How to measure engine compression


The compression indicator of the cylinder-piston group allows you to determine the state internal combustion engine or its individual elements. Most often, this parameter is replaced when the power of the power unit has noticeably decreased or when there are difficulties with starting the engine.

Let us consider for what reasons the pressure in the cylinders may drop or even disappear, how to check this parameter, what tool is needed for this, as well as some of the subtleties of this procedure.

What the compression measurement shows: the main malfunctions

Before considering how to measure compression, you need to understand the definition itself. It is often confused with compression ratio. Actually, the compression ratio is the ratio of the volume of the entire cylinder to the volume of the compression chamber (the space above the piston when it is at top dead center).

How to measure engine compression

This is a constant value, and it changes when the parameters of the cylinder or piston change (for example, when replacing a piston from a convex to an even one, the compression ratio decreases, since the volume of the compression chamber increases). It is always denoted by a fraction, for example 1:12.

Compression (more accurately defined as end-of-stroke pressure) refers to the maximum pressure that the piston creates when it reaches top dead center at the end of the compression stroke (both intake and exhaust valves are closed).

How to measure engine compression

Compression depends on the compression ratio, but the second parameter does not depend on the first. The amount of pressure at the end of the compression stroke also depends on additional factors that may be present during the measurement:

  • pressure at the beginning of the compression stroke;
  • how the valve timing is adjusted;
  • temperature during measurements;
  • leaks in the cylinder;
  • crankshaft starting speed;
  • dead battery;
  • excessive amount of oil in the cylinder (with a worn-out cylinder-piston group);
  • resistance in the intake manifold pipe;
  • engine oil viscosity.

Some mechanics try to increase engine power by increasing the compression ratio. In fact, this procedure only slightly changes this parameter. You can read about other ways to add "horses" to the engine. in a separate article.

How to measure engine compression
Changed compression ratio

What does the pressure at the end of the compression stroke affect? Here are just a few factors:

  1. Cold start of the engine. This factor is especially important for diesel engines. In them, the air-fuel mixture is ignited due to the temperature of the highly compressed air. For gasoline units, this parameter is equally important.
  2. In some cases, a decrease in compression causes an increase in crankcase gas pressure. As a result, a larger volume of oil vapor gets back into the engine, which leads to an increase in the toxicity of the exhaust, as well as contamination of the combustion chamber.
  3. Vehicle dynamics. With a decrease in compression, engine throttle response drops noticeably, fuel consumption increases, the oil level in the crankcase drops faster (if lubricant leaks through the oil scraper ring, the oil burns out, which is accompanied by blue smoke from the exhaust pipe).

There is no universal value for the pressure at the end of the compression stroke, since it depends on the parameters of the individual power unit. In view of this, it is impossible to name a universal compression value for all power units. This parameter can be found from the technical documentation of the vehicle.

When a change in pressure is detected during measurements, this may indicate the following malfunctions:

  • Worn pistons. Since these parts are made of aluminum, they will wear out over time. If a hole forms in the piston (burns out), the compression in that cylinder can be greatly reduced or practically disappear (depending on the size of the hole).
  • Burnout valves. This often happens when the ignition is set incorrectly. In this case, combustion of the air-fuel mixture occurs when the valve is open, which leads to overheating of its edges. Another cause of valve seat or poppet burnout is a lean air / fuel mixture. Loss of compression can also be due to valves not seating tightly (deformed). Clearances between the valve and its seat cause premature gas leakage, which causes the piston to be pushed out with insufficient force.How to measure engine compression
  • Damage to the cylinder head gasket. If for any reason it bursts, gases will partially escape into the resulting crack (the pressure in the cylinder is high, and they will surely find a "weak point").
  • Piston ring wear. If the rings are in good condition, they will regulate oil flow and seal the sliding movements of the piston. Their other function is to transfer heat from the piston to the cylinder walls. When the tightness of the compression pistons is broken, the exhaust gases penetrate into the crankcase to a greater extent, rather than being removed into the exhaust system. If the oil scraper rings are worn, more lubricant enters the combustion chamber, which leads to increased oil consumption.

Also, during measurements, it is worth paying attention to the extent to which the pressure in the cylinders has changed. If the procedure showed a uniform decrease in the indicator in all cylinders, then this indicates natural wear of the cylinder-piston group (or some of its parts, for example, rings).

When the pressure at the end of the compression stroke of one cylinder (or several) differs significantly from the compression in others, then this indicates a malfunction in this unit. Among the reasons are the following:

  • Burnt out valve;
  • Sagging piston rings (mechanics call it “rings stuck”);
  • Burnout of the cylinder head gasket.

Self-measuring equipment: compressometer and AGC

An engine compression measurement is carried out to identify indirect engine malfunctions. The following tools are used for accurate diagnosis:

  • Compressometer;
  • Compressor;
  • Cylinder tightness analyzer.


It allows for a budget check of the status of the CPG. The cheap model costs around $ 11. It will be enough for several measurements. The more expensive version costs about $ 25. Its kit most often includes several adapters with hoses of different lengths.

How to measure engine compression

It is worth paying attention to the fact that the device can be with a threaded lock, or it can be clamping. In the first case, it is screwed into the plug hole, which makes the procedure easier and more accurate (small leaks are excluded). The rubber bushing of the second type of devices must be firmly pressed against the hole of the candle well.

This appliance is an ordinary pressure gauge with a check valve, which allows you not only to see the indicator, but also to fix it for some time. It is advisable that the check valve be separate, and not be content with the one that the pressure gauge is equipped with. In this case, the measurement accuracy will be higher.

There are also electronic compressometers. This is a motor tester that allows you to measure not only the pressure in the cylinder, but also the changes in the current at the starter during idle cranking of the motor. Such devices are used at professional service stations for deep vehicle diagnostics.


How to measure engine compression

This is a more expensive version of the compression gauge, which not only measures the pressure in an individual cylinder, but also generates a graphical report for each node. This device is classified as professional equipment. Its cost is about $ 300.

Cylinder Leakage Analyzer

This device does not measure the compression itself, but the vacuum in the cylinder. It allows you to assess the condition:

  • cylinders;
  • pistons;
  • piston rings;
  • intake and exhaust valves;
  • valve stem seals (or valve seals);
  • liners (wear);
  • piston rings (coking);
  • valves of the gas distribution mechanism.
How to measure engine compression

The tool allows you to measure indicators without disassembling the engine.

For self-checking at home, a budget compressor is enough. If he showed a low result, then it is worth contacting the service station so that the specialists identify the problem and carry out the necessary repairs.

Measurement of compression of a gasoline and diesel engine

Compression measurements on gasoline and diesel engines are different. In the first case, the procedure is much easier than in the second. The difference is as follows.

Petrol engine

The pressure in this case will be measured through the spark plug holes. Compression is easier to measure on your own if there is good access to the candles. For the procedure, a conventional compressometer is sufficient.

How to measure engine compression

Diesel engine

The fuel-air mixture in this unit ignites according to a different principle: not from the spark generated by the candle, but from the temperature of the air compressed in the cylinder. If the compression in such an engine is low, the engine may not start because the air has not been compressed and heated to such an extent that the fuel ignites.

Measurements are made with preliminary dismantling of fuel injectors or glow plugs (depending on where it is easier to get to and on the recommendations of the manufacturer of a particular motor). This procedure requires certain skills, so the owner of a car with a diesel engine is better off contacting a service.

How to measure engine compression

When purchasing a compressor for such a motor, you need to determine in advance how the measurement will be made - through the hole of the nozzle or glow plug. There are separate adapters for each of them.

Compression measurements in diesel engines do not require pressing the gas pedal, since most modifications do not have a throttle valve. The exception is the internal combustion engine, in the intake manifold of which a special valve is installed.

Fundamental rules

Before taking measurements, you should remember the basic rules:

  • The engine is warmed up to a temperature of 60-80 degrees (the motor runs until the fan turns on). To diagnose problems with a "cold" start, first measure the compression in a cold engine (that is, the internal combustion engine temperature is identical to the air temperature), and then it is warmed up. If the rings are "stuck" or the parts of the cylinder-piston group are very worn out, then the indicator at the start "on cold" will be lower, and when the engine warms up, the pressure rises by several units.
  • The fuel system is disconnected. On a carbureted engine, you can remove the fuel hose from the inlet fitting and lower it into an empty container. If the internal combustion engine is injector, then you can turn off the power supply to the fuel pump. Fuel must not enter the cylinder so that it does not wash out the oil wedge. To shut off the fuel supply to the diesel engine, you can de-energize the solenoid valve on the fuel line or move the high pressure pump shut-off lever down.
  • Unscrewed all candles. Leaving all the spark plugs (except for the cylinder under test) will create additional resistance when turning. crankshaft... Because of this, the compression measurement will be made at different speeds of rotation of the crankshaft.How to measure engine compression
  • Fully charged battery. If it is discharged, then each subsequent rotation of the crankshaft will occur more slowly. Because of this, the end pressure for each cylinder will be different.
  • To crank the crankshaft at a constant speed at the workshop, starting devices can be used.
  • The air filter must be clean.
  • In a gasoline engine, the ignition system is turned off so that the battery does not consume excess energy.
  • The transmission must be in neutral. If the car has an automatic transmission, then the selector must be moved to the P (parking) position.

Since the maximum pressure in the cylinder of a diesel engine exceeds 20 atmospheres (often it reaches 48 atm.), Then an appropriate pressure gauge will be required to measure the compression (increased pressure limit - most often about 60-70 atm.).

How to measure engine compression

On gasoline and diesel units, compression is measured by cranking the crankshaft for several seconds. The first two seconds the arrow in the gauge will rise, then stop. This will be the maximum pressure at the end of the compression stroke. Before you start measuring the next cylinder, the pressure gauge must be reset.

Without compressometer

If the motorist's toolkit does not yet have a personal compression meter, then you can check the pressure without it. Of course, this method is inaccurate and cannot be relied upon to determine the state of the engine. Rather, it is a way to help determine whether the loss of power was due to a motor malfunction or not.

How to measure engine compression

To determine if sufficient pressure is created in the cylinder, one plug is unscrewed, and a wad from dry newspaper is inserted in its place (a rag gag does not work). With normal compression, when the crankshaft cranks, the high pressure gag should shoot out of the spark plug hole. A strong pop will sound.

In case of pressure problems, the wad will still jump out of the well, but there will be no cotton. This procedure should be repeated with each cylinder separately. If in one of them the gag popped out not so "effectively", then the car needs to be taken to a minder.

Using a compressometer

In the classic version, measurements of compression at home are carried out using a compressometer. For this, the motor is warming up. Then all the candles are unscrewed, and instead of them, using an adapter, a hose connected to the pressure gauge is screwed into the candle well (if a pressure gauge is used, then it must be tightly inserted into the hole and held so tightly so that air does not leak out of the cylinder).

How to measure engine compression

The assistant should depress the clutch pedal (to make it easier for the starter to turn the flywheel) and throttle (to open the throttle fully). Before measuring the compression, the assistant tries to start the engine to remove soot and deposits from the cylinder.

The starter is twisted on attraction for about five seconds. Usually this time is enough for the gauge needle to rise and stabilize.

Compression and throttle

The position of the throttle valve changes the compression ratio, therefore, for an accurate diagnosis of the malfunction, the measurement is first carried out with the throttle fully open, and then with the closed one.

Closed damper

In this case, a small amount of air will enter the cylinder. The end pressure will be lower. This test allows you to carry out fine diagnostics of faults. This is what low compression with closed throttle can signal:

  • Valve stuck;
  • Worn out cam camshaft;
  • Not tight fit of the valve to the seat;
  • Crack in the cylinder wall;
  • Rush of the cylinder head gasket.
How to measure engine compression

Such problems can arise as a result of natural wear and tear of some parts. Sometimes such malfunctions are the result of poor-quality ICE repairs.

Open damper

In this case, more air will enter the cylinder, so the pressure at the end of the compression stroke will be noticeably higher than when measuring with a closed damper. With minor leaks, the indicator will not differ much. In view of this, such a diagnosis allows one to determine more severe defects in the CPG. Possible malfunctions include:

  • The piston is burnt out;
  • Rings coked up;
  • The valve is burnt out or its stem is deformed;
  • Ring burst or deformed;
  • Seizures have formed on the cylinder wall mirror.
How to measure engine compression

The dynamics of increasing compression is also important. If it is small during the first compression, and jumps sharply on the next, then this may indicate possible wear of the piston rings.

On the other hand, a sharp formation of pressure during the first compression, and during subsequent compression, does not change, may indicate a violation of the tightness of the cylinder head gasket or valve. It is only possible to pinpoint the malfunction using additional diagnostics.

If the car owner decides to use both methods of measuring compression, then the procedure should first be performed with the throttle valve open. Then you need to screw in the candles and let the motor run. Then the pressure is measured with the damper closed.

Compression measurement with the addition of oil to the cylinder

If the pressure in one of the cylinders drops, the following method can be used, which will help to more accurately determine which malfunction has occurred. After the "problem" cylinder has been identified, 5-10 milliliters of pure oil is poured with a syringe. You need to try to distribute it along the walls of the cylinder, and not pour it on the piston crown.

How to measure engine compression

Additional lubrication will strengthen the oil wedge. If a second measurement showed a significant increase in compression (maybe even higher than the pressure in other cylinders), then this indicates a problem with the rings - they are stuck, broken or coked.

If the compression ratio after adding oil has not changed, but remains still low, then this indicates problems with a violation of the tightness of the valves (burned out, incorrectly adjusted clearances). A similar effect is caused by damage to the cylinder head gasket, a crack in the piston or its burnout. In any case, if there is a discrepancy between the readings of the meter and the data in the technical documentation of the car, you must contact the specialists.

We evaluate the results obtained

If the indicator of pressure in the cylinders differs slightly (the spread of indicators within one atmosphere), then, most likely, this indicates that the cylinder-piston group is in good condition.

Sometimes in a separate cylinder the compressor shows more pressure than in the others. This indicates a malfunction in this node. For example, the oil scraper ring is leaking some oil, which "masks" the problem. In this case, oil carbon deposits will be noticeable on the electrode of the candle (you can read about other types of carbon deposits on candles here).

How to measure engine compression

Some motorists take measurements of the compression on the engine of a car, motorcycle or walk-behind tractor to calculate the time remaining until the overhaul of the power unit. In fact, this procedure is not so informative.

The relative error of such a diagnosis is too large for the compression ratio to be the main parameter that allows you to establish the exact state of the CPG. Compression is influenced by many additional factors, indicated at the beginning of the article... Normal blood pressure does not always indicate that CPH is normal.

Waters is one example. High mileage car. The motor is carbureted, the compression in it is about 1.2 MPa. This is the norm for a new motor. At the same time, oil consumption reaches two liters per 1 kilometers. This example shows that compression measurements are not a "panacea" for solving all problems with the motor. Rather, it is one of the procedures that is included in a complete engine diagnosis.

How to measure engine compression

As you can see, you can check the compression in the cylinders yourself. However, this will help in determining whether the car really needs to be taken to a minder. Only professionals can carry out competent engine diagnostics, and determine which part needs to be changed.



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