Checking the compression in the engine cylinders
Content
Modern car engines are very reliable and in caring hands are able to work out more than one hundred thousand kilometers without major repairs. But sooner or later, the operation of the power unit ceases to be flawless, there are problems with starting, power drops, and fuel and lubricant consumption increases. Is it time for a refurbishment? Or maybe it's not that serious? It's time to measure the compression in the engine cylinders. This will allow you to assess the health of your engine without disassembling it, and even determine the most likely sores. And then, perhaps, it will be possible to do without a major overhaul, limiting itself to decarbonizing or replacing individual parts.
What is called compression
Compression is the maximum pressure in the cylinder during the movement of the piston to TDC on the compression stroke. Its measurement is made in the process of idling the engine with a starter.
Immediately, we note that compression is not at all identical to the degree of compression. These are completely different concepts. The compression ratio is the ratio of the total volume of one cylinder to the volume of the combustion chamber, that is, that part of the cylinder that remains above the surface of the piston when it reaches TDC. You can read more about what the compression ratio is in.
Since compression is pressure, its value is measured in the appropriate units. Auto mechanics commonly use units such as the technical atmosphere (at), bar, and megapascal (MPa). Their ratio is:
1 at = 0,98 bar;
1 bar = 0,1 MPa
For information about what should be the normal compression in the engine of your car, look in the technical documentation. Its approximate numerical value can be obtained by multiplying the compression ratio by a factor of 1,2 ... 1,3. That is, for units with a compression ratio of 10 and above, the compression should normally be 12 ... 14 bar (1,2 ... 1,4 MPa), and for engines with a compression ratio of 8 ... 9 - approximately 10 ... 11 bar.
For diesel engines, a coefficient of 1,7 ... 2,0 must be applied, and the compression value can be in the range from 30 ... 35 bar for old units to 40 ... 45 bar for modern ones.
How to measure
Owners of cars with a gasoline engine can easily measure the compression on their own. Measurements are carried out using an instrument called a compressometer. It is a pressure gauge with a special tip and a check valve that allows you to record the measured pressure value.
The tip can be rigid or have an additional flexible hose designed for high pressure. Tips are of two types - threaded and clamping. The threaded one is screwed in instead of a candle and allows you to do without an assistant in the measurement process. Rubber when measuring will have to be pressed tightly against the candle hole. One or both of them can be included with the compression gauge. This must be taken into account if you decide to purchase such a device.
A simple compression gauge can be bought at a very affordable price. More expensive imported devices are equipped with a whole set of adapters that allow measurements in any motor of any manufacturer.
Compressographs are much more expensive, allowing not only to take measurements, but also to record the results obtained for further analysis of the state of the cylinder-piston group (CPG) by the nature of the pressure change. Such devices are mainly intended for professional use.
In addition, there are electronic devices for complex engine diagnostics - the so-called motor testers. They can also be used to indirectly evaluate compression by recording changes in starter current during idle cranking of the motor.
Finally, you can completely do without measuring instruments and approximately estimate the compression manually by comparing the forces required to crank the crankshaft.
For use in diesel units, you will need a compression gauge designed for a higher pressure, since their compression is much higher than that of gasoline ones. Such devices are commercially available, however, to take measurements, you will need to dismantle the glow plugs or nozzles. This is not always a simple operation that requires special tools and skills. It is probably easier and cheaper for diesel owners to leave measurements to service specialists.
Manual (approximate) definition of compression
You will need to remove the wheel and remove all the candles, leaving only the first cylinder. Then you need to manually turn the crankshaft until the end of the compression stroke in the 1st cylinder, when its piston is at TDC.
Do the same for the rest of the cylinders. Each time, only the spark plug for the cylinder being tested should be screwed in. If in some case the forces required for turning turn out to be less, then this particular cylinder is problematic, since the compression in it is lower than in others.
It is clear that such a method is very subjective and you should not rely on it entirely. The use of a compression tester will give more objective results and, moreover, will narrow the circle of suspects.
Preparation for measurement
Make sure the battery is in good condition and fully charged. A dead battery can reduce compression by 1 ... 2 bar.
A clogged air filter can also significantly affect measurement results, so check it and replace if necessary.
The motor should be warmed up before reaching the operating mode.
Shut off the fuel supply to the cylinders in any way, for example, remove power from the injectors, turn off the fuel pump by removing the appropriate fuses or relays. At the mechanical fuel pump, disconnect and plug the pipe through which fuel enters it.
Remove all candles. Some unscrew only one, but the result with such a measurement will be inaccurate.
The manual transmission lever must be in the neutral position, if the automatic transmission is in the P (Parking) position. Tighten the handbrake.
For each cylinder, it is desirable to take measurements both with the damper open (with the gas pedal fully depressed) and closed (the gas pedal is not pressed). The absolute values obtained in both cases, as well as their comparison, will help to more accurately identify the malfunction.
Compressometer application
Screw the tip of the measuring device into the spark plug hole of the 1st cylinder.
To measure with an open damper, you need to turn the crankshaft with a starter for 3 ... 4 seconds, pressing the gas all the way. If your device has a clamping tip, then an assistant is indispensable.
Look at and record the readings recorded by the device.
Release the air from the compression gauge.
Take measurements for all cylinders. If in any case the readings differ from the norm, take this measurement again to eliminate a possible error.
Before starting measurements with the damper closed, screw in the spark plugs and start the engine to let it warm up, and at the same time recharge the battery. Now do everything as with an open damper, but without pressing the gas.
Measurement without warming up the motor
If there are difficulties with starting the engine, it is worth measuring the compression without preheating it. If there is serious wear on the CPG parts or rings are stuck, then the pressure in the cylinder during a “cold” measurement can drop by about half the normal value. After warming up the engine, it will increase noticeably and may even approach the norm. And then the fault will go unnoticed.
Analysis of the results
Measurements taken with the valve open help to detect gross damage, since the injection of a large volume of air into the cylinder more than covers its possible leaks due to defects. As a result, the decrease in pressure relative to the norm will not be very large. So you can calculate a broken or cracked piston, coked rings, a burnt valve.
When the damper is closed, there is little air in the cylinder and the compression will be low. Then even a slight leak will greatly reduce the pressure. This can reveal more subtle defects associated with piston rings and valves, as well as the valve lifter mechanism.
A simple additional check will help clarify where the source of the trouble lies. To do this, apply a little oil (about 10 ... 15 ml) to the walls of the problematic cylinder so that the lubricant clogs possible gas leaks between the piston and the cylinder wall. Now you need to repeat the measurement for this cylinder.
Significantly increased compression will indicate leaks due to worn or stuck piston rings or scratches on the inner wall of the cylinder.
The absence of changes means that the valves do not close completely and need to be lapped or replaced.
If the readings increased by a small amount, the rings and valves are to blame at the same time, or there is a defect in the cylinder head gasket.
When analyzing the measurement results, it should be taken into account that the pressure in the cylinders depends on the degree of engine warm-up, lubricant density and other factors, and measuring instruments often have an error that can be 2 ... 3 bar. Therefore, not only and even not so much the absolute values of compression are important, but the difference in the measured values \uXNUMXb\uXNUMXbfor different cylinders.
If the compression is slightly below normal, but in individual cylinders the difference is within 10%, then there is uniform wear of the CPG without obvious malfunctions. Then the reasons for the abnormal operation of the unit must be sought in other places - the ignition system, nozzles and other components.
Low compression in one of the cylinders indicates a malfunction in it that needs to be fixed.
If this is observed in a pair of neighboring cylinders, then it is possible.
The following table will help to identify a specific malfunction in a gasoline engine based on the results of the measurements and additional signs.
In some cases, the results obtained may seem illogical, but everything can be explained. If the engine of solid age has high compression, you should not conclude that it is in perfect order and there is nothing to worry about. The point may be a significant amount of soot, which reduces the volume of the combustion chamber. Hence the increase in pressure.
When the reduction in compression is not too great and the engine’s standard service life has not yet been reached, you can try to carry it out, and then take measurements again a couple of weeks after that. If the situation improves, then you can breathe a sigh of relief. But it is possible that everything will remain the same or even get worse, and then you need to prepare - morally and financially - for the assembly.
