few minutes you can take readings from all cylinders without unscrewing the spark plugs.

However, readings in this device are taken not in pascals, but in percentage terms. The cylinder that works more efficiently has a higher percentage.

RVS Master additives for two-stroke engines: many advantages

RVS compositions are really good. And we want to explain this in more detail. RVS Master, consisting of high-quality materials, does not enter into chemical reactions with fuel and oil and retains their original composition. The additives do not in any way affect the condition and operation of seals and gaskets, and after application they add up to 60,000 km to the engine. As a result, fuel consumption is reduced by approximately 10%. What’s interesting is that conventional additives only last for a refill. With each new refueling it is necessary to add them again. And this is many times worse than in the case of RVS.

Despite the higher cost of RVS Master compared to other additives, complex and long-term use of RVS is slightly cheaper. The second use of RVS does not require additional purchase of supporting products. The RVS additive is cheaper in 2 cycles and has a long-term effect.

From words to deeds

Before checking engine compression, you need to prepare the car.

To do this you should:

1. Warm up the engine to 80 - 90 degrees, this can also be done on a cold engine, but more on that later;
2. Turn off the fuel supply to the cylinders. For your decision, for example, turn off the fuel pump;
3. Be sure to turn out ALL the spark plugs, not just a few, but ALL of them, this is important;
4. Check the battery charge, it should be fully charged.

Engine compression can be measured in two ways:

1. With closed throttle;
2. With the throttle valve open.

In the first case, the pressure in the cylinders will not be high, in the range of 0.6 - 0.8 MPa, since air will not flow in large quantities. Therefore, compression in our case will decrease if the slightest misdensity exists in the system. This will allow us to identify hidden defects in the CPG.

In the second case, the air flow will be large, and accordingly the pressure in the cylinders will increase and can reach 0.8 - 0.9 MPa.

The second method is more suitable for identifying serious malfunctions in the engine, which we have already written about above, see Air leakage.

But, no matter what method you use, you need to remember one important nuance - watch the rate of increase in pressure, as this is a kind of “litmus” when diagnosing breakdowns.

For example, if the pressure gauge reading on the first stroke, during measurement with a compression meter, was equal to 0.3 MPa, and then sharply jumped up on the next strokes, 99.9% this is a problem with the piston rings.

A radically opposite situation also occurs, when the pressure gauge readings at the first step during measurements are 0.8 MPa, and then do not increase. In this case, you should pay attention to the integrity of the head gasket or the serviceability of the compression gauge valve.

Naturally, after this testing stage, other diagnostic tools should be used to ensure more reliable detection of faults.

Measurement on a “cold” engine

If there is a problem with starting the engine, or the car does not start at all, then the engine compression can be checked “when it is cold”.

We take a gasoline engine. In this case, if, when checking for a “cold” engine, the compression level indicators are at least two times lower than optimal, compared to checking on a warm engine, and this is in the range from 4.5 to 5.5 atmospheres, then this is primarily due to the presence of the rings in the pistons, which is already a malfunction.

We take a diesel engine. It is not a fact that you will perform the check, but you need to know that it is not possible to start a diesel engine if the compression readings in it are no more than 17 atmospheres. And if not more than 24 atmospheres, then you cannot do without a major overhaul of the diesel engine.

Also, when checking compression in a diesel engine, do not allow the presence of oil in the cylinders, let the engine settle so that the oil flows into the crankcase, then the pressure gauge readings will be realistic.

Checking compression when hot

First, let's answer the question - why is compression checked on a warm engine? The bottom line is that when diagnosing, it is important to know what maximum compression is possible in the engine at the peak of its power. After all, the lower this value is, the worse the condition of the engine. On a cold engine, compression is checked only if the car is difficult to start when cold, and all elements of the starting system have already been checked.

Before performing an engine compression test, you need to know what it should ideally be for the engine being measured. This information is usually found in the repair manual for the car or its engine itself. If such information is not available, then the compression can be calculated empirically.

How to find out approximately what compression should be

To do this, you need to take the value of the compression ratio in the cylinders and multiply it by a factor of 1.3. The value will be different for each engine, but for modern cars with a gasoline engine it is about 9.5...10 atmospheres for 76 and 80 gasoline, and up to 11...14 atmospheres for 92, 95 and 98. th gasoline. Diesel engines have 28...32 atmospheres for engines of older designs, and up to 45 atmospheres for modern engines.

The difference in compression between the cylinders can differ by 0.5...1 atmosphere for gasoline engines, and by 2.5...3 atmospheres for diesel engines.

How to measure compression when hot

When performing an initial hot engine compression check, the following conditions must be met:

Universal compression meter

• The engine must be warmed up; on a cold engine the value will be underestimated.
• The throttle should be fully open (gas pedal to the floor). If this condition is not met, the combustion chamber at top dead center will not be completely filled with the air-fuel mixture. Because of this, a slight vacuum will arise and compression of the mixture will begin at a lower pressure compared to atmospheric pressure. This will underestimate the compression value when checking.
• The battery must be fully charged. This is necessary so that the starter rotates the crankshaft at the required speed. If the rotation speed is low, then some of the gases from the chamber will have time to escape through leaks in the valves and rings. In this case, the compression will also be reduced.

After performing the initial check with the throttle open, you need to perform a similar check with the throttle closed. The conditions for its implementation are the same, but you don’t need to press the gas pedal.

Analyzing the results obtained

Analysis of the results obtained comes down to comparing the pressure gauge readings in all cylinders. It is worth paying attention to the cylinder whose pressure level differs from the pressure readings in other cylinders; it can be either less or more.

If the difference in readings does not exceed one atmosphere, then there is no point in sinning on the CPG. Return to diagnosing the car's ignition system, check the spark plug. Also check the fuel supply to the cylinders.

Indications in more than one atmosphere are also typical for cars with a mileage of more than 50 thousand km.

The reason for the distinctively higher pressure in the cylinder may be the presence of a large amount of oil in it, the cause of which must be clarified separately. Turn out the spark plug and see if it is in oil, then this is our case.

Main regulatory indicators

All necessary data can be taken from the maintenance documentation for a particular car brand.

Basic data, taking into account testing only on a warm engine, is presented below.

Standard compression level:

1. Modern gasoline cars that are filled with A-95, 98 gasoline - from 11 to 14 atmospheres (range of readings from 0.5 to 1.0 atmospheres);
2. Old cars filled with gasoline A - 80, A-92 - from 9 to 10 atmospheres (range of readings from 0.5 to 1.0 atmospheres);
3. Modern diesel cars - about 45 atmospheres (range of readings from 2 to 3.0 atmospheres);
4. Old diesel cars - about 28 atmospheres (range of readings from 2 to 3.0 atmospheres).

Depending on the installed diesel engine, standard readings can vary from 28 to 45 atmospheres.

Let's sum it up

When checking engine compression, try to adhere to the rule that all data obtained during measurements will be only relative. Yes, they should be taken into account during the analysis, but the main thing for you will be a comparative analysis of the data taken from each cylinder.

The difference in values ​​will be a fundamental factor for you in identifying a faulty cylinder and quickly carrying out other necessary diagnostics to identify the causes of the malfunction.

And in order to rely on the absolute standard compression value, it is necessary to have data from early measurements taken at different times during the vehicle’s operation, which is not always done.

Moreover, the data archive, in addition to the results of basic measurements, should contain: under what temperature conditions the test was carried out, temperature and oil viscosity, vehicle mileage and its general technical condition, etc.

For a simple driver, this is all difficult, so to check engine compression, you should not neglect the services of a car service center, and save the data obtained until the next check.

Video - determination of compression in engine cylinders.

The cylinder-piston group (CPG) and the gas distribution mechanism (GDM) are subject to wear during the operation of the internal combustion engine. The condition of these engine parts affects compression. Engine compression is one of the important parameters for the normal operation of the unit.

Read in this article

How to determine the pressure in diesel cylinders

To organize the procedure, you will need to prepare a well-charged battery and a compression gauge for working with diesel engines (it has special attachments and a measurement limit of up to 60-70 atmospheres). Engine compression is measured after it has warmed up. Following actions:

• remove the glow plug;
• screw the compression gauge into the vacated hole;
• turn off the diesel fuel supply (remove the power plug from the solenoid valve);
• ask a friend to press the accelerator (if the diesel engine has a valve that creates a vacuum used in the vacuum brake booster and regulator) and turn on the starter with the “start” button;
• remember (write down) the data obtained using the device;
• check the obtained values ​​with those recommended by the car manufacturer (the necessary information is in the service book).

The compression value in a diesel engine depends on the brand of engine and is in the range of 28-40 kg/sq.m. see. An interesting dependence of compression in a diesel engine on the possibility of starting it at a certain outside temperature. Below are the compression values ​​when the engine starts without problems:

• up to 28 kg/sq. cm: air temperature not lower than minus 15 degrees;
• up to 30 kg/sq. cm: normal start at minus 20;
• up to 32 kg/sq. cm: the engine will start at minus 25;
• up to 36 kg/sq. cm: start-up can be done at minus 30;
• up to 40 kg/sq. cm: diesel is able to start at 35 degrees below zero.

A drop in compression in diesel engines is accompanied by the appearance of a bluish exhaust from the muffler pipe, which is associated with incomplete combustion of fuel due to low temperature. Another consequence of a decrease in pressure in the cylinders is poor atomization of diesel fuel in the chambers, leading to a drop in power of the power unit.

Decoding the results

If there is normal compression, the pressure in the cylinder will gradually and evenly increase to a value corresponding to the operating data specified by the engine manufacturer.

• The compression of a working gasoline engine is usually 12-15 bar. But everything is very individual, so in any case you need to look at the instructions and technical characteristics of the engine installed in your car. Note that the difference in values ​​between the cylinders should be minimal (no more than 1-1.5 bar).
• On a working diesel engine, a compression of 25-35 bar is considered normal. The difference between the cylinders should not exceed 10%.
• Increased compression in the engine usually indicates the presence of carbon deposits in the cylinder.
• If the test indicates low compression in one or more cylinders, proceed to the next section of the article.
• Having a 10-15% difference in readings between two or more cylinders usually indicates worn components such as valves, rings, cylinder.
• Low compression in two adjacent cylinders may indicate a breakdown of the cylinder head gasket between them. We recommend taking measurements with oil (see the next section of the article).
• Low compression in all cylinders usually indicates a worn timing chain or a stretched belt. Another option is wear of the cylinders or rings, which can only be “treated” by a major engine overhaul.

Once you have reached preliminary conclusions, you need to measure the compression in the cylinders in more detail to establish an accurate verdict.

Note. After checking the compression, the Check Engine light may come on on the instrument panel. You can reset the error using a special scanner. Sometimes disconnecting the battery helps.

Common causes of low engine compression:

• crack in the block or cylinder head;
• problems in the gas distribution mechanism;
• problems with the valve or valve seat;
• worn rings or cylinders.

What does a compression test show: main faults

Measuring engine compression with your own hands is the simplest and most accessible way that allows you to identify various malfunctions of the power plant. The main task when independently measuring the compression of a diesel or gasoline unit is to identify deviations regarding the compression pressure in the engine cylinders from the permissible value.

The solution to measure compression allows you to determine a malfunction in both one and several cylinders. Additionally, measuring engine compression can indicate problems with valves and valve seat tightness. Compression testing can be done in different ways. Let's look at how to correctly measure the compression of a diesel and gasoline engine.

Compression ratio and compression: what is the difference

Compression is the value of the maximum pressure of the fuel-air mixture in the engine cylinder.

Obviously, the higher the compression ratio, the higher the normal compression values.

But the interesting thing is that in a working engine, the compression values ​​are always greater than the compression ratio.

This happens because when compressed, the gases inside the cylinder heat up, and when heated, their pressure also increases.

So, for VAZ engines the compression ratio is 9.6:1, and the compression is 11–13 bar (or the familiar “atmospheres”, expressed in kgf/cm2).

On average, compression is 1.2 times higher than the compression ratio on gasoline engines, and 1.8 times higher on diesel engines.

Bigger is not always better

Why can't you increase the compression ratio and thereby increase engine power?

Because due to the same heating during compression, the fuel-air mixture can ignite spontaneously before the piston approaches the top point of its movement (experts call it “top dead center” - TDC), and the oncoming impact of gases expanding during combustion will prevent proper movement of the piston and lead to the destruction of parts of the crank mechanism.

The compression ratio is defined as the ratio of the total volume of the cylinder (Va) to the volume of the combustion chamber (Vc)

Equipment for self-measurement: compression meter and AGC

To measure compression yourself, a simple compression meter is most often used. It is worth adding that measuring in this way is not very informative, but it can clarify the general picture of the state of the CPG. Also, diagnostics of the cylinder-piston group is carried out using special equipment that is available at the service station.

The main recommendation when choosing a compression gauge is the presence of a threaded tip. This tip allows you to screw the compression gauge into the spark plug hole. If the compression gauge does not have such a tip, then the measurement accuracy is reduced. It is also advisable to have a compression gauge with a pressure fixing valve in the head itself, which screws into the spark plug hole. Devices with a valve located under the pressure gauge are less accurate and tend to underestimate the final readings.

To assess the condition of the CPG and other elements of the internal combustion engine, you can also use a special device called a cylinder leakage analyzer (CTA). The measurement method is similar to working with a compression meter, but the device does not record the compression pressure in the cylinder, but the vacuum parameters.

Using this device, the presence of a leak through the combustion chamber, the effectiveness of valve fit is determined, burnout of the piston bottom is detected, and the condition and degree of wear of the liner is assessed. Also, AGC allows you to assess the condition of the piston rings, detect coking or stuck rings without disassembling the engine.

Causes and consequences of reduced compression

The main reason for compression liquefaction is friction. Of course, this is counteracted by the protective film formed by the oil on the walls of the cylinder and piston. However, despite this protection, gradual wear still occurs. Layer by layer, the cylinder self-bores. As a result, leaks form between the piston rings and the cylinder. At the moment of compression, the fuel mixture leaks into the crankcase through these leaks. Engine efficiency decreases due to the fact that not all of the fuel is converted into useful energy.

Compression measurement of gasoline and diesel engines

The list of general requirements in order to accurately measure compression includes:

1. The engine must be warmed up to 80-90 degrees;
2. The fuel supply must be turned off;
3. All candles without exception must be unscrewed;
4. The starter must be absolutely in good working order;
5. The battery must be fully charged;

• In the case of a manual gearbox, you can engage the upper (over) gear and also prevent the vehicle from moving. To do this, use the parking brake and/or wheel chocks. It is also possible to measure compression in the “neutral” position of the manual transmission lever. In cars with an automatic transmission, the selector is set to parking mode (P).
• The next step is to check the condition of the air filter. Before measuring compression, a dirty filter is replaced with a new one. The second method is to temporarily disconnect the air supply line from the inlet.
• In gasoline internal combustion engines, the ignition system will need to be turned off. To do this, it is necessary to disconnect the wiring from the breaker to supply low voltage, which is supplied to the ignition coil. Motors equipped with an ignition distributor will require removal of the main high-voltage wire from the coil. The specified wire is then connected to ground.
• To effectively shut off the fuel supply in designs that include a mechanical fuel pump, the fuel line will need to be disconnected. Next, the pipe is turned off. For an electric fuel pump, it will be enough to remove the contact from its relay.
• Afterwards, the places for installing the spark plugs are cleaned of dirt and the tips of the high-voltage wires are removed from the spark plugs. Then the candles are unscrewed.
• With the spark plugs removed, you will need to turn the engine over several times with the starter. The procedure is necessary to clean the cylinders from possible carbon deposits and other deposits.
• The last step is to connect the compression gauge to the spark plug well of the cylinder. It is better to measure compression with a partner who will press the accelerator pedal all the way. The gas pedal is depressed so that the compression measurement is carried out with the throttle valve open, and due to this, the intake resistance is reduced. With the pedal depressed, the engine is turned with the starter for about 5 seconds. It is necessary to monitor when the highest value appears on the scale. The indicated value is noted and further measurements are taken in other engine cylinders.

As for the features of measuring compression on a diesel engine, then:

• Turning off the fuel supply in a diesel engine is achieved by pressing down the cut-off lever. This lever is located on the injection pump. The second way is to turn off the solenoid valve, which will also block the fuel supply to the diesel internal combustion engine. This valve is located on the fuel line, which is responsible for supplying fuel.
• The compression gauge in a diesel engine is connected through the holes of the injectors or glow plugs. This depends on which elements are more convenient to reach, as well as according to the recommendations for a particular type of diesel engine.
• To measure compression on a diesel engine, you will need a special compression meter, which will have an appropriate tip for secure fastening in the hole of the injector or glow plug. The maximum measurement threshold for such a compression meter for a diesel engine should be higher compared to its gasoline counterpart. A diesel compressometer should be able to record a pressure of about 60 atmospheres.
• When measuring diesel compression, there is no need to press the gas pedal. The fact is that most diesel engines do not have a throttle valve. Only diesel engines that have a special valve in the intake stand separately. The valve is necessary to create vacuum. The vacuum will ensure the functionality of the vacuum regulator and brake booster.

If you take measurements by unscrewing only the spark plug in one cylinder, then there is a noticeable increase in the resistance to crankshaft rotation. When the starter cranks, this resistance leads to a decrease in speed. If the battery is faulty or has a weak battery, you can use a jump starter, although this method is not recommended.

Why add oil to the cylinders?

A problem with rings, pistons, valves or gaskets is more accurately diagnosed by filling the cylinder under test with about 3-5cc. see butter. An immediate increase in compression after adding oil already on the first stroke will indicate piston ring failure. If the compression is initially low, then oil was added, but it was not possible to increase the compression, then leaks through the cylinder head gasket are likely.

Most often, compression drops in only one cylinder. About 5 “cubes” of clean engine oil should be poured into this cylinder with the lowest compression. During the filling process, the oil should not fall on the piston bottom, but on the walls of the cylinder liner. Next, the compression measurement is repeated. If the pressure increases relative to the readings in other cylinders, it is likely that the piston rings are coked, stuck or destroyed.

Also, the absence of changes in the readings after adding oil may indicate that the valves are not completely seated against the seats. The valve may burn out or the valve seat may be damaged; the valve clearances may be incorrectly adjusted.

To diagnose cylinder head gasket damage, piston burnout or cracks in the piston body, it is necessary to install the piston closer to top dead center (TDC), which corresponds to the compression stroke. Next, you need to supply a portion of air into the cylinder under a pressure of about 2 - 3 atmospheres.

If the head gasket is damaged, then the sound of air escaping from the spark plug well located nearby will be heard. Air escaping through the carburetor means the intake valve is not seated properly. Additionally, the oil filler plug is removed. If air comes from there, a burnout or crack in the piston is likely. Air in the exhaust tract will indicate a loose or burnt exhaust valve.

For maximum accuracy, it is recommended to use a combined method of measuring compression (measuring “cold” and “hot”, with the throttle valve open and closed). At the initial stage, you need to measure the compression in the cylinders of a gasoline unit with the throttle open. Then the spark plugs, glow plugs or diesel injectors (depending on the type of internal combustion engine) are put in place and the engine starts. It is necessary to warm up the engine, and then measure the compression (with the throttle closed for gasoline engines).

You need to measure compression on a cold engine when you cannot start the engine. The engine does not start “when cold” due to critical wear of the CPG, as well as as a result of stuck piston rings. On a cold internal combustion engine, the pressure in such cases drops by two or more times. Warming up the engine allows you to increase compression, after which the unit is acceptable.

This time is necessary for the remaining oil to completely drain back into the engine crankcase. A drop in the compression rate for a cold diesel engine below 18 atmospheres will mean that starting such an engine is almost impossible. If the indicator after warming up does not rise above 24 atmospheres, then the engine must be repaired.

What affects compression

Before taking measurements, it is recommended to warm up the engine until the cooling fan turns on, to a coolant temperature of +80°C...+90°C.

The difference in compression between cold and hot is that for an unheated engine its value will always be lower than for a warmed one. This is explained quite simply. As the engine warms up, its metal parts expand, and accordingly, the gaps between the parts decrease and the tightness increases.

In addition to engine temperature, the following factors also affect the engine compression value:

• Throttle position . When the throttle is closed, the compression will be lower, and accordingly, its value will increase as the throttle opens.
• Air filter condition .
  Compression will always be higher with a clean filter than if it is clogged. A clogged air filter reduces compression
• Valve clearances . If the clearances on the valves are larger than required, a loose fit in their “seats” contributes to a significant reduction in engine power due to the passage of gases and compression decreases. With small cars it will stall completely.
• Air leak . It can get sucked in in different places, but in any case, when it gets sucked in, engine compression decreases.
• Oil in the combustion chamber . If there is oil or carbon deposits in the cylinder, the compression value will increase. However, this actually harms the engine.
• Excessive fuel in the combustion chamber . If there is a lot of fuel, it dilutes and washes away the oil, which plays the role of a seal in the combustion chamber, and this reduces the compression value.
• Crankshaft rotation speed .
  The higher it is, the higher the compression value, since in such conditions there will be no leaks of air (fuel-air mixture) due to depressurization. The crankshaft rotation speed depends on the charge level of the battery. This may affect the results in absolute units up to 1...2 atmospheres downward. Therefore, in addition to measuring the compression while it’s hot, it is also important that the battery is charged and the starter turns well when checking.

If the engine is working properly, then compression on a cold engine should increase very quickly as it warms up, literally in a matter of seconds. If the increase in compression occurs slowly, this means that the piston rings are most likely burned out . When the compression pressure does not increase at all (compression is the same on cold and hot), but sometimes, on the contrary, drops, then the cylinder head gasket is most likely broken . So if you are wondering why there is more compression on cold than on hot, perhaps this is how it should be, then you should look for the answer in the cylinder head gasket.

Checking hot compression in different operating modes allows you to diagnose breakdowns of individual components of the engine cylinder-piston group (CPG). Therefore, when checking the condition of the engine, technicians always first recommend measuring the compression in the cylinders.