Help with pressure malfunctions

Too low pressure

A compression pressure of 13.0 to 14.0 bar on an 8 valve engine or 14.0 to 15.0 bar on a 16 valve engine indicates good engine condition. If readings are below 10.0 bar (8 valve engine) or 11.0 bar (16V), the engine needs to be repaired. Opel workshops allow a pressure tolerance of 1 bar between cylinders. The same low pressure in the cylinders does not have to be an alarm; the reason may be the tolerance of measurements between different measuring instruments. It is of concern when there is a difference of more than 3 bar between the measurements of the four cylinders. This could mean:

• Wear of pistons and piston rings
• «Occurrence» piston rings as a result of sludge formation
• Non-round cylinders as a result of piston seizure
• Deposits on valve stems or seats due to residual products of combustion 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 technique: drop a little viscous oil into the spark plug hole and measure the compression pressure again.

• If the readings are still poor, then the cause is in the valves.
• If you get higher pressure readings then it's the piston rings and maybe the cylinders as well. The filled oil briefly increased the tightness 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 design by the length of the piston stroke, the shape of the piston base and the volume of the combustion chamber. In contrast, compression or compression pressure may vary depending on the condition of the engine.

Pressure loss test

More accurate information is given by a pressure loss test, which can be carried out by some workshops. The test apparatus 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 tester and, in addition, to an indicator scale. If the combustion chamber under test loses pressure, this can be seen on the scale. A more severe loss of compression can be detected by listening to:

• 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 if the cylinder head is cracked, compressed air escapes through an adjacent spark plug hole or through an open coolant expansion tank.
• Worn cylinder walls, piston guides, or piston rings release pressure into the crankcase and it escapes through an open oil filler tube or dipstick tube.