Tested 7 devices to assess compression of the engine
Checking the compression in the cylinders is the easiest way to diagnose the mechanical part of the engine. It can be done by any motorist himself. Except that not all compressometers are equally convenient and accurate.
The term “compression”, in general, is not quite professional – it is more correct to talk about the pressure at the end of the compression stroke. However, “compression” is so firmly embedded in our lexicon that we will use this word in the future.
How does compression differ from compression ratio?
Everything! Compression is the pressure that is created in the cylinder when the ignition is turned off (or when the fuel supply is turned off in a diesel) with the piston at the top dead center. Compression depends on many parameters: compression start pressure, valve timing, temperature at which the measurement is made, and the wear of the engine parts themselves. The compression is an indicator of the real health of the engine, which deteriorates with age.
The compression ratio, on the other hand, does not change over the years. This is a dimensionless parameter, laid down by the engine designer. It characterizes the geometry of the cylinder, more precisely, the ratio of its total volume to the volume of the compression chamber. It is obvious that compression depends on compression ratio, but compression ratio does not depend on compression!
Compression chamber or combustion chamber?
We are talking about the same thing: the volume of the space above the piston at its top dead center (TDC) position. It is sometimes also called the end of compression volume. The term “combustion chamber” is not quite correct, since combustion takes place almost in the whole volume of the cylinder.
If the compression is low…
If the compression is low, the engine will be worse to start. This is especially noticeable on diesels, where the compression end pressure and temperature determines whether or not the diesel will ignite.
In gasoline engines, with reduced compression, the fuel entering the cylinder begins to evaporate worse, remaining in the form of non-flammable droplets. Crankcase gas pressure increases – toxicity increases, and the compression chamber becomes contaminated at an accelerated rate. And if the compression has become irregular in cylinders, the engine can begin to vibrate, especially at idle.
How and with what to measure the compression
To measure it, you need a compressometer – a device that measures the pressure of the working mixture at the end of compression in the cylinder. For examination we have bought seven tools of similar purpose, but we will return to them a little later – it is necessary to prepare the engine for measurements beforehand.
The battery must be fully charged. We conduct the test on the engine warmed up to working temperature. While getting ready for the test, the temperature will drop a little, but it must not be lower than 50-60 °C. To avoid washing the oil film from the walls of the cylinders it is desirable to disconnect the fuel supply. To do this, disconnect either the common connector of the injectors harness, or connectors from each injector.
Then disconnect the wiring harness from the ignition module or individual ignition coils. Having prepared access and cleaned the recesses in the cylinder head around the plugs, unscrew all the plugs. If you don’t clean, don’t be surprised if the compression soon drops: sand trapped in the cylinders will quickly destroy them.
It is necessary to ensure free passage of air into the cylinders. For this purpose, on cars with cable-operated throttle valve it is enough to press the accelerator pedal to the floor. But with the electronic drive, it is desirable to verify whether the choke opens when the ignition is turned on, the gas pedal is pressed and cranking the starter. To do this, you will have to disconnect the pipe that supplies air to the throttle assembly from the air filter. If the choke does not open, it is necessary to dismantle the throttle assembly. The second variant – at your own risk, having disconnected electrical connector from throttle unit, try to rotate flap very carefully with your finger until it is fully opened. It is spring-loaded, and therefore it will have to be held during all measurements.
To measure, insert or screw in the tip of the compressometer into the spark plug hole. Turn the engine with the starter until the pressure gauge readings stop increasing. Fix the result and reset the pressure in the compressometer.
How to estimate results
The compression of the serviceable petrol engine must be within 11-14 bar, and the difference of readings on the cylinders must not exceed 1 bar.
To find out the causes of low compression pour into the cylinder through the spark plug hole 5-10 cm ³ engine oil and repeat the measurement. If the compression has increased by more than a couple of bars, the wear, lodging or damage of piston rings is most likely. If the reading did not increase, the valves are probably loose against the seats. This happens when the thermal gaps in their actuator are disturbed, and also when the valve poppets or seats are worn, burned out or damaged.
Why does the compression value differ from the compression ratio?
An engine with minimal piston ring leakage and tight valves has higher compression than the geometric compression ratio because the air in the cylinder is heated during compression – this additionally increases its pressure. And low compression can be at significant leaks.
Checking compression in engine cylinders – methods with and without a device
Checking engine cylinder compression is recommended in almost any instructions when looking for malfunctions: when the engine has oil, when it stalls under any circumstances, when the machine works poorly at idle or just a drop in its power. In many cases, the cause of such problems can be poor compression in the engine, and in order to identify the cause, you need to know what the compression should be specifically in the engine of your car model and how to check this very compression. Compression is the pressure of the compressed air in the cylinder at the very end of the compression stroke, when this pressure is maximum.
We decided to write these instructions because measuring compression in the cylinders is a very simple task. You will need to measure the compression in each cylinder in turn while the engine is cranked by the starter, but not running on its own on fuel. Then you will only have to compare the pressure readings in all cylinders between each other and with the pressure specified in your car’s specifications (the instruction manual or any other reliable source of your car’s extended engine specifications).
How to check the compression in the engine?
To check you will need an indispensable attribute of our work – a compressometer, and you will have to buy it in the nearest auto store or just ask your car mechanic, hoarder or inveterate car enthusiast or rent one. Compressometer is a very inexpensive thing and costs a little more than a pressure gauge, although prices for the former vary much more. In general, the cheapest compressometer will cost you 400-1000 rubles, but there are also more professional devices worth up to 10-15 thousand rubles, but you hardly need them.
When selecting a compressometer, make sure that its tip is screwed into the spark plug passage (there are some which mercilessly clog there), so that the tip and the gauge itself are connected by a flexible hose rather than a pipe, as in the latter case you may have to remove something else from under the hood, as it will simply get in our way when installing the compressometer. In addition, look for a maximum pressure jackal of at least 20 kg/cm2.
Compressometer with nozzle adapters for different types of plugs
In addition to the compressometer, you’ll need an assistant to crank the starter with the ignition key from the cabin, and a spark plug wrench. Also, your car should have a serviceable starter and, most importantly, excellent battery condition, since it will have to work quite a bit.
For an accurate and correct measurement, you will need to do the following few steps:
- First, start the engine and let it run for as long as it needs to until it reaches normal operating temperature (90º), then shut it off and remove the wires from all the spark plugs. As you remove them, mark each wire and its corresponding spark plug with a different color of paint (marker or marker pen) for each spark plug so you don’t mix them up.
- Now you need to prevent fuel from flowing into the engine. The easiest way to do this is to disconnect the gasoline pump from it. Remove the plug wires by grasping the caps without pulling on the wires, which could damage them. Label the gauge wires if necessary to avoid confusion when putting them back in place.
- Clean the areas around the spark plugs to prevent dirt from entering the engine. Unscrew the spark plugs with a head or special spark plug wrench.
- Each compressometer has a pressure relief valve – remember to reset the tester before testing each subsequent cylinder. Screw the tip of the compressometer into the first spark plug hole.
- Ask an assistant to push the accelerator pedal all the way down to open the throttle fully and begin cranking the starter with the ignition key for about 6-10 seconds. At this time the pressure with each revolution of the engine in the compressometer will rise and will reach a certain maximum level – then you must turn off the starter.
- Be sure to record the result of the compression test in the cylinder. You need to write down what compression was obtained as a result of the compression measurement and how long it took approximately in seconds to reach the maximum pressure on the pressure gauge.
- Check all the other cylinders in the same way, also writing down the engine compression reading on a piece of paper.
Order of connection of compressometer to check compression in the engine: take off high-voltage wires from plugs; unscrew all plugs; screw in instead of plugs in turn compressometer
The compression rate is different for each engine, but in general it is considered normal for a gasoline engine to be about 10-12 kg/cm2. However, check this information for your car model and specification.
If any cylinder has lost compression
If one or more cylinders show significantly lower compression than the others, we will need to do an additional test to determine if the low compression is due to leakage through worn piston rings or other microscopic holes or cracks in the cylinder.
- Pour a tablespoon (about 10 ml) of clean engine oil into the suspected cylinder.
- Connect a compressometer and crank the engine again according to the above instructions.
- If this resulted in a noticeable increase in engine pressure, it most likely means that the piston rings are worn or there is some other air leak in the cylinder, and you need to go to an auto mechanic. The fact is that the engine oil in such a leak will rush to the leak and, once there, will clog it and keep the air from escaping.
- If the compression reading did not change during the test, then most likely the leak is so large that the engine oil simply could not “seal” it. The engine valves or badly leaking gaskets could be the culprit.
The cylinder head must be removed to check the valves, and this is also not an easy procedure. A leaking gasket, however, will show symptoms other than loss of compression. A leak between cylinders will cause the engine to run unevenly, and there may also be a loss of coolant into the crankcase, which will show itself as white emulsifying foreign fluid on the oil check dipstick and a generally high oil level.
Table: Compression deviations and possible engine faults
|Engine compression, kg/cm2 (with throttle pedal fully depressed/open)||Possible malfunction||Possible additional symptoms|
|12-15||(Increased compression) Fouling in combustion chamber with worn intake plugs||Blue colored exhaust and high oil consumption|
|6,5-9,5||Piston rings or pistons are worn, or there is damage to the cylinder walls||Blue colored exhaust and high oil consumption|
|7-8,5||Damage to camshaft cam, causing valve not to open/close properly||Complete or partial cylinder failure|
|6-8||Cracked piston jumper||Blue exhaust, higher oil flow, higher oil pressure|
|5-8||Valve does not return to original position||Complete or partial cylinder failure|
|4,5-6||Piston overheats and burns out||Unstable engine idle speed, increased oil consumption|
Specialists recommend measuring the pressure in cylinders after 3-5 thousand km run for cars of domestic production and after 10-20 thousand km for foreign cars. It is necessary to measure the compression at an open and closed throttle, it will give an opportunity to determine the cause of engine malfunction more accurately.
The check should be carried out with the engine warmed up. If the machine has been at rest, the engine must be started and warmed up to operating temperature. After warming up, the ignition must be turned off and the fuel supply cut off. The battery must be fully charged for proper testing. Insufficient charge of the battery will not allow the starter to crank to full power, which can seriously distort the test readings. In addition, you should check that the starter is in good working order.
Diagnosing Engine Performance
The main part of the check
Checking the pressure in the cylinders consists of the following steps:
- First you need to disconnect the spark plug wires.
- Next, you should unscrew the spark plugs with a spark plug wrench or head. At the same time it is necessary to monitor the cleanliness of the spark plug holes. If necessary, they must be cleaned of dust and dirt, and then make sure that no dirt gets into the holes.
- The gear lever should be set in neutral position.
- Then it is necessary to screw a compressometer meter into one of the holes.
The readings taken need to be compared with the standard data for this make of car and mileage. Knowing what the compression should be and considering the mileage of the car, it is necessary to make a comparative analysis of the data obtained.
Compression measurement with oil
If the tests showed too low compression in one or more cylinders, measure with oil to more accurately determine the cause of the problem.
Basically, this method of checking is no different from the previous one, except for the moment when you add 20-30 ml of clean engine oil to the cylinder to be checked. Use an ordinary syringe to do this, adding oil through the spark plug hole.
One of two things will eventually happen:
- The compression will not change at all, indicating an air leak through the cylinder-head gasket or valve (a cracked cylinder head or the block itself could also be the culprit, but this is rare);
- the compression in the cylinder will increase, indicating an air leak through a worn cylinder or piston ring.
Warning. Do not use more oil in a gasoline engine, 20-30 ml is enough. Otherwise the data obtained may be inaccurate. In the case of a diesel engine, a large amount of oil can damage its components at all.
The concept of compression
Translated into Russian, compression means “compression”. If there is no mixture leakage, the engine runs at full power. Thus, compression is the pressure in the combustion chamber occurring at the highest point of the compression stroke. Compression can be measured in pascals, kilograms per centimeter, but the generally accepted unit of measurement is atmospheres.
Pressure reading on a pressure gauge
The optimum cylinder pressure is determined by the specifications stated by the manufacturer. If the engine is running at full power, the right amount of air enters the cylinders as the piston moves downward, then the piston begins to move upward and fuel is injected into the chamber. When a spark occurs, the mixture detonates and should burn without residue. This releases exhaust gases, the amount of which should be minimal.
A compression check is the most practical way to determine the condition of the mechanical parts of the engine without having to disassemble it. Such diagnostics will help you detect problems that negatively affect engine performance. Once you get accurate results, it will be easier to make an informed decision about what to do next.