My 1999 grand prix gtp hesitates on acceleration when boost guage kicks in i replaced plugs and plug wires and the mass air flow sensor also cleaned the throttle body out its throwing multiple misfire

Asked by Nov 06, 2014 at 07:33 PM about the 1999 Pontiac Grand Prix 4 Dr GTP Supercharged Sedan

Question type: Maintenance & Repair

my 1999 Pontiac grand prix gtp is running poorly when boost kicks in...it throws multiple misfire codes and nothing else..i replaced the plugs and wires with iridium plugs and mid grade wires also replaced the air filter and mass air flow sensor as well as the air filter what could be the issue....the car idles somewhat normal after it warms up but runs rough when first started in the morning...could it be a cat...or a bad plug or what could cause this issue...as long as I don't push the car it does fine but under high load it boggs down and acts like it misses

4 Answers

70

Almost seems like a vacuum leak, low fuel pressure, or your intake hose not seated properly into the super charger.

70

My fault, typing before I think, intake hose onto the throttle body.

70

It is a 3800 engine ?

4,655

Yes Steve, they are all 3.8s. I did have to have the blower to intake gasket changed on one of my supercharged 3.8s years ago. I can't remember what the symptoms were but if Steve is on the right track with vacuum/air pressure issues that might be another place to look.

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