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Posted by D Sherman [72.47.9.228] on Thursday, October 07, 2010 at 22:35:22 :
In Reply to: Re: low compression posted by Glen Idaho [64.126.154.235] on Thursday, October 07, 2010 at 21:43:56 :
I like to do it with the gears in neutral because that way, if the air pressure doesn't rotate the engine, I know I have it at TDC. If it's a little off from TDC, the air pressure may rotate the engine as you described. The problem is if it's a little off from TDC and you have the engine blocked from turning, and you hear a leak, you don't know if it's because of a burned valve, or because the valve is open a hair due to the position of the crank. It seems like half the engines I've worked on have timing marks that I can't find or can't believe, so doing the leakage test this way give me some assurance that the engine is in the position where the valves ought to be closed.