Low Compression Symptoms in Engine Cylinders and How To Check

by | Apr 1, 2020 | News | 0 comments

If you have a car, then you must have heard more than once about the term “engine compression check”. The fall of this compression itself is fraught with a decrease in power and an increase in fuel consumption. And such an engine is often much more difficult to start. In this article, in simple words, we explain the concept of “compression”, and talk about the reasons for its decline and the consequences for the engine. Also find out how often you need to check engine compression.

What is compression and what are its norms depending on the type of engine

Compression of engine cylinders is a key indicator of the quality of a motor unit. These words mean the maximum air pressure that is created in the combustion chamber at the moment when the piston is at the highest point of the compression stroke. Multiple units of measure for this parameter have been accepted. The most common is kg / cm2 (the number of kilograms per unit area per square centimeter). It also happens that compression is designated in atmospheres.

According to the compression value, earlier the engines were divided into forced (more than 9 kg / cm2), medium-forced (7–9 kg / cm2) and unforced (6–7 kg / cm2).

Almost every day we see how an undetected and uncorrected problem leads to an expensive repair, and even to neglected cases, to accidents (God forbid, of course!).
We believe that it is CRITICALY IMPORTANT for everyone to know about the 3 most common problems that motorists face most often (below you can download a document that details all the possible problems):
  1. Dashboard Warning Icons: Engine Check Indicator The “Engine Check” icon on the dashboard is the most common problem for cars and trucks. This indicator on the tidy lights up when the computer (electronic engine control unit) detects an error code in the system in the operation of any system. In total, there are more than 200 possible error codes, in order to identify the cause, you need to do a full-fledged electronic diagnosis in the car service nearest to you (most often it is free).
  2. High fuel consumption When the engine is operating normally, it burns fuel at a speed that helps to efficiently consume fuel (we get fuel economy). However, some parts of the fuel system, such as fuel filters, air filters, mass air flow sensors and oxygen sensors (lambda probe), become dirty and worn out over time. If they are not replaced (and this is the most common reason for increased fuel consumption), this will lead to the fact that the engine will consume more fuel than usual.
  3. Squeaking brakes Brakes in a car are sacred. It is from them that the safety of our car, and sometimes life, often depends! Therefore, when you notice any signs of problems, such as a squeal, a creak, or if, for example, the brake pedal becomes soft, you should, of course, show the car mechanic as quickly as possible. Most often, a creak or squeal suggests that it is necessary to replace brake pads or brake discs. Some car breakdowns are not critical and allow you to operate the car, while others, on the contrary, require immediate intervention.

checking engine compressionThe diesel engine has outstanding compression values ​​- from 28 kg / cm2 and higher. Of course, diesel engines with a more modest 20 kg / cm2 were previously produced. A compression spread in cylinders of 2-3 kg / cm2 was allowed.

As for gasoline engines, at present a value of 10–13 atmospheres is a generally accepted normal value. This is true for both imported and domestic cars. On some cars, for example, a VAZ of the tenth series, the compression in the cylinders is 11 atmospheres and above.

Causes and effects of low compression in cylinders

The main reason for compression liquefaction is friction. Of course, this is opposed to the protective film formed by the oil on the walls of the cylinder and piston. However, despite this protection, gradual wear and tear still occurs. Layer by layer, the cylinder self-develops. As a result, leaks are formed between the piston rings and the cylinder. At the time of compression through these leaks, the fuel mixture leaks into the crankcase. Efficiency of the engine is reduced due to the fact that not all fuel is converted into useful energy.

  • carbon deposits on the walls of the cylinder and piston;
  • burnout of the piston bottom due to high temperatures;
  • deformation of the valves, including their burnout;
  • cracking in the cylinders;
  • cylinder head gasket wear;
  • air filter contamination.

What are the consequences of a drop in compression! This directly affects the operation of the machine unit:

  • difficulty starting the engine;
  • failures occur in different operating modes;
  • bumps and pops appear that can be easily heard;
  • fuel consumption is steadily increasing;
  • increased pressure in the channels of the engine cooling system;
  • black smoke may come out of the chimney.

bad fumes

It can be concluded that engine compression can be significantly reduced for a number of reasons. What can an ordinary car owner do without the skills of an auto mechanic! Having noticed at least one of the above signs, you need to go to the diagnostic station as soon as possible to check the compression in the engine cylinders. The price of the service may seem high, but do not regret the possible costs. Operation of such an engine without repair can lead to much greater problems in the future.

Do-it-yourself engine compression test without a compressor

There are only two tools for checking this important parameter of the motor:

  • pressure gauge for checking engine compression;
  • own hands of the car owner.

Undoubtedly, these two methods are incomparable in the accuracy of the obtained values. No professional will not check compression by hand. However, you can use this method in a situation when there is no station or device nearby. After all, even the approximate value of compression will help assess the condition of the engine.

Despite the simplicity of manual compression measurement, it is still required to develop a small skill in this matter and have certain knowledge. In view of the above, the procedure is even for beginners.

Have low compression in one cylinder!

The procedure for checking compression in the engine cylinders is as follows:

  1. The first thing to do is unscrew the spark plugs, except for the spark plugs of the first cylinder.
  2. Turn the crankshaft, trying to put the piston in the first cylinder to the upper point of the compression stroke.
  3. Reconciliation with marks will help determine the exact position of the piston.
  4. Remember the effort required to manually scroll the crankshaft.
  5. Unscrew the candle from the first cylinder and put in place the candle of the second cylinder. Then repeat all over again.

The purpose of these measures is to determine by means of an exception which cylinder is lower in compression than in others. This will be clearly visible if the crankshaft scrolls easier when checking this cylinder than in other cases.

The method does not claim to be absolutely accurate. However, even with such a simple method, one can quite clearly determine which cylinder needs repair. Specific pressure values ​​can only be obtained using the intended for this device – a pressure gauge for checking compression.

How to check engine compression without a gauge.

Diagnostic stations ask for a lot of money for their services. However, an experienced car owner is able to independently diagnose his engine for compression in the cylinders. To do this, you need a special device – a compressometer that looks like a pressure gauge.

A very useful part of this device is a threaded fitting, with which a manometer is screwed in instead of a candle.

  1. First of all, unscrew the spark plugs. This is done without heating, on a cold unit. Unscrew the candles carefully, without damaging the thread. Further, to facilitate the process, it is necessary to lubricate the thread on the candles with anti-seize paste. Then the candles are screwed back, and the tightening torque is set exactly using a torque wrench. The tightening torque can be found in the machine manual.
  2. A great influence on the reliability of measurements is exerted by the amount of battery charge. That is, if during the test the battery charge is significantly reduced, then the results of such measurements will be incorrect.
  3. Prepare the appropriate appliance. For example, checking the compression in the cylinders of a diesel engine requires a compressometer designed for more pressure than in the case of a gasoline engine.
  4. Then the engine is warmed up. It doesn’t matter how – on the road or at idle. But the cylinders and valves must come into a hot state that is working for the engine.
  5. Having turned off the engine, they begin to measure. Open the throttle to prevent the motor from running out of air. It can be fixed either with a screwdriver or by squeezing the gas pedal all the way for the duration of the test. You also need to disconnect the air filter at the point of its attachment to the carburetor or throttle.
  6. It is necessary to prevent the fuel mixture from entering the engine by removing the fuel pump fuse or injection system fuse. This is an electric fuse. Determine his position will help all the same manual on the machine.
  7. Then start the engine. It must work until it has consumed all the fuel from the line, then it will die out.
  8. All high-voltage wires of the spark plugs are removed in turn. In this case, it is necessary to mark which wire from which candle was removed to put everything later in its place.
  9. The place where the spark plug is installed – the well, is cleaned with compressed air and a soft brush. This is necessary so that debris does not get inside the cylinder and does not cause damage during operation.
  10. Next, remove the spark plugs using a special tool.
  11. Disconnect the ignition system by removing the wire or the entire connector from the coil.
  12. Then set the compressor in the first cylinder in place of the spark plug. Quality appliances have a threaded connection. Cheap Chinese counterparts are equipped with a rubber fitting. They must be held pressed during compression measurement.
  13. When measuring, it is desirable to have an assistant. It is he who must crank the engine with the ignition key. After several cycles of the motor, the pressure reaches the maximum value that the device displays. This figure must be recorded.
  14. The compressor has a special valve to reset the readings. Using it, zero the device. Then he is ready for a new measurement cycle.
  15. The above three steps are repeated for each cylinder.
  16. They process the obtained results, comparing them with the minimum and maximum ones from the vehicle operation manual.

The second way to check the compression of a gasoline engine using a compressor

There is an alternative way to check compression in gasoline units. To do this, do the following:

  • a small amount of engine oil is poured into the cylinder;
  • carry out routine measurements with a compressometer;
  • if the results indicate increased pressure, then we can definitely conclude that the piston rings are malfunctioning. In this case, it is necessary to replace them;
  • if pressure increase is not observed, then head wear is possible. Or, this phenomenon may indicate leakage of valves. If these parts are in good condition, the rubber gasket may be to blame.

causes of poor engine compression

The most serious problem with a lack of compression are burnt pistons. It also happens that pieces of rings or crumpled valves get stuck in the pistons. A working carburetor can also indicate a malfunction in the cylinder. Thus, periodic air emissions from it indicate an incomplete fit of the check valve. If there is a crack in the cylinder head, then it will release air through the radiator.

Checking the compression in the cylinders of a diesel engine is characterized by a number of nuances. So, the device for such a procedure should have an extended measurement range. In diesel engines, there is usually no throttle, therefore, an assistant for pressing gas is not required. If there is such a damper, thoroughly clean it from debris and other contaminants before taking measurements.

A diesel engine is usually more complex than a gasoline engine. And for each model, there is a normal compression value. Therefore, it would be advisable to contact the service, whatever the price of checking engine compression.

  • The pressure in the cylinders is checked both with the hole for the candles and at the outlet of the nozzle. It’s easier, of course, to use a candle hole.
  • The engine is brought to operating temperature before checking. The cold state of the engine will lead to a large measurement error.
  • If there is a throttle, it must be fully opened.
  • The requirements for the serviceability of the starter and battery of the car remain unchanged.
  • Glow plugs must be removed before the measurement cycle.

The compressor is installed instead of the spark plug and the engine starts with a starter, while the air damper is always open. After two or three revolutions of the crankshaft, the ignition is turned off and the readings of the compressor are recorded. Then go to the next cylinder.

Based on the obtained verification values, a conclusion is made about the condition of the engine cylinders. If the measurements are within acceptable limits, there is nothing to worry about, the motor is working. Slightly underestimated readings can be corrected with an additive suitable specifically for this engine. If the compression drop is significant, then repair of the power unit cannot be dispensed with.

Is the engine compression test “cold” or “hot”!

Many drivers ask if compression testing on a cold engine is possible. If the engine starts hard, then this procedure is not only allowed, but also recommended. In this case, the compression test is performed at an engine temperature equal to the ambient temperature.

Problems with starting the engine may indicate a strong wear of the piston group parts. The pressure in the cylinders of a gasoline engine can then drop to 6-7 atmospheres (half the nominal value). When heated, the compression increases by several atmospheres. Checking “hot” and “cold” will clarify these nuances and accurately establish the cause of the malfunction.

Diesel engines are even more cranky about CPG wear than gasoline engines. So, an engine with compression “on cold” below 17-18 atmospheres cannot be started anymore. Diesel engines have another nuance – the amount of oil in the cylinders directly affects the pressure. Therefore, before measuring the compression, it is recommended to let the engine stand so that the oil glass in the crankcase. Compression values ​​below 24 directly indicate the need for a major overhaul of the power plant.

Dmitry Petrov

Dmitry Petrov is an engineer who specializes in materials science, and has a deep passion for all things related to automotive technology. He is a true motorhead at heart, and spends much of his free time tinkering with engines and studying vehicular dynamics.


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