Re: Chasing Rough idle...compression?

Posted by ineffabill On 2011/5/27 14:38:30

Getting back to the original thread, I would perform a running compression test on this engine. Cranking compression test measures the result of gross cylinder sealing & cam timing. A running test compares each cylinders' ability to "breath" at idle.

Run the engine until it reaches operating temp & shut down. Screw compression gauge into a plug hole & ground that cyl plug wire. Start & idle the eng- release gauge press then watch it build. Do this several times & note the highest reading & how long it takes to build.

Repeat this procedure on all cylinders. You want to compare all cyl to each other. Since eng is compressing fuel/air mixture at 18" of [vacuum] or so, your reading is going to be about 1/3 of your cranking result. This test helps to find intermittant exhaust valve problems. A worn guide or rusty/carboned up valve stem will usually deal fine at cranking speed, but act up when moving faster & hot.