Question about 1999 Pontiac Grand Prix

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I have a 1999 grand prix se. it idles rough as heck but smooths out with gas. i am throwing code PO300 (multiple misfire) but cant figure out why. i have checked all hoses and electrical conectors and all are fine. i had to pull heads off and send in to shop and replaced all new gaskets, new push rods and lifters. after doing the work it started doing it. i have gone over and over this and cant figure it out. . please help.

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Check engine compression--all cylinders. Rockers may have been overtightened eliminating valve lash on some cylinders. I've done it myself at times. If compression isn't right, pull valve covers and see if you can twist each pushrod in your fingers when lifter is off the cam lobe. If not, reaccomplish rocker tightening procedure carefully.

Posted on Sep 09, 2010

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Check the mass air flow sensor,it is common on these and sounds like your problem!

Posted on Oct 12, 2010

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If you are still haveing probs,check the oil pressure,with a manual screw in type oil guage if it is running too high it will pump up the lifters, and it will start running worse with valves not seating ,looseing compression. I had this problem on a 327 vet engine years ago.

Posted on Sep 11, 2010

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