Question about 1985 Pontiac Firebird
I have an 85 Pontiac Trans Am. I have done virtually every test and replaced everything possible that was faulty that I can think of on the car. The vacuum tests out good. It seems as though the engine runs ok once you get it off an idle. It is much worse when the car is cold. I'm not sure what else to try. Any suggestions??? Thank you very much for your help!!!
Have you done an exhaust back pressure check? The brief description of your symptoms could be a clogged catalytic converter....which might be worse when cold than warm. If you've got any reason to believe the catalytic converter has been damaged (incorrect fuel or additives, physical damage/crushing, excessive heat, etc) I'd definitley eliminate it as a potential cause. It's tricky to check exhaust back pressure....but here's some ideas that might confirm or deny.
1. Is there a hissing or whistle from the tail pipe at idle? might indicate an obstructed exhaust causing a hiss or whistle from gases trying to force around/through the obstruction
2. Does the car initially fire up fine but start to loose a few RPM within a few seconds of idling? possibly indicating back pressure build up starting to hurt engine performance.
3. The best method is probably to disconect the exhaust from the engine and use a leaf blower or similar to blow a large volume of air through the exhaust....the problem then is determining how much back pressure is "normal", but generally the back pressure or resistance to air flow through an exhaust should be low - a few psi differential at most.
4. The most accurate method would be to set the engine up with one cylinder exhaust valve fully open, pull that spark plug, plumb in a differential pressure guage and compressed air source and put air into the cylinder. The pressure guage should indicate how much back pressue it is present in the exhaust system. The pressure differential will rise with the pressure and volume of air supplied to the cylinder, but if it's possible to build up more than 1-2 psi in a cylinder with the exhaust valve open, the most likely cuase is obstructed exhaust. The most common cause of obstructed exhaust (after physical crush damage) is a clogged catalytic converter.
Posted on Jul 06, 2009
What type of transmission is it?
Posted on Jun 20, 2009
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Posted on Jan 02, 2017
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