prowlingleather

Hey guys. New to the LS1, so hopefully this question doesn't come off as too dumb; I've looked at other threads (on multiple sites) and tried a few of the recommended fixes, but it hasn't seemed to help.

I recently got a 1998 Trans Am and I've been fixing her up for a while now. She idles fine while cold, but she's had a hard time idling completely evenly (the RPMs bounce about a 100 each way) once she warms up; drives well when she idles fine, drives worse when the idle surges. Pulling the TPS fixed it briefly, but I replaced it and, after it warmed up, the problem's back. Any help or suggestions would be much appreciated!

What I've done since I got her:
Replaced fuel pump and sender
New fuel filter
Cleaned out all the rust in the tank (temp fix)
Replaced coils/plugs/wires
Replaced the PCV hoses and valve
Cleaned the throttle body and IAC
New TPS
Got a smoke test done (fixed a cracked EGR leak)

RPM WS6

iTrader: (3)

I had a similar idle surge issue with mine many years ago (mostly it would hang about 100rpm too high, but sometimes it would surge), it was occasionally bad enough to trip a P0507 code. The problem was intermittent but I eventually learned what sort of driving conditions were most likely to cause the problem. Tried all kinds of stuff to fix it, including several of the things you mentioned above. Replacing the EVAP purge solenoid seemed to be the fix for mine (this is the one that connects to the intake manifold on the driver's side, just behind the throttle body).

One thing that was very consistent about mine (not sure if this is true for yours as well) is that I could immediately correct the issue by turning the engine off and then right back on. This seemed to "reset" whatever was causing the issue. I also took note of the fact that IAC counts would be at 0 (IAC closed) on my scanner when the condition was happening, meaning that the PCM was effectively no longer in total control of the idle speed. So air had to be coming from somewhere, and it was something that could be reset with a simple key cycle. Even though I wasn't getting any EVAP codes, I went ahead and replaced that solenoid since it was the only other device that connected to the intake which I hadn't yet checked/tested/replaced/etc.

prowlingleather

Thanks for the suggestion! I'll get my hands on the car again next week, and I'll see if swapping out the EVAP solenoid helps at all.

The issue generally goes away every time I let the car sit long enough for her to cool off; when I start her up after that, the idle is fine and she drives normally (so not exactly what you had but similar).

blunder

Maybe try cleaning the maf.

RPM WS6

iTrader: (3)

That's a great suggestion. For some reason I thought it was already listed in the things that the OP has tried above, but now I see it's not there. This should definitely be done.

The issue could even be more than just the need for cleaning (MAF might be failing, but not enough to cause an SES). He could unplug the MAF (forcing SD mode) and see if the problems go away.

prowlingleather

I actually cleaned the MAF a month or so ago (the car didn't come to me in great shape so I figured it was as decent a place to start as any) so it should still be alright unless it's failing; completely skipped my mind to include it in the list of things I've tried.

I'll unplug it when I'm home and see if that helps any. Thanks!

pdxmotorhead

My jeep did that and it was a combination of the IAC valve sticking at low RPM and a tired Crank Position sensor that the signal would get weak on when it warmed up..

prowlingleather

Swapped EVAP solenoid, seemed to get it to idle a little lower. After cycling it about two or three times, SES light came on with the same damn P0507 that it would periodically flash and she's been doing the same things.

After about a hundred miles after reset every time, she's started stumbling--I'll give her gas and the engine just won't go until I step off and back on the throttle a few times (it sounds like a hiccup and the idle gets much more burbling and uneven when I finally get to a stop; RPMs at red lights drop considerably). OBD2 doesn't show any other codes and everything shows as ready for emissions testing except for the EVAP system. She used to do this before the fuel pump/sender/filter was replaced, but I don't see why she should be doing it now. Eugh...

Popped in a new IAC and it seemed to idle even worse than usual? Is there something I have to adjust after installing a new IAC or?

Haven't given the MAF sensor a go yet (ended up just fiddling with what was fastest and most immediate (we had both an IAC and the EVAP solenoid in the garage)) but I'm about to.

Thanks again for the suggestions guys

prowlingleather

Ended up pulling the IAC out of curiosity, and it seemed to help. Popped the new one in. Got worse. Pulled the plug. Idled fine.

Possibly a wiring issue to the IAC (or I just managed to get a dud one)? Not exactly sure what goes on with the PCM once the IAC is pulled, but the IAC being unplugged definitely helps.

RPM WS6

iTrader: (3)

Have you tried unplugging the MAF sensor yet? I would reinstall (and plug in) whichever IAC motor was working better (I think that would be the previous one?) to eliminate that as a variable first, and then unplug the MAF and see what happens to the condition.

prowlingleather

I did; didn't seem to help, sadly. However, I took a look at Torque because I was wondering if something may be up with the TPS or the TB since I replaced the sensor; it shows the throttle position as 0 when it idles well, and it bounces between 0 and 0.4 (I believe) when it idles rough. Should that be happening?

RPM WS6

iTrader: (3)

0.4 is still a normal value at idle. I've seen mine at that number many times without any issues (as a percentage [%]....but what sort of voltage does it show in the idle position? About 0.5v perhaps?) But it's odd that the only time you see this fluctuate is when the idle issue is occurring.

You mentioned that you replaced a "sensor"....did you replace the TPS? Or did you just mean the IAC (motor)? Perhaps there is an issue with the TPS or with the signal (anything from wiring to system voltage.)

prowlingleather

I replaced the TPS. I'd pop in the old one to see if maybe it's a faulty sensor, but the damned thing crumbled to pieces when I pulled it so I just threw it out. It's late here so I'm gonna spare the neighbours and wait until tomorrow to check voltage.

If it's worth mentioning, the IAC has been caught in a fully closed position the last two times I pulled the plug (while the engine was still on), but the first time I pulled the plug to clean the IAC (when the engine was off), the pintle was in a much more open position.

The pulled IAC seems to do the trick (no shop here has an AC Delco replacement so I'm waiting for it to come in; maybe it'll work better than whatever I have); could an issue with the TPS wiring or voltage cause an issue with the IAC as well?

RPM WS6

iTrader: (3)

Have you been able to look at IAC counts at all? Would be interesting to see if they were at 0 during the surge events (meaning that PCM was commanding it closed due to some other air source or because the IAC wasn't responding to commands). You might have better luck with an OEM/GM piece here.

It would also be interesting to see TPS voltage at idle and if there are fluctuations in this during the surge condition similar to the angle pecentage. It should be right around 0.4-0.5v at idle and about 4.2-4.5v at WOT (unless someone has ground the bump stop to allow further opening of the throttle blade, in which case it can go to about 4.70v before setting a code). You can test this with the engine off (key on), though you may want to see if anything about the idle voltage changes with the engine running.

RevGTO

iTrader: (12)

Could it be running fine cold because it's in open loop, but start running badly when it goes to closed loop? You mentioned rust in the tank. Could your injectors be gummed up, and thus causing rich or lean conditions that the PCM is busy trying to compensate for? A speculative stretch, I know. Getting a good scanner on it may be the only pathway to the right answer.

prowlingleather

I don't actually have a scanner that can read IAC counts, unfortunately, so I've been playing everything by ear really.

The voltage seemed to be within the range you mentioned, and it does not seem to fluctuate with the surging. Just to be sure, I swapped the TPS with another one along with the IAC to see if she evens out after a drive later today. I also had a transmission jerk from reverse into drive that I figured was at least partially caused by the high idle/tps possibly malfunctioning, so I'll see if that gets any better as well.

Fingers crossed.

RockinWs6

When I had rust in the fuel injectors screens from rusted fuel lines not only did the idle surge but it surged at light throttle. Once the screens blocked more it didn't drive very well, pretty easy to diagnose at that point. Took a new fuel pump, all the fuel lines, injectors and fuel rail to fix it. Ethanol from sitting over the winter just ate everything plus I found out later the place we were buying all our fuel was getting low level auction gas that had sea water in it. Sea water and ethanol doesn't play nice together. Real Mess.

RPM WS6

iTrader: (3)

After 20 years of annual winter storage with E10 fuel, my '98 has never suffered a single issue like this. Other than the fuel filter, my fuel system is 100% assembly line original. The key seems to be keeping the tank as full as possible during winter storage (less air = less moisture). But I think your issue was probably due mostly to that low quality fuel with high water content.

RockinWs6

I'm absolutely certain it was the sea water in the fuel. Unfortunately its very common if buy fuel from discount places or mini marts that don't sell brand name fuel to get the auction fuel. If you plug in a scanner you should be able to watch the fuel trims and get some idea if its fuel related.

prowlingleather

Sorry for the radio silence; had to fix a botched brake job...

Ended up throwing in the towel and just handing her over to my mechanic so I could at least know what system or part I'm looking at; turns out to be the PCV system. I had taken it somewhere to get it redone after a buddy I had talked to (who had owned a '98 Trans Am) mentioned it being a pain in the ***. Gonna wait and see what my mechanic finds once he takes a closer look.

Thanks for the suggestions, anyway, guys!