|Wet and dry
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?
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
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
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!