My 99 grand prix cranks and backfires when letting off key from trying to starting. I have fuel pressure 45 psi. I have spark showing on ignition tester at the three coils i guess thats what they are called. My compression is all around 150 on all cylinders. Starting to wonder if it jumped time time. Is there an easy way to check since i cant start it? Not sure where timimng marks are..![]()