Fits of Misfiring

[Roger Davis]

Engine Performance problem
1991 Pontiac Trans Am V8 Two Wheel Drive Manual 60,000 miles

I have a 1991 Firebird with 60K original miles, manual 5 speed. I make a daily commute of about 35 miles one way. It drives fine for about the first 2 ~ 5 minutes, but after then it SOMETIMES misfires and jerks under load (acceleration or uphill). Sometimes badly enough to buck strongly at low speed in higher gears under light load.

No backfiring. Constant highway speed on level ground is ok. If I rev it in lower gears, sometimes at least one or two or more cylinders quit only at high rpm (or some similar feeling power loss), but sometimes this will remove the problem. Also, after about 20 minutes of normal freeway running, it usually will clear up on it's own. Most of the time, it just works fine at all speeds.

The fuel filter might be original, I plan to change it soon. I reason that it might be passing small cruddies, causing one or more injector(s) to act up sometimes. But wouldn't this cause problems at idle and constant speed too?

Some other sensor or system might be causing it, but not sure which one has a capability to "fix itself", work fine for a few days, then get bad enough to make me wonder if I'll make it to my destination.

How about any connections that are known to go intermittant, wires that are known to break or ground, brain box gets forgetful, then remembers, or ???

Vacuum leak?

[d00012]

Check your fuel pressure, it could be low enough to run fine at idle. Then run lean on highway but seem ok. Happened to my camaro, gas mileage actually went up over 30 mpg with my V6 because it was running lean. So, change that filter, and if it doesn't help, run Seafoam through the tank.

Also, check the Ignition Control Module, from what I have learned, it can fail intermittently. Check the ignition timing, too. Mine was about 8-10* retarded on my Firebird, and it would surge under load.

Engine check light coming on?

Low fuel pressure is what I think it could be. Seems to be the common problem in all third gens. I had to drop the tank in both of mine.