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Wet and dry compression

Question : You talked about wet and dry compression in CBT the other day. Could you explain what it is, and where can I get it done on my Ford Telstar 90?

Answer : The dry and wet compression test is a standard test used to determine the condition of the pistons and rings, cylinder, valves and seats, and even the head gasket, and is carried out as follows:

1. Using an air line, blow away all dirt and debris from around the spark plugs.

2. Start the engine and allow it to warm up to operating temperature.

3. Remove all spark plugs and disable the high tension coil and the fuel pump.

4. Screw the compression tester into each cylinder in turn and crank the engine, making sure the throttle is kept fully open. Crank the engine for about four compression strokes and record the readings.

5. Check the readings. The lowest reading cylinder should not be less than 75% of the highest reading. Some manufacturers specify a minimum reading and any reading lower than that is unacceptable. You can check the vehicle service manual for these figures.

6. If all the readings are within specifications, there is no need to do the wet test but if there is variation between cylinders, then squirt about a tablespoon of lubricating oil into each cylinder and repeat the test. If the pressures increase appreciably and come up to the manufacturer's specifications, then you can be sure there is piston ring and/or cylinder and piston wear.

7. If after adding the oil, the pressure does not go up, your valves may not be seating properly and that could be for a variety of reasons. If there is no pressure rise in only one cylinder, it could be because of a blown head gasket.

Please take note that you never do a wet test with a compression ignition engine, i.e. diesel engine, because the additional oil may ignite on compression!


Dry or wet test

Drop in compression

Wet and dry compression

Grind the valves

Compression test



Spark plugs
Timing belt


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