Compression values for gasoline engines are shown in Table. 5.1.
Table 5.1 Compression values for gasoline engines
The pressure difference in the cylinders must not exceed 1.5 bar. In older engines, compression is reduced, however, if the difference does not exceed acceptable limits, this is not considered a malfunction. As soon as the compression reaches the lower limit, the engine should be repaired or replaced.
If the cylinder compression differs by more than 1.5 bar, the cause should be sought in the following:
- wear of pistons or piston rings;
- sticking of rings in pistons;
- cylinder wear (often due to sticking rings);
- soot on valves and seats;
- damaged valve;
- burnt valve (with too little clearance).
Compression measurement
1. Warm up the engine to operating temperature. After that, it is necessary to drive about 5 km to ensure sufficient oil temperature.
2. Stop the engine by turning off the ignition.
3. Open the hood.
4. Brake the vehicle with the parking brake.
5. Set the shift lever to neutral position (or in position «R» - on a car with an automatic transmission).
6. Remove the spark plugs.
Attention! Do not crank the crankshaft with the starter so that fuel does not enter the cylinders and damage the compression recorder.
Pic. 5.3. Compression check: 1 - compression gauge; 2 - sealing cone
7. Insert compression tester 1 (pic. 5.3) into the threaded part for the spark plug, pressing it with the sealing cone 2.
8. Connect the recorder:
- unscrew the fastening of the cover of the control unit;
- in the control unit housing, disconnect the terminal connector «50» and connect pin 1 of the connector to the control circuit wire of the recorder;
- remove the positive terminal cover «30» on the left wheel arch in the engine compartment and connect the power cable of the recorder to the positive terminal «30».
9. Ask the assistant to press an accelerator pedal against the stop and to hold it in this position throughout all check.
10. Turn the crankshaft with the starter several times while pressing the contact switch of the recorder to eject carbon deposits and soot from the cylinder.
11. Pressing the contact switch of the device, turn the engine crankshaft with a starter (at least 9 turns), until the reading stops changing.
12. Write down the resulting compression value.
13. Check successively all cylinders and compare the received results with nominal sizes.
14. Screw in the spark plugs.
15. Connect the terminal connector «50» control box and install the cover.
16. Close the positive terminal cover «30» on the wheel arch.
17. Close the hood.
Differences in the sequence of work for the V6 engine
1. Remove the air filter with the mass air flow sensor.
2. Remove from the left side of the ignition coil.
3. Remove one spark plug from each cylinder.
4. After checking the compression, have the engine trouble code memory read at a specialized auto repair shop and, if necessary, erase its contents.
Reasons for low compression
With low compression, drop a small amount of oil into the nozzle hole. Check compression again.
If the measured values do not increase: Valves, valve seats, valve passages, cylinder head or cylinder head gasket may be damaged.
If you get better results, then the cause should be sought in the piston rings or in the cylinders themselves. The oil improves the sealing of the pistons and cylinder walls for a short time, so the compressed air resistance becomes stronger.