I pulled a glow plug from the drivers side and put a compression test on it .!what I found confuses me. It has compression but i cannot get a reading cause th e gauge bounces up to 100 or so and drops back to zero after the compression stroke.up and down.granted I have a home made rig made from a old glow plug and a brazed piece of brake tubing however it worked before. I see the rocker arms moving up and down. Could it be a burnt valve ? Or what else could it be?