Compression testing, an alternative.

Posted by Mike Smith on May 22, 1998 at 13:11:56

In Reply to: Been there, seen that!
posted by MRR on May 22, 1998 at 12:03:02

" On the dark side, your engine may be worn enough to allow "blow-by" to cause the problem.If this is the case your looking at a more challenging task.Check your compression."
An alternative method of checking for blow-by is to get an old spark plug. Break or remove the porcelain so that you have the threaded body with a hole through it. Braze or solder a copper or steel pipe ofabout 1/4" O.D. into the body so that there is little projection through the threaded end and about2" out of the top end, nothing critical about this. Now you need access to a compressor, low presure needed about ten or twenty pounds is plenty. Remove the plugs from your engine and hand turn it to TDC on #1, if your not sure where this is then take the distributor cap off and turn the engine 'till the rotor is coming around to the #1 plug lead position, stop short of this and then bring the engine on untill the piston is absolutely on top, a rod or screw driver in the plug hole will help. It must not be before or after TDC. Insert the adaptor you have made and fit the compressor airline onto the pipe. Open the air flow and you will be able to tell if you have any major blow by. A bad exhaust valve will be indicated by air coming from the exhaust pipe, A bad inlet valve shows from the Carb Intake and if the Rings are bad you can hear the air flow, by taking the Oil filler cap off. In all cases you are bound to get a certain leakage but any major fault will be evident to you. Repeat this on all cylinders. Hope it helps find the fault, if you have one.

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