Too low pressure readings
Compression pressure from 13.0 to 14.0 bar in an 8-valve engine or from 14.0 to 15.0 bar in a 16-valve engine indicates good engine condition. If the readings are below 10.0 bar (8-valve engine) or 11.0 bar (16V), the engine needs repair. Opel workshops allow a tolerance of 1 bar in the pressure between the cylinders. Equally low pressure in the cylinders does not necessarily have to be an alarm signal; the reason may be in the measurement tolerance between different measuring devices. It is a matter of concern when there is a difference of more than 3 bar between the measurements of four cylinders. This may mean:
- Wear of pistons and piston rings
- "Stuck" piston rings due to sedimentation
- Non-round cylinders as a result of piston seizure
- Deposits on valve stems or seats due to residual combustion products of fuel or oil
- Bent valves
- Burnt valves.
Troubleshooting
In order to localize the defect when the compression pressure is too low, use the following method: drop a little viscous oil into the spark plug hole and measure the compression pressure again.
- If the readings are still bad, the valves are the culprit.
- If you get higher pressure readings, the piston rings are the cause, and maybe the cylinders as well. The added oil briefly increased the seal between the pistons and the cylinder walls, so the compressed air-fuel mixture could not leak out.
Tip: Compression pressure should not be confused with compression ratio. The compression ratio is determined by the design through the piston stroke length, the shape of the piston base, and the volume of the combustion chamber. In contrast, compression or compression pressure can vary depending on the engine's condition.
Pressure loss test
More accurate information is provided by a pressure loss test, which some workshops can perform. The test device consists of two chambers, one of which is under constant pressure. The second chamber is connected by a hose through the spark plug hole to the combustion chamber, by a nozzle to the first chamber of the test device, and also to an indicator scale. If the combustion chamber being tested loses pressure, this can be seen on the scale. A more serious loss of compression can be detected by listening:
- A bubbling noise in the muffler suggests that the exhaust valve is leaking.
- If compressed air comes out of the air filter housing, the inlet valve is faulty
- If the cylinder head gasket is defective or the cylinder head is cracked, compressed air will escape through the adjacent spark plug hole or through the open coolant expansion tank.
- Worn cylinder walls, piston guides or piston rings release pressure into the crankcase, and it escapes through the open oil filler pipe, or through the tube with the dipstick for measuring the oil level.
[Original posted on the website: opelbook.ru]
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