I'm entirely stumped on this one, if anyone has any idea what could be happening I would greatly apprcaite it. I drive a 1992 Firebird GTA.

Just recently my car has developed a slight stutter when both accelerating and maintaining a speed. I can't really feel the stutter as much as hear it, and it doesn't seem like enough to stall the engine. Here's what happens:

If I rev the engine while in park or neutral, it never happens. I was able to maintain 3000 RPMs for over 30 seconds without a hiccup or stutter.
Also, if I put the car in gear, hold my foot on the brake and rev the engine, it does not occur.
The stutter is most noticable when accelerating.
It is also noticable when I get up to speed, most recently at around 2000 RPMs or 45-50 MPH.
It does not happen every time. I could go out and drive around the block right now and it may or may not stutter at all.

I ran out the tank of gas I was running on when this first started and put a few gallons of gas in from a different gas station, thinking it was just some bad gas.
A few months ago I ran some Seafoam through the gas tank. The fuel filter is also not terribly old, maybe about 4 months or so.
The Service Engine Soon light has not come on yet. No error code has been generated from this.

Since most of my car is handled by electronics, I'm at a complete loss as to what's happening. I keep thinking if it was the fuel pump or fuel delivery it would happen weither or not there's load on the engine (in neutral as opposed to accelerating). Same goes for timing, else it would happen consitantly. But if its not any of those what else could it be?

 

check for vacuum leaks - bad vacuum can make any car run really bad, or occasionally stutter - I'd start there

 

Ok just went out there and started her up.. Used a plastic cup with the bottom cut off to listen- I could not hear any vacuum leaks, especially one big enough to cause the engine to stutter. Anything else it could be?

 

Plastic cup huh? That's the test? Never heard of it. And I've seldom ever heard a vacuum leak actually make noise. I may be old fashioned - but the only way I ever found a vacuum leak was by looking for it - tracing all the vacuum lines, and physically checking them for splits, tears, cracks, etc. These cars have several hard lines from the factory - trapped under the hood for at least 15 years of heat has surely ruined at least one.

I'm just not sure how a plastic cup will help - but whatever.

 

It could be the vac line to the TV valve on the trans. The valves go bad and suck trans fluid into the engine.

 

That sounds about right. I had to have transmission work done to the car a year ago and considering the condition of the car at the time I would not doubt if that's the problem. Concerning listening for a vacuum leak, this was advice from a friend however I intend to inspect what I can.

 

Your initial post ruled out a vacume leak. Your problem is either from the tranny back or in the suspension.
And if you PM me your # ill call you and give you directions to the house. I work for BUD Light.

 

It could be fuel delivery, since it only happens under load. Running the tank dry can hurt the in-tank electric pump - if it was happening before the tank was run dry, it might just be worn out. Getting a fuel pressure gage on it would be the way to eliminate/confirm that. If this is TPI (should be since it's a GTA), tapping in to get a fuel pressure reading is pretty simple. You need an EFI fuel pressure gage (about $40-$60), and you need to be able to monitor it while the symptoms are occurring (you may get lucky and be able to see low fuel pressure at idle).

If it is a mechanical problem, the most likely would be a broken transmission mount, worn-out u-joints, or a bad differential bearing.

By the way, a TH700R-4 doesn't have a vacuum line going to it.

 

One thing I noticed yesterday, for the past few days I thought the tachometer was twitching because it would randomly seem to just slightly jump. Turns out everytime it was doing that there's a "puff" sound coming from the exaust, but it's so subtle I didn't notice it until I was standing outside my car while it was running. Of course the engine was in park with no load on the transmission. It never seems to dip, only jump up. Now I'm more confused. I'll be doing a fuel pressure test tomarrow, here's hoping its not the fuel pump. Thanks for everyone's help.

 

Ignition. Most likely in the pick-up system (module) or coil.

 

Thank you, that's something else I'll look into getting replaced. I was able to test the fuel pressure. With the key on but engine not started it went up to 42 psi. When the engine was started it dropped to 32 to 34 psi and didn't really move from there, even if the engine was put into gear. The psi didn't jump up and down at all though, and certainly not at the same time the tachometer jumped. According to my Haynes manual the psi should drop anywhere from 3 to 10 psi when there's load on the engine (if I'm reading that right) so I guess this rules out the fuel pump. Now to replace the ignition control module to see if that clears it up.