Jwooky

I have a new issue where the engine will not idle down below ~2000 RPM until I make a complete stop. Once I have zero speed it ideas normally as it did before.

The Speedo with takes its signal form the the PCM is working normally, so the VSS is obviously working.

just started doing this. I have to put in neutral to get it to stop. Even if I just start to roll away it jumps up.

It’s a twin turbo LS T56 with a GM 411 PCM with speed Density tune.

no faults.

I am not a tuner, and don’t have HP tuner to take a log.


I have checked for vacuum leaks, no the I can find.


Any thoughts will be appreciated.

QwkTrip

iTrader: (23)

Guessing too much timing advance or too much air. Both are adjusted with the tune.

Jwooky

Nothing has changed to either of those.

New issue just started after I got the car out for the year.

Jwooky

No one?

beau.

Are you sure this is a new problem that you didn't have 1yr ago? Did you change anything in your setup since you last started it 1yr ago? Does the problem happen upon startup or just after you've been moving?

I'd check to see if it's a mechanical issue with your throttle body sticking since you don't have HP tuners.... but if you had a scanner I'd say to start there to check TPS % to make sure it's operating properly, reference TPS position when the issue occurs, and adjust/disable throttle cracker to see if the problem stops. There's also a good chance it's airflow related if it is within the tune as aforementioned.
-Beau

Jwooky

Yes absolutely sure. It’s horribly annoying and plain embarrassing. It’s fine at startup and idle. It jumps up soon as I start rolling, then won’t drop down and idle till I come to a COMPLETE stop.

No changes. Throttle body is ok, checked it.

looked for vacuum leaks, nothing yet.

trying to get help with a log, but weather is crappy right now.

G Atsma

I bet it has to do with the speed sensor. Stopped, OK. ANYTIME rolling, 2k RPM

QwkTrip

iTrader: (23)

Idle Air Control has a factor that depends on ground speed (trans speed sensor). This is adjustable in the tune. I think it's called the Throttle Cracker table or something like that (I've never tuned with a GM ECM so not sure of the particulars).

Jwooky

I had exactly the same thoughts.

However, my Speedometer is taking the signal directly from the PCM, so it is getting it properly if the Speedo is.

Jwooky

That is my understanding too.

Just doesn’t make sense it just started doing this. Nothing has changed in the tune.

QwkTrip

iTrader: (23)

You're basically looking for anything (physical or in the tune) that could increase air flow or advance timing. Those are the two things that make an engine hang. It's more likely air flow since the idle control will try to reduce timing to bring the engine down.

My throttle body stop screw would move every time I went racing and mess up idle quality. It was fine on the street all year, but go to the track and I was hosed. Ended up wrapping the screw with teflon tape so the nut had more friction and wouldn't unwind.

G Atsma

Many of you are missing one little piece of this puzzle. The high idle is TRIGGERED by ANY movement. No movement, no high idle. I still say the speed sensor is involved here. What else detects movement??

kdick91

iTrader: (1)

It can't hurt to replace the IAC and TPS and do a TPS reset. If it doesn't fix it, just return the parts. It's really hard to diagnose without HPTuners or a datalogger.

I had a very similar issue when my ECU was failing. Car would drive awesome till I went WOT and then it would hang idle so bad it'd get the car cruising to 60mph without any throttle. When I came to a stop, it'd die. I've now had two 09354896 ECUs die randomly with weird symptoms. They look exactly the same as the 0411 ECUs, but, from my understanding, are a little less robust. Maybe something to look in to. Would be great to know which OS you're running on your ECU as well.

minytrker

iTrader: (13)

With out a high end scanner or hp tuners its going to really hard troubleshoot it.

Jwooky

Sure is. Very frustrating.

Since the tune hasn’t been touched, I keep thinking it’s got to be something physical but I sure can’t find it.

I did check basic ODB faults and nothing there.

I hate to start throwing parts at it without a lead at all.

Jwooky

Found the issue.

The intake split at the back.