[W126 Coupe] compression

[Jim Ham]

There are two general methods of testing compression.

The first is with a "compression checker". This is with a pressure gage 
that has a built-in check valve. You take out the spark plug and screw in 
the gage with the check valve. Then you use the starter motor to turn over 
the engine a few time so that the gauge registers the maximum pressure on 
the measured cylinder. The rule of thumb is that if all cylinders are 
within 10% all is OK.

The second is called a leak down test. With this method you need a source 
of compressed air and a "leak down tester". The leak down tester has a 
pressure regulator, a small orifice  to supply air to the measured cylinder 
and a pressure gage to measure the resulting cylinder pressure. You put the 
measured cylinder at TDC with the valves closed. Then you attach the tester 
to the spark plug hole and turn on the air. You read the resulting pressure 
from the gauge. This method measures the leakage from the cylinder through 
the valves and rings. It's much more repeatable then the "compression 
checker" method above. It's commonly used with aircraft engines.

I don't know the preferred method by Mercedes.

Regards,

Jim Ham


At 04:22 PM 5/1/2005, you wrote:
>can someone please explain how compression is measured? not tested, but 
>measured.. thanks
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