Compression control values
The compression values for your vehicle vary slightly depending on the compression ratio. Presented in table. 4.2 guide values are valid for engines in perfect technical condition. When analyzing compression, more importance is given not to the absolute highest values, but to the same values in all cylinders. Deviations up to a maximum of 2 bar are allowed. If deviations exceed the allowable value, it is necessary to find out the reason for this deviation. As a first step, your specialist will advise you to take a pressure loss measurement.
Older engines create less compression
In older engines, compression is significantly reduced, and this is a normal phenomenon associated with natural engine wear. Therefore, there is no particular cause for concern, it depends less on the absolute highest pressure, and more on the same values in all cylinders. Only when the measured values reach the maximum permissible values, it is necessary to think about the upcoming engine repair or replacement with another one. If the difference in values is more than 3 bar, practice indicates the following reasons:
- wear of pistons or piston rings;
- jamming of worn piston rings in the piston grooves caused by the deposition of combustion products;
- damage to the working surfaces of the cylinders as a result of slight seizing of the pistons or jamming of the piston rings;
- residues of combustion products and oil in the form of plaque on the valve mechanism or on the working surfaces of valve seats;
- cracked valves;
- valve burnout due to insufficient valve clearance or thermal overload.
Compression can be measured on your own. First of all, for this you need an assistant who turns the crankshaft with a starter, and a pressure measuring device. In well-equipped workshops, a compression tester is a basic piece of equipment. Remove all spark plugs first (in a diesel engine - fuel injectors) from the cylinder head and check that the valve clearance is correctly adjusted. During the check, the assistant must fully press the clutch and accelerator pedals, you check the compression in each cylinder with a device. Since diesel engines constantly suck in the same amount of air, the accelerator pedal does not need to be depressed. Start the measurement from the first cylinder and then measure successively in all other cylinders. Count the number of revolutions of the crankshaft until maximum pressure is created, this number will be a criterion for subsequent cylinders: the faster compression is created, the «healthier» your engine. IN «healthy» engine compression should be created after 6-8 revolutions of the crankshaft.
Reliable measurement is based on a good starter motor and a fully charged battery
Despite the fact that this is an elementary truth, we still remind you once again that the basis for measurement is a starter with good traction and a fully charged battery. If the crankshaft rotates very slowly, the compression in the cylinder also occurs slowly - the compression measurement loses its meaning. If you get large deviations in the measurement results, perform a pressure reduction test. The handling of the compression loss tester requires some experience, so we recommend having a compression loss test carried out by a specialist.
You can find out the reason for the decrease in pressure as follows.
- At low compression through the spark plug hole (in a diesel engine - in the hole of the injection nozzle) put some engine oil into each cylinder, then check again. The oil improves the seal between the mating surfaces of the pistons and cylinder walls.
- If the compression has increased slightly, then the leak is through the valves or the cylinder head gasket. Leakage through valves can be caused by burnt valve seats and/or valve lips, or by deformed or cracked valve stems.
- If, after the introduction of oil, the compression increased, then we can make an unambiguous conclusion that the piston rings are worn out.
Compression measurement
SEQUENCE OF WORK
1. Before measuring, warm up the engine to operating temperature. This creates optimal clearances between all moving parts.
2. Turn off the ignition (terminal «15»), remove the spark plug caps and remove all spark plugs.
3. Apply the parking brake, place the shift lever in neutral, depress the clutch and accelerator pedals.
Pic. 4.13. When installing the compression tester, the rubber cone plug must tightly close the spark plug hole
4. Firmly insert the rubber cone plug of the compression tester into the spark plug hole (in a diesel engine - in the hole for the fuel injector). Use a suitable adapter if necessary (pic. 4.13).
5. Engage the starter and rotate the crankshaft for at least eight compression strokes while observing the compression gauge. It must be borne in mind that the best filling of the cylinders occurs only when the accelerator pedal is fully depressed.
Pic. 4.14. After measuring the compression on all four cylinders, compare the curves recorded by the instrument's recorder
6. Write down the highest compression value. Repeat the measurement procedure for the rest of the cylinders and compare the results with the technical data (pic. 4.14).
PRACTICAL ADVICE
Compression reduction
A decrease in compression accompanied by air noise can be caused by the following reasons:
- noise in the suction pipe or noise insulation of the pipe indicates a leaky inlet valve;
- if air comes out of an open radiator or coolant expansion tank, then the cylinder head gasket is broken or there is a crack in the cylinder head;
- if air comes out of an open oil filler neck or engine oil level indicator tube, it means that the cylinder walls, piston working surfaces or piston rings are worn out;
- if air comes out of the exhaust pipe, then the exhaust valve is leaking.
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