It does not really matter how the compression test is done as long as everybody does it the same. A warm engine is called for to get as close as possible to actual operating conditions. The ring gap does go down as the engine temp goes up, so compression will go up too. The same with the wide open throttle. As long as everyone does it the same we get a meaningfull test. With the throttle open the engine will "pump up" pressure slightly faster. With experance you can tell from how fast (how many crank revs) a little about the engines condition. If it goes right up to 180 on the first turn then its great. If it takes 6-10 turns then even thou it gets to 180 there is signs of a problem. The engine should also crank slightly faster with the throttle open. When I check my 5 cylinder Eurovan engine I start with a warm engine but with the time it takes to unscrew the tester from one cylinder to the next the engine has cooled slightly and I get different readings depending on if I start on #1 to #5 or #5 to #1. Not a significant difference, maybe 5 psi, but detectable. Oil is supposed to help the rings seal better and even with a flat engine can help get a better reading and tell if its the rings or the valves. oil will not help valve sealing at all.
Pawling NY Cars I have: 93 Eurovan MV Cars I had: 89 GTi 16v 75 Westy 87 Syncro 73 Super 83 Rabbit GTi 82 Vanagon Diesel (w 83 GTi engine) |
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