E38 - Runaway idle - 2000rpm and climbing!

Hi everyone! Long time lurker, first time poster. First I'd like to thank everyone for all contributions to this site as everything I've learned about HPTuners pretty much came from here! Great stuff.

Ok, so onto the business...I've got a 2007 Silverado 6.0L (L76) with a Maggie on it. The truck has a mild cam, LS9 injectors, stock exhaust for now, and nothing much else special. I had the motor out and apart after sticking an AFM/DOD lifter so I got rid of the AFM and the VVT in the process of adding the Maggie. After getting the thing fired up I just wanted to get the idle squared away and do some closed loop tuning until I get a wideband installed. Long story short, I've been waiting for my headers for 4 months now :mad: and wasn't planning to mess with wideband in stock exhaust, much less reinstall the stock exhaust. Anyway, I get the truck fired up and everything is going great until after it's up to temp and the idle starts climbing, and climbing! I tried blipping the throttle a couple times to see if that would bring it back down but only made it worse. I finally shut the truck down after the idle was over 2000rpm and no way to bring it back in check.

Yeah, I know the MAF tune is way out in the log. I did go back and get it within a couple % on the trims and still does the same thing.

Here's what I can see:

Timing - it looks like the idle timing is doing it's job as it's backed off to 0degree or negative timing in spots trying to bring the idle down. Finally once it gets up around 2000rpm it looks like the timing gives up and goes back to normal. lol

Throttle - it looks like the throttle keeps creeping open for some reason. As far as I can tell, it's doing exactly as it's commanded but why is it commanding to do that!?! I can't figure this out! HELP!

Update - the problem seems to be fixed but I'm even more confused.

Fix - while it doesn't make sense, I found an open vacuum/boost gauge line coming off the blower. This is a tiny 1/8" ID hose. I'm not ready to declare 100% victory yet, but in the little bit of run time after plugging the hose, the idle didn't do anything unexpected.

Question - with a small vacuum leak, wouldn't the throttle blade be trying to close extra far? Mine was doing just the opposite, opening the throttle blade PLUS the small vacuum leak. I'm curious to understand what was going on. Any ideas?

You need to change your percent max and percent max brake - should be closer to 2.3 or 2.47ish... This is the max allowed closing on the TB... You'll also need to dial in your minimum airflow - it's temperature and rpm control over the TB... There are a lot of other TB controls like tq and so on too...

GHuggins, thanks! I'll start there plus try to digest all the information that you, mowton and bigmike laid out in the other E38 idle threads I just found. I assume you meant '...max allowed OPENING on the TB....' in your post above? This makes sense, but why would lowering min airflow do anything here? At the end of the log, idle was hanging around 2000rpm and 175ish lb/h airflow. The min airflow at 2000rpm is only 111 lb/h in the tune file. Is there any way to correlate Percent Max (%) to TPS (%)?

Nope - max allowed closing :) In other words these settings control how much the throttle plate is allowed to close shut... So when it was idling up - these settings would have help to allow the throttle to close...

Hi, I'm back again. Instead of starting a new thread, I figured revive this one. The original idle hanging problem hasn't re-occured since I plugged the boost gauge line.

My new question is this... WHAT CONTROLS THROTTLE BLADE OUTSIDE (ABOVE) ADAPTIVE IDLE CONTROL?

If you'll look at the attached log, you'll see at 33.3 sec I disabled adaptive spark control with the scanner. Everything was happy and it idle right along at 615rpm like it always does. I then gave a little throttle blip and things went silly again. By 35.5 sec I had given it a blip and the pedal is back to 0%. Now you can see spark is way up there and throttle isn't closing. Spark I think I understand as it's referencing main spark table because it's beyond adaptive control, even if it's turned off. That's not what concerns me. I can't figure out why the throttle blade is hanging open and not returning to where it was before the blip. If you look at the MAF it's somewhere around 190 lb/hr at 2200rpm, which should be wayyy above any base airflow stuff in the tune. Base airflow should be around 120lb/hr if I'm reading the tune correct.

Attachment 70039

Should I be logging a different airflow number, like Dynamic Air? Maybe that is calculating a completely different airflow number? Disclaimer: I haven't tuned VE with MAF disabled but I did use the VE calc method based on fuel flow rate, injector PW, etc. It's probably not exact, but it shouldn't be too far into left field.

As a side note, this all came about while I was messing around with the changes suggested above. Lowering Percent Max and Throttle Follower Torque didn't seem to make much difference on the drawn out return to idle after a throttle blip. You can see an example of this around 16.8sec in the log. I made the Torque negative in neutral to compare to park. It didn't really seem to do much. It comes back to idle with no overshoot, or hunting, but I'd like to speed up the process some. How do I make that RPM downslope steeper?? It seems like the throttle isn't closing until after adaptive spark is bringing the idle down.

My other question is.... WHICH THROTTLE POSITION CHANNEL TO LOG? There are at least 4 different throttle positions channels and they're all different. Which one corresponds to the TPS fields in the Editor for the E38 ECM?

Throttle Position (SAE)
Throttle Position
Relative Throttle Position (SAE)
Accelerator Pedal Position - this one is clear
Commanded Throttle Position - this one is clear