Can't figure it out and it's driving me crazy. Mostly stock 2000 m6 Trans am with a magnaflow cat back. ( with the exception of a4 to m6 swap) Idles normal on start up. But after driving it climbs to 850 870 on a scan tool. It's seems to bounce around a lot on the tool I'm reading. I have cleaned the maf replaced the iac and tps. I put it on a smoke machine, found and fixed one vacuum leak in the pcv grommet. Seems to have no other leaks. I can't figure it out. It only does it after driving then while stopped obviously. After hard runs it will bounce around a little higher between 860 and 910. Any help is greatly appreciated,thanks!! Oh and no engine codes.

 

When it's idling high, if you turn the engine off and then right back on again, will the idle return to the commanded speed (basically "reset" itself)?

 

Yes sometime it will instantly return if I key it off and on. But if it is warmer , it will have to sit for a sec then it will return when I fire it back up. The other thing is when running any heat ,defrost vents ,or air ,it will idle low. I only bring that up because I thought it was supposed to idle higher not lower when running any hvac.

 

You said you found one vacuum leak with a smoke machine, but have you rechecked it since fixing that leak?

Also, have you looked at short and long term fuel trim data on a scanner?

 

Yes I rechecked it after the leak was fixed and couldn't locate another. The fuel trims seem to both be -4 or -5 . Ltft that is. What driving conditions should I use to get an accurate fuel trim reading? Thanks a lot for your help by the way. I am a total newb and learning a ton.

 

Well the negative fuel trims would also suggest that their are no further vacuum leaks, so that's a good thing. Since the issue is at idle, the idle fuel trim cells would be the ones to watch.

I fought a very similar (and hard to diagnose) issue like this once with one of mine, but I could always get the issue to clear up with a simple key cycle (after which it wouldn't return until the next cold start), which suggested some sort of electronic part failure rather than a hard vacuum leak. But it sounds like yours isn't corrected with a restart. I narrowed it down to an intermittent failure of the EVAP purge solenoid, but I'm not convinced that yours is the same problem.

What do the IAC and TPS values look like at idle when the condition is happening? And what do those values look like when the idle is normal?

The lower idle when the A/C compressor is running might have something to do with how it was tuned for the M6 swap. Perhaps some portion of the A4 tune is still in place, which would command lower idle speeds than the standard 800rpm for an M6, even with the compressor running.

Have you checked commanded idle speed vs actual engine rpm to make sure there isn't some tuning related issue?

 

I would disconnect the battery for 10 minutes and let it idle for while. See if this helps the idle "re-learn" after idling for about 10 minutes. Also check for debris on your MAF screen and make sure there is no tears in your intake tubing.

 

Yeah the weather here is really bad so I haven't checked the fuel trims again . Tomorrow I'll take the fuel trims while its ideling high for about 10 mins. And my scanner doesn't show me iac position . Just tps ,maf ,o2s,fuel trims ,and almost everthing except iac. I had it tuned to a stock 2000 m6 . I had no idea where it was at from the previous owner. The tuner said he had the fueling set all wrong. So I wanted to start back stock and go from there as I add mods slowly. So it should be set to idle at 800rpm I would think. I'll either go see him tomorrow morning or next Saturday and find out. I have to have him delete my egr and set it to ready plus I figured out I have 3.23 gears not 3.42. Lame. So I have to have him fix that cause my speedo is a little off. Door code was for 2.73 gears so I figured dude I bought it from at least upgraded to 3.42 . That's what I get for assuming! And can no longer get ahold of him so he can't be of help.Oh yeah, the tps read 0 at idle . I'll check it again to make sure.

 

Yeah I've done about 10 idle relearns by pulling the appropriate fuses. It didn't help. Once I drive for a few it starts ideling high again. I've cleaned the maf a ton of times. Plus I drove with it unplugged and my car still does it so that eliminates the maf. Now the intake tubing I'm pretty sure is fine.Even though smoke came out of there when I did the test but that's where all the smoke is entering so I don't think its that.

 

Ok went to see the tuner and he wasn't there so I took the fuel trims again and this is what I came up with. I turned the engine on and it was idleling at 810 to 860 or so rpm. Fuel trims after 25min of idle : ltft b1 +2.3. Ltft b2 +0.8 they slowly climbed to this lean reading. Drove it back home a 25 mile ride then took them again after 15 more min of idle they are:ltft b1 0.0. Ltft b2 -2.3 . Stft seem to stay between 0 and +3.0 lean. Tps is 0 at idle. And I think the longer I let it idle it will climb lean again to about the same as the first reading. Rpm moves between 800 and 860 but drops to about normal whenever the fans kick on. And while driving at stops it takes a sec to come down then it will idle between 870 and 915 rpm . Kind of confusing I know.

 

Those fuel trim readings are fine for a stock tune, usually you can expect to see anything between -10% to +10% at idle, depending on conditions, engine load, ambient and coolant temp, and how long it's been idling. Also up to a ~3% spread between banks is usually considered normal for a stock tune.

Ideally, you'd obviously want them negative all the time to ensure they lock at 0% at WOT, and this is what the aftermarket tuners shoot for. But again, with a factory tune your readings are fine and to be expected.

TPS being at "0" may or may not be normal depending on what this reading is indicating. Is this for Throttle Position Percentage, or voltage? TPS angle at 0% for idle is fine, but there should still be a voltage reading above 0 (something like 0.4-0.6v).

What I'd really like to see are IAC values and commanded idle speed. It's possible that 800rpm is the commanded speed, but that the IAC is not able to maintain that value due to another air source. The fuel trims don't indicate a vacuum leak, but if the throttle body blade is open too far or the hole in the blade has been enlarged (both would be a metered source of air and shouldn't affect fuel trims either way, but would affect idle speed), then the condition you have described would be possible. If we could see IAC counts during the high idle, then we'd know if the IAC was doing everything possible to bring the idle down (meaning fully closed, 0) or not. If engine rpm at idle is higher than the commanded idle speed, then IAC counts should always be 0, and this would indicate that the IAC has lost control of idle speed due to some other source or air. If the idle speed creeps up much further than it already has you'll get a P0507 code (I think this trips at 150rpm above commanded idle speed).

 

Tps is for percentage. My scantool doesn't have iac counts it sucks. Throttle blade is closed all the way. I'm sitting here at idle and with the scanner and defrost on it idles perfect. Doesn't make sense. I can't talk to the tuner till next week so I'm just gonna have to live with it. Heat defrost or air puts it right by 800 rpm . I don't understand. And that throttle blade hole is stock size.

 

With all the resets and battery disconnects you've done, you may not have allowed enough time for the idle relearn process to complete. Anytime I reflash the computer or disconnect the battery, I suffer with high idle and "cruise control effect." It takes about six weeks of regular driving for it to go away. And I've never had any luck with drive cycles or other techniques speeding up the process.

It happened to me once with what turned out to be a leak in the PCV system. I started a thread about it in the Tuning forum. Just like the above cases, it took six weeks of driving to resolve.

 

Wow that is a shockingly long period of time for an idle re-learn. With all of my LS1s, whether it was a battery disconnect or a hard reset from a PCM reflash, it's never taken more than one complete idle relearn process to get the idle back in line - regardless of model year.

 

I never had to wait that long before. And the idle was fine after the original re flash to stock a few months ago.Yeah I did my last relearn 5 or 6 weeks ago so that will make no difference. I just don't understand why with the compresser running it idles perfect. Hopefully the tuner can shed some light on it Saturday. I'm fighting the urge to go buy an oem idle air control valve. Maybe the borg Warner one isn't working right. Warranty the old one and pay the 60 buck difference at oreilly. But the only time doing that has ever worked was when I took back the two RPT maf sensors because both of the would not work at 4000 rpm. The stock worked fine. I'd probly just be throwing more money away though . And I've done enough of that this month.

 

Well I believe I fixed it. I readjusted the set screw to close the throttle blade more and reset the tps. Seems to have worked. Staying right around 800 now and returning to idle much quicker. I thought the blade was closed ,I guess it wasn't. Thanks a bunch for all your help RPM WS6!!

 

Glad you got it figured out!