borrone821

iTrader: (3)

Hey guys,
I'm workin on my dad's 94 trans am LT1 A4 with 37k original miles.

It's got the wonderful stumble/bog from 1200-2k rpms. If I downshift, I can get out of it and not worry about it, but it's certainly an issue. Also idles pretty rough, not in park or neutral, but when it's in drive or reverse. When in park, I can rev all the way to redline w/o issue

I've taken apart the intake, cleaned the MAF and TB. I don't think there are any vacuum leaks because the RPMS seem consistent.
I didn't see any arcing running in the dark. All of the injectors measured 12.3 or 12.4 ohms.
I'm really hoping it's not the optispark since after 2k RPMs the car is fine. I'd assume that a distributor issue would get worse at higher RPMs.

So I put a fuel pressure gauge on the shrader valve tonight. Getting 36 psi at idle. When my wife revved it, pressure dropped into the low 30s then back up to 36 when she let off. When I pulled the FPR vacuum, went straight to 44 and stayed there. Is this an issue? Any way to check if your fuel filter is bad, or just replace it?

I really don't want to throw money at this since it's not even my car. Any and all help would be greatly appreciated.
Thanks
Jim

jlpz

iTrader: (8)

With the vac line unplugged it should be at 43.5psi. Don't know if there is a way of checking the fuel filter other than to cut it in half to see the filter element. I'd just replace it if it hasn't been changed.

myltwon

iTrader: (1)

Sounds like a vacuum or iac issue. I wouldn't think its fuel or spark since a greater load on the motor clears up the problem.

borrone821

iTrader: (3)

So 36psi at idle and 30s while revving is ok?

And could the IAC cause stumbling issues at 1200-2k rpms?

myltwon

iTrader: (1)

If its not opening properly yes. Iirc fuel pressure around 35-40 at idle is fine, free reving will be lower unless there is a load on the motor.

borrone821

iTrader: (3)

Thanks for getting back to me on that..

So I'll clean the IAC to hopefully fix the idle but I'm still confused how that will affect a 1200-120k stumble.

borrone821

iTrader: (3)

Alright, so I put an IR gun on the exhaust manifolds where I could.

After a minute or two, driver side exhaust is running around 240 +/- 10 on all outlets.
Passenger side, I couldn't get to the rear manifold section, but the front 3 were all ready over 400 degrees.
I'm assuming having one side of the motor run 150 degrees hotter than the other is an issue. Any idea what could be causing this?

I know this has nothing to do with the idle (or shouldn't) but I was hoping to see a cylinder misfire from time to time that could cause the stumbling.

Any thoughts, or I am just running around in circles?

Thanks guys
Jim

jlpz

iTrader: (8)

Check 02's and for vacuum leaks. check the egr if its stuck open, even just a little, it will cause a miss at idle or low rpm.

borrone821

iTrader: (3)

Alright how do I check the O2s and how do I check the EGR? Stumble and idle issues are the whole time, not only when engine is warm.
Can the O2s have any effect on the motor when it's in open loop?

borrone821

iTrader: (3)

alright, so I just pulled apart the intake and cleaned the IAC and TB really well with qtips.

No change. Idle in park or neutral is OK but you can hear random misfires on the exhaust. In any gear, RPMs are in the 600 range is it's jumping a little bit, definitely a rough idle. I haven't taken it for a ride yet to see if it's still stumbling at low rpm, but I'm assuming it's still there.

So my issue is at all times, not only open or closed loop.

I'm going to replace the fuel filter when I get a chance.. should I replace plugs and wires just as maintenance? Car has 37k original miles and I'm starting to feel like i'm chasing my tail

borrone821

iTrader: (3)

EGR does not appear to be the issue.... when i push in the diaphragm, the motor stalls out. When I pull the vacuum line off, nothing happens... at idle when the motor is coldish, there isn't any vacuum going to the EGR, or at least nothing my fingers can feel.

So after cleaning the IAC and resetting it, idle appears much smoother, but still feel a miss. Idle in gear is around 400-500rpm... this seems low, right?

And after going for a drive today, the bogging/stumble issue is apparent at cold, but much much worse after driving for a few minutes... Does that lead to a plug/wire issue that gets progressively worse as the motor heats up?
The O2s don't come into effect until closed loop, so they shouldn't impact anything until it gets up to temp... correct?

Any and all help would be greatly appreciated.
Thanks
Jim

cchvyd28

iTrader: (2)

Just solved this issue on my 96. Had a stumble that i would notice on take off or shifting at low rpm's. I took the vacuum line off of the egr and plugged it, took the car for a spin and it was completely gone.

Pushing the plunger in on the egr while the engine was running did kill the engine, but blocking off the vacuum line solved my issue. From what i was told the egr could be functioning properly, but if it is carboned up it could be sticking open a small amount causing the stumble.

borrone821

iTrader: (3)

alright, so now the issue only starts happening after about 3 or 4 minutes of driving, as soon as the temp needle start moving.

What are the chances it's the Opti? From what I've read, the Opti either works or doesn't, and won't produce a "stumbles from 1200-2000rpm" type issue.

Should I just go ahead and replace the plugs and wires? they've got 35k on them, completely stock.

Oh, and I tried the vacuum trick on the EGR with no effect.

Looking for any and all other suggestions.

Thanks
Jim

Geneticallybroke

iTrader: (3)

I had the same issue, it started to stumble until almost shutting off, but ran good at high rpms, mine turned out to be the maf