You are right, the first time I checked it was 170 and the second time 180 but both times were within half an hour. I have not checked it again. I returned the tester to oreilly but I can get them again If I need to. I do not know whether would be easier if we find out the problem now or after dismantling the engine again....Are those numbers from the most recent compression tests? You mentioned #3 was at 170 n 180 psi a few posts back, not 200!