Hard Cranking When Hot

Also check to make sure your gas cap is venting properly. If you pop the gas cap after it's hot and get alot of pressure release it may be causing the compression issues you spoke of. I just recently troubleshot two blown diaghrams in two new mechanical fuel pumps on my 454 chevy(yeah I know, It's for sale now)The pressurue would get so high it actually prevented the engine from turning over, I replaced the battery too thinking it was a battery issue. In the end it was the gas cap vent was stuck closed. I never would have thought that a fuel pump could lock up a motor.....but it did. I also had to drain the oil both times since the blown diaghram poured gas into the oil pan while I was cranking it over.good luck finding the culprit.