after car warms up it runs badly what could cause this


Asked by Apr 11, 2010 at 10:02 PM about the 1994 Pontiac Firebird Formula

Question type: Maintenance & Repair

When I first start my car up in the morning it runs great for about 5-10 mins or until warm.  After this it starts to run rough and backfire at low rpms.  At high rpms it seems to be missing and has lowered power.  It is an LT1 with headers and a cold air intake(slp) with dual cones.  It has new plugs and wires from last summer(car has too much power to drive in pa winters) I ran it for two days before this happened.  It also has bigger injectors and fuel pump.  It also gets a little hard to start once it is warmed up as well.

8 Answers

Larry Quigley

Sounds like an electrical problem. Check you plug wires along with the distributor cap and the coil. Look for any burning or loose connections.

4 out of 4 people think this is helpful.
Best Answer
Ray Goines

coils are bad

1 out of 1 people think this is helpful.

Thanks guys, turns out that my headers burnt two of the spark plug wires. I also found out that the ground for my fuel pump wasn't right, so I am glad it went to the shop and I found out before the fuel pump burned up.

1 out of 1 people think this is helpful.
Jon Romero

Same thing happened to the 94 LT1 Camaro we built. Routing wires in f-bodies suck. Are you running the stock opti? You run the quarter with it?

1 out of 1 people think this is helpful.

Yes it is the stock opti, but it has other things done to it. The friggin headers are what keep burning the wires, and when burned I have to put the car on a lift to replace because the car is only about five inches off the ground(3 inch exhaust is only about 3) The first time I ran it with the new stuff on it I ran 12:07sec in the quarter. The 4.10 rear and the racing clutch make all the difference in the world. I think I am gonna go with a 3.73 and a supercharger next. That way I can go faster on highways, but who knows. Exhaust, suspension, cam, intake, and all the SLP stuff made a big difference. As well as the 800lbs less weight, the opti and piston are about all that is left that is stock.

Jim Morrison

I would look for a vaccum leak, causing a lean situation

1 out of 1 people think this is helpful.

i had a 1994 firebired that did the samething as your after it warms up it ran poorly and had no power and backfired. it turns out that cadalletic converter was plug up and need to be replaced. After i replaced it she ran perfect.

2 out of 2 people think this is helpful.

check r timing,or this might sound crazy,but a 40 year mechanic told me u might have too much iol in r pan.sounds nuts,but check r oil anyway

1 out of 1 people think this is helpful.

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