0Compression (1)
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How to measure engine compression

The compression ratio of the cylinder-piston group allows you to determine the condition 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 difficulties are observed with starting the engine.

Let us consider for what reasons the pressure in the cylinders may or may not disappear, how to check this parameter, which tool will be needed for this, as well as some of the subtleties of this procedure.

What compression measurement shows: basic malfunctions

Before considering how to measure compression, you need to understand the definition itself. Often it is confused with the degree of compression. In fact, 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).

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

Compression (a more precise definition is the pressure of the end of the compression stroke) means the maximum pressure that the piston creates when it reaches top dead center at the end of the compression stroke (both valves are closed - both the inlet and the outlet).

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Compression depends on the degree of compression, but the second parameter does not depend on the first. The 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 the measurement;
  • leakage in the cylinder;
  • crankshaft starting revolutions;
  • a dead battery;
  • excessive amount of oil in the cylinder (with a worn cylinder-piston group);
  • resistance in the intake manifold pipe;
  • engine oil viscosity.

Some mechanics are trying to increase the power of the motor 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.

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Modified compression ratio

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

  1. Cold start engine. This factor is especially important for diesel engines. In them, the air-fuel mixture ignites due to the temperature of highly compressed air. For gasoline units, this parameter is no less important.
  2. In some cases, a decrease in compression causes an increase in crankcase gas pressure. Because of this, a greater volume of oil vapor enters the engine, which leads to increased toxicity of the exhaust, as well as to pollution of the combustion chamber.
  3. Car dynamics. With a decrease in compression, the engine speed decreases noticeably, fuel consumption increases, the oil level in the crankcase drops faster (if grease 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 of the end of the compression stroke, since it depends on the parameters of an individual power unit. In view of this, it is impossible to name the universal value of compression for all power units. This parameter can be found in their vehicle technical documentation.

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

  • Piston wear. Since these parts are made of aluminum, they wear out over time. If a hole forms in the piston (burns out), the compression in that cylinder can greatly decrease or practically disappear (depending on the size of the hole).
  • Valve burnout. This often happens when the ignition is set incorrectly. In this case, the combustion of the air-fuel mixture occurs when the valve is open, which leads to overheating of its edges. Another cause of burn-out of the valve seat or "plate" is the lean air-fuel mixture. A decrease in compression may also be due to the fact that the valves are loose in the seats (deformed). The gaps between the valve and its seat cause premature gas leakage, causing the piston to be pushed out with insufficient force.4 Progorevshij Valve (1)
  • Damage to the cylinder head gasket. If it bursts for some reason, the gases will partially show through into the crack (pressure in the cylinder is high, and they will certainly find a “weak spot”).
  • Piston ring wear. If the rings are in good condition, they regulate the oil flow and seal the sliding movements of the piston. Another function is to remove heat from the piston to the cylinder walls. When the tightness of the compression pistons is violated, the exhaust gases penetrate the crankcase to a greater extent, and are not removed to the exhaust system. If the oil scraper rings wear out, a larger amount of lubricant enters the combustion chamber, resulting in 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 the natural wear of the cylinder-piston group (or some of its parts, for example, rings).

When the pressure of the end of the compression stroke of one cylinder (or several) is significantly different from the compression in others, this indicates a malfunction in this node. Among the reasons, the following can be distinguished:

  • Burnt valve;
  • Sagged piston rings (mechanics call it "ringed rings");
  • Burnout of cylinder head gasket.

Self-measurement equipment: compressometer and AGC

Measurement of engine compression is performed to detect indirect engine malfunctions. For accurate diagnosis, the following tools are used:

  • Compressometer
  • Compressograph
  • Analyzer of tightness of cylinders.

Compressometer

It allows you to conduct a budget check of the state of the CPG. A cheap model costs around $ 11. It will be enough for a few measurements. The cost of the more expensive version is about $ 25. Her kit most often includes several adapters with hoses of different lengths.

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

This device is a conventional pressure gauge with a check valve, which allows not only to see the indicator, but also to fix it for a while. It is desirable that the non-return valve be separate, and not be content with the one 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 current changes on the starter during the "idle" scrolling of the motor. Such devices are used at professional service stations for in-depth vehicle diagnostics.

Compressograph

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This is a more expensive version of the compressor, which not only measures the pressure in a separate cylinder, but also creates a graphical report for each node. This device belongs to the category of professional equipment. Its cost is about 300 dollars.

Cylinder tightness 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);
  • sleeves (their wear);
  • piston rings (their coking);
  • valve timing.
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The tool allows you to measure performance without disassembling the engine.

For an independent check at home, a budget compressor is enough. If he showed a low result, then it is worth contacting a service station so that 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 candle holes. Self-compression is easier to measure if there is good access to the candles. A conventional compressorometer is sufficient for the procedure.

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Diesel engine

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

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

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

Compression measurements in diesel engines do not need to be depressed on the gas pedal, as in most versions there is no throttle. An exception is the internal combustion engine, in the intake manifold of which a special valve is installed.

Fundamental rules

Before taking measurements, remember the basic rules:

  • The engine warms up to a temperature of 60-80 degrees (the engine runs until the fan turns on). To diagnose problems with a "cold" start, the compression in the cold motor is first measured (that is, the temperature of the internal combustion engine is identical to the air temperature), and then it is heated. If the rings “lie down” or the details of the cylinder-piston group are very worn, then the indicator at start-up “to the cold” will be lower, and when the engine warms up, the pressure rises by several units.
  • The fuel system is off. In the carburetor 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 of the fuel pump. Fuel must not enter the cylinder so that it does not erode the oil wedge. To shut off the fuel supply in a diesel engine, you can de-energize the solenoid valve on the fuel line or move down the shut-off lever of the high-pressure pump.
  • Unscrewed all the candles. If you leave all the candles (except the tested cylinder), this will create additional resistance when turning crankshaft. Because of it, the measurement of compression will be performed at different speeds of rotation of the crankshaft.11Candles (1)
  • Fully charged battery. If it is discharged, then each subsequent rotation of the crankshaft will occur more slowly. Because of this, the pressure of the end of the compression stroke in each cylinder will be different.
  • To crank the crankshaft at a constant speed at the service station, 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 gearbox must be in neutral. If the car has an automatic transmission, then the selector must be moved to position P (parking).

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

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On a gasoline and diesel unit, compression is measured by cranking the crankshaft for several seconds. For the first two seconds, the needle in the gauge will rise, then stop. This will be the maximum pressure at the end of the compression stroke. Before proceeding with measurements of the next cylinder, the pressure gauge must be reset.

Without compressor

If the motorist’s tools do not yet have a personal compressor, then you can check the pressure without it. Of course, this method is inaccurate, and this indicator cannot be relied on to determine the condition of the engine. Rather, it is a way to help determine whether power loss has occurred due to a motor malfunction or not.

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To determine whether sufficient pressure is created in the cylinder, one candle is twisted and a wad from a dry newspaper is inserted in its place (a rag gag does not fit). Under normal compression, when the crankshaft is cranking, the gag should shoot out of the spark plug hole due to high pressure. Strong cotton will sound.

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

With a compressometer

In the classic version, compression measurements at home are carried out using a compressor. For this, the engine warms up. Then all the candles are unscrewed, and instead of them, with the help of an adapter, a hose connected to the manometer is screwed into the candle well (if a clamping analog is used, it must be firmly inserted into the hole and held so that air does not leak out of the cylinder).

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

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

Compression and throttle

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

Closed damper

In this case, a small amount of air will enter the cylinder. The pressure of the end of the compression stroke will be less. This check allows you to fine-tune the troubleshooting. This is what low compression can indicate when the throttle is closed:

  • Valve hang
  • Worn cam camshaft;
  • Not tight fit of the valve to the seat;
  • Crack in the cylinder wall;
  • Gasket cylinder head gasket.
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Such problems may arise as a result of the natural wear and tear of some parts. Sometimes such malfunctions are the result of poor-quality engine repair.

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 the shutter closed. With minor leaks, the indicator will not differ much. In view of this, such a diagnosis allows us to identify more severe defects of the CPG. Among the possible malfunctions:

  • The piston burned out;
  • Rings coked;
  • Valve burned out or its core deformed;
  • The ring has broken or deformed;
  • Scuffing formed on the mirror of the cylinder walls.
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Equally important is the dynamics of increasing compression. If it is small during the first compression, and jumps sharply at the next, this may indicate a possible wear of the piston rings.

On the other hand, a sharp pressure build-up during the first compression, but not changing during the subsequent ones, may indicate a violation of the tightness of the cylinder head gasket or valve. Accurately determine the malfunction is possible only with the help of additional diagnostics.

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

Measurement of compression with the addition of oil to the cylinder

If pressure drops in one of the cylinders, you can use the following method, which will help to more accurately establish what kind of malfunction has formed. After the “problematic” cylinder has been determined, 5-10 milliliters of pure oil is poured with a syringe. It is necessary to try to distribute it on the cylinder walls, and not pour it on the piston bottom.

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Additional lubrication will strengthen the oil wedge. If the repeated 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 buried, 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 gives damage to the cylinder head gasket, a crack in the piston or its burnout. In any case, if there is a discrepancy between the meter readings and the data in the technical documentation of the car, you need to contact specialists.

We evaluate the results

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

Sometimes in a separate cylinder, the compressor shows more pressure than in the rest. This indicates a malfunction in this node. For example, the oil scraper ring passes a certain amount of oil, which “masks” the problem. In this case, oil soot will be visible on the electrode of the candle (for other types of soot on candles, you can read here).

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Some motorists take measurements of compression on the engine of a car, motorcycle or walk-behind tractor in order to calculate the time remaining before 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 rate 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. Not always normal pressure indicates that CPG 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 included in a complete engine diagnosis.

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As you can see, the compression test in the cylinders can be performed independently. However, this will help in determining whether the car really needs to be taken to the minder. Competent engine diagnostics can only be carried out by professionals, and determine which part needs to be changed.

Measurement of compression for cold or hot

Measurements of the compression of a diesel engine are made in a slightly different way, since this power unit works according to a different principle (air and fuel are mixed directly at the moment of diesel fuel injection into the chamber, and due to strong air compression, this mixture ignites spontaneously). By the way, since the air in the cylinders of a diesel engine must heat up from compression, then the compression in such an engine will be higher than that of a gasoline analogue.

First, the valve that opens the fuel supply is turned off in the diesel engine. The fuel supply can also be shut off by squeezing the cutoff lever installed on the injection pump. To determine the compression in such an engine, a special compression meter is used. Many diesel models do not have a throttle valve, so there is no need to press the accelerator pedal during measurements. If a damper is still installed in the car, then it must be cleaned before taking measurements.

Based on the results, the state of the cylinder-piston group of the unit is determined. Moreover, it is necessary to pay more attention to the difference between the indicators of individual cylinders than to the average compression value in the entire engine. The degree of CPG wear is also determined taking into account the temperature of the oil in the internal combustion engine, the incoming air, the speed of rotation of the crankshaft and other parameters.

An important condition that should be taken into account when measuring compression, regardless of the type of power unit, is the warming up of the engine. Before connecting the compressor to the cylinders, it is necessary to bring the internal combustion engine to operating temperature. This will ensure the proper oil back up, just like when the car is moving. Basically, the desired temperature is reached at the moment when the cooling system fan turns on (in case the engine thermometer scale has no numbers, but only divisions).

On a gasoline engine, as in the case of a diesel engine, it is necessary to shut off the fuel supply. This can be done by de-energizing the fuel pump (this applies to the injectors). If the car is carbureted, then the fuel hose is disconnected from the carburetor, the free edge is lowered into an empty container. The reason for this procedure is that the fuel pump in such a car has a mechanical drive and will pump gasoline. Before connecting the compressor, it is necessary to burn all the fuel from the carburetor (let the machine run until the engine stalls).

How to measure engine compression

Next, ALL ignition coils are unscrewed (if the machine uses individual SZ for each cylinder). If this is not done, then during the procedure, they will simply burn out. Also, ALL spark plugs are unscrewed from the cylinders. A compressor is connected to each cylinder in turn. It is necessary to crank the crankshaft several times with the starter (until the pressure on the scale stops increasing). The results are compared with the factory value (this information is indicated in the instructions for the machine).

There are two opposing opinions among motorists as to when to test compression: cold or hot. In this regard, the most accurate indicator will be a measurement taken after the vehicle has been preheated, since in a cold unit there is no oil film between the rings and the cylinder wall. Naturally, in this case, the compression of the internal combustion engine will be less than after warming up. If this "drawback" is eliminated, then when the unit heats up, as a result of the expansion of the ring, the cylinder mirror will be damaged.

But when the engine does not start at all, then it is worth checking the compression for a cold one in order to identify or eliminate problems with the cylinder-piston group. In the process of performing this procedure, it should be borne in mind that measurements are taken on a cold one, therefore the ideal indicator should be lower than that specified by the manufacturer.

Another factor to consider, regardless of when compression is tested, is the battery charge. The starter must provide high-quality cranking, which can give incorrect results on a dead battery. If the battery is "living out its last days", then in the process of measuring the compression, a charger can be connected to the power source.

Signs of decreased compression

Due to a decrease in the compression ratio, the following problems with the motor may occur:

  • The motor has lost traction. Exhaust gases and a partially combustible mixture enter the engine crankcase. Because of this, the piston is not pushed to top dead center with such force;
  • The oil needs to be changed, even when the car does not maintain the prescribed mileage (the lubricant becomes less fluid and darkens decently). This is due to the fact that a certain amount of an air-fuel mixture seeps into the lubrication system, and subsequently the oil burns out faster;
  • Fuel consumption has increased markedly, but the driver did not change the driving mode, and the car does not transport more cargo.

If at least one of the listed symptoms appears, it is not recommended to continue operating the vehicle until the cause of these symptoms has been eliminated. First, it is economically unjustified. Secondly, due to the problems that have arisen, sooner or later, other related breakdowns of the unit will appear along the way. And this will also negatively affect the thickness of the motorist's wallet.

The reasons for the decrease in compression in the cylinders

The compression in the motor decreases for the following reasons:

  • Due to the formation of carbon deposits on the inside of the cylinders and pistons, they overheat (heat exchange is worse), and as a result of this, burnout of the piston may occur or carbon deposits will scratch the mirror of the cylinder wall;
  • Due to disturbed heat transfer, cracks can form on the parts of the CPG (severe overheating without proper subsequent cooling);
  • Burnout of the piston;
  • The cylinder head gasket is burnt out;
  • The valves are deformed;
  • Dirty air filter (the proper amount of fresh air is not sucked into the cylinders, which is why the air-fuel mixture is not compressed so well).

It is impossible to determine why the loss of compression has occurred, visually without disassembling the motor. For this reason, a sharp decrease in this indicator is a signal for diagnostics and subsequent repair of the motor.

At the end of the review, we offer a short video on how the compression of an internal combustion engine is measured:

Life is pain when compression is zero

Questions and answers:

How to measure compression on a carburetor engine. This will require an assistant. Sitting in the passenger compartment, he fully depresses the accelerator pedal and cranks the starter, as when starting the power unit. Typically, this procedure requires a maximum of five seconds of starter operation. The pressure arrow on the compressor will gradually increase. As soon as it reaches the maximum position, the measurements are considered complete. This procedure is carried out with the candles turned inside out. The same steps are repeated on each cylinder.

How to check compression on an injection engine. The basic principle of checking the compression on the injector does not differ from the identical operation with the carburetor unit. But you need to take into account a couple of nuances. First, it is necessary to disable the crankshaft sensor so as not to damage the ECU controls. Secondly, it is necessary to de-energize the fuel pump so that it does not uselessly pump gasoline.

How to measure cold or hot compression. Compression measurement on a cold and hot engine is no different. The only real value can be obtained only on a heated internal combustion engine. In this case, there is already an oil film on the cylinder walls, which ensures the maximum pressure in the cylinders. On a cold power unit, this indicator should always be lower than the indicator specified by the automaker.

One comment

  • Joachim Uebel

    Hello Mr. Falkenko,
    You did really well. As a German teacher, I teach professional language courses and have chosen the profession of mechatronics technician for further training. I used to repair cars and tractors myself. I would like to change the German in your article a bit, at no cost to you. An example: You write “and the car no longer transports freight” would mean “and the car no longer pulls properly” in German. For example, the word “node” should be replaced by “area”, etc. But I could only do that during the summer holidays. Please contact me. And I'll say it again clearly for everyone: Your site is great.

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