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Published on Nov 16, 2009
Make sure the throttle is wide open while doing a compression test. If the throttle is closed, the engine is being strangled and is trying to suck in and compress a partial vacuum. With a closed throttle, the reading depends mainly on how worn the valve guides are and is not very useful as a comparison to another engine of the same make and model.
However, testing with the throttle closed on a multi-cylinder engine may tell you if one cylinder has a loose inlet valve guide (it's reading will be higher).