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I've been fighting, and I mean fighting, a P0171 and rough idle for a while on the 96 SSEi. It's been there since the misfire catastrophe that was occuring when I got it, now that's the only thing left to fight (read: no more misfires). I have already: replaced the front O2 sensor, cleaned and tested all grounds, checked for vacuum leaks, replaced the MAF, and replaced the fuel pressure regulator. Next on my list to test is fuel pressure, along with injectors and PCM. However the PCM swap never changed anything before, so I've left the stock 96 PCM in it. Every time I change something it seems to get slightly better, but it still has a rough idle (around 5/600 rpm) and then it surges back up to about 1k rpm where it smooths back out, only to idle down and stumble again. P0171 code persists, I'm assuming they're related. Anyone else have any other suggestions?

Cracked EGR tube?

Sticky or bum injector? Could be putting too much fuel in, and too much fuel could cause stumbling? When is your rough idle, only when warm, or all time?

What's your consumption like?

What do your plugs look like?

EGR tube is not damaged, it was perfectly in tact when I did LIM gaskets on it last year, and it remains in tact. Injector is a good possibility. I'm obviously not hoping it's that, but fearing a little. Lol. I plan to check those the next time I get a chance, maybe tomorrow between shifts.

Fuel consumption: normal to low, keep in mind I only drive this car when I'm delivering pizzas now, so it's all in town whalings upon, anything above idle it runs and drives well enough to not notice there's any issue. Unless you rev it in park and hold it steady, the revs will then fluctuate.

I just checked the plugs over the weekend, they're still looking good. They were done less than 10k miles ago if my memory serves me correct.

As for when it happens, I notice it in both closed and open loops, meaning from the first few seconds it's running, all the way until I shut it off.

If it's during open and closed loop, it's probably something that's not caused by the PCM.

Tried a different IAC valve?

Otherwise, fuel pump, filter, injectors, o rings, all could be causing issues.

Don't rule out power, I had a failing alternator and old battery in my BMW, it's amazing how many systems were better once I replaced them. A Simple voltage check would have told me that a while ago...

I swapped on a spare alternator, and a spare IAC from a TB assembly I had. The grounds being cleaned did have an impact on my electrical system, but not on this. I'm going to check fuel pressure and injectors next, it almost has to be in that area, i would think. Th o-rings passed a brake parts cleaner test, so I can rule that out.

Thanks for the help, at least I know I'm heading in the right direction. Haha

Don't ask me how I've never noticed this, in the multitude of times I've had various parts removed from the top half of this engine, exposing the issue right in front of me. At least it's an easy fix.

Well, turns out that injector connector didn't change anything.

This, however, concerns me. I think I found my issue. Just my luck, the spare TB I have is also missing the screen.


Pardon the shaky cam pic. Been sick for a few days.

Have you checked for unmetered air entering the cylinders through
gasket leaks in the intake manifold or throttle body or PCV? Getting
more air than the MAF is registering will cause a lean condition, especially
at idle when the throttle is almost closed. When running, it will be less
of an issue because the open throttle is dumping a lot of air and a small
leak doesn't make a huge difference.

I know you said you checked them, but I missed them when I had the p0171 a while back. It turned out that my rear bank injector o-rings would leak when the motor warmed up, but sealed fine when it was cold. They passed the carb spray test once, but I got lucky checking it a second time and found out that they were indeed leaking. I'm not sure if the maf screen would cause a lean condition other than maybe letting the maf get dirty quicker, since that extra air would still be pre-maf and metered.

I recall that Nate (LakevilleSSEi) had an idle problem (idled as if it had a cam) on his 95. He couldn't find the problem. When he traded it to someone else, the new owner found the MAF screen was missing. When it was replaced, the idle smoothed out.

Well, to update this, I haven't driven the car much lately, life changes I'll update in a new thread, but I put a MAF screen in it and it idles mostly smooth now, still persisting with a P0171 though, and the idle is slightly rough. Pretty sure I have a vacuum leak in the HVAC harness, as I hear a whistling when it idles rough(ish) and at the same time the HVAC controller is trying to make the totally jank HVAC function like it should. (Needs blend door actuators and probably an HVAC vac harness (still available from the General ).