LS 5.3 High Idle

[Elliot W]

I'm having a high idle issue, seems to be a common problem (according to the internet) but I haven't seen anything posted in these threads with all of your expert assistance. Embarrassingly enough, I think this has been an issue since swapping the motor in and I just never really picked up on it because I knew I had massive vacuum leaks (and codes) that I didn't address until recently. After fixing all vacuum leaks, codes still remained (p0332, p0154, p0507). I replaced the knock sensors and harness, but still get these codes in some variety when driving the truck after clearing codes. I have to turn the key twice for the truck to start (minor backfire on first turn that I thought was a result of bad timing due to bad knock sensor). Second turn it starts up and immediately revs to ~3k rpms, then slowly drops to ~1k rpms. The exhaust does sound a bit gurgled here (it is a stock motor). About 30 seconds later, the gurgling stops and the rpm creeps up to ~2000. It will stay at 2000 rpm when in idle or neutral (5 speed NV3500 trans).

I have tried unplugging the VSS connection but it makes no difference. After reading online, I figured the tps is wrong and needs to be reset. I tried unplugging the batt for 30 min, no change. I tried unplugging tps, then key on engine off for 30 seconds, key off then re plugging. This seemed to make a difference at first (engine sounded better on start up, first turn) and ran at low RPM for a few minutes but reverted back to high rpm a few minutes later. I tried the same procedure with the IAC, both the IAC and TPS, and the IAC/TPS/MAF but cannot replicate the better start up/run or fix the problem. My next step is to bring to a tuner to have him look at it and/or do a TPS reset/relearn.

Anyone else seen this issue before? Harness is from PSI performance, same with PCM, stock except for VATS delete and delete the after cat o2 sensors.

 

[Nasty-LSX]

Need to make sure the tune is correct for the start up and idle before messing with everything else.
Dont take this the wrong way, but if the person who tuned it the first time cant fix it, take it to someone
else to look at it that is a tuner. I learned to always have a tune from someone who has been doing for
Years and has a good rep with the public.

 

[TotalyHucked]

I have a PSI harness as well. Honestly, they've rubbed me the wrong way and given me bad info a few times now, I wouldn't trust their tune as far as I could throw the computer.

That said, I don't think you have a tune problem, or at least that's not the only problem. What did you do to the motor to put it in? New gaskets? Have you tried the brake clean around the intake gaskets/throttle body trick? If it revs up, there's your leak.

Throw up some pictures of your engine bay so we can see. Another set of eyes might spot something else

 

[Elliot W]

I checked for vacuum leaks but attaching my air compressor to the brake booster line (5-10psi) with a latex glove over the intake at the MAF. The glove stays inflated for a long time when I turn off the air. Leaks were identified with soapy water. I'm confident there are no vacuum leaks (even checked the brake booster connection).

Here is a photo of my engine bay. Sorry to hear you had a bad experience with PSI. I thought their customer service has been as good as it can be for trying to diagnose problems over the phone. But maybe I should try them for advice. Also, if it matters, the throttle is by cable.

You must be registered for see images attach

 

[TotalyHucked]

Is that the MAF that's so close to the TB? That needs to be much further away than that, though that's probably not the issue.

Who's knock sensors and harness did you use? I've never had any luck with anything but genuine GM, code always remained with any others I tried.

Also, how many miles were on the motor? Did you put new injectors in it or have the flow tested? You may have one or more that are stuck wide open which could explain your backfire. If that's the case, that could also hydrolock the motor if you're not careful.

 

[Elliot W]

Thanks for the input. I'll see about moving the MAF further from the throttle body, but the wire on the harness is only so long. The knock sensors were Amazon specials. Did you get you knock sensors from the chevy dealership? No info on the motor's miles. Came with the C10 (in the bed) and was told it came from an 03 Avalanche. Any way to test the injectors while installed? I did have issues with 2 injectors initially when I put the motor in (stuck shut I believe, not operating). My friend was able to beat them into submission and work (or as you put it, maybe just enough to get stuck open). I got a new set of injectors (Amazon special, again, looks like they have 4 injection holes a piece), but I have not installed them yet. If it were the injectors, would the truck be running as well as it is otherwise?

 

[Bextreme04]

Thanks for the input. I'll see about moving the MAF further from the throttle body, but the wire on the harness is only so long. The knock sensors were Amazon specials. Did you get you knock sensors from the chevy dealership? No info on the motor's miles. Came with the C10 (in the bed) and was told it came from an 03 Avalanche. Any way to test the injectors while installed? I did have issues with 2 injectors initially when I put the motor in (stuck shut I believe, not operating). My friend was able to beat them into submission and work (or as you put it, maybe just enough to get stuck open). I got a new set of injectors (Amazon special, again, looks like they have 4 injection holes a piece), but I have not installed them yet. If it were the injectors, would the truck be running as well as it is otherwise?

The 02 sensor not responding and the high idle could be related. Do you have a way to log live data? I would go look at what the 02 is actually doing, then see what IAC is doing, what TPS is reading, and what make sure you are getting data on important sensors like IAT and CTS. The computer is obviously trying to adjust idle down or it wouldn't trigger the code, so it could be as simple as a bad ground or wire or a bad or sticky IAC valve. Did you actually verify the harness before installing it? I highly recommend ringing out any harness before you install it. That means getting the P01 or P59 pinout for your ECU(LT1swap.com is a good source) and actually check that the correct wire at the connector goes to the correct pin on the PCM/Trans or that it has continuity to ground on a GND pin and 12v on a power pin. Failure to do this can cause the magic blue smoke to escape from important and expensive electronics.

Once you let the smoke out, it's really expensive to get back in.

Live data would let you narrow this down pretty quick. Exhaust leak could cause the 02 issue, but wouldn't cause the hard start. Injector could cause both the hard start and 02 issue. Wiring issue could cause all of your issues.

 

[Hunter79764]

As for live data, the Torque app and a bluetooth adapter is a good option if you don't have HPTuners or similar. I don't have any other input on the issue, sounds like good advice so far. As for cleaning injectors, a small bucket of gasoline and seafoam to soak them in has helped me out, I had similar issues of injectors glazed over and not operating after the motor sat for a while.

 

[Elliot W]

Thanks guys. A coworker with HP tuners may be able to help me out with getting data before I head to the tuner in a week.

One more thing I forgot to mention…it’s also doing what I think is called rpm hang. When I give it some gas in neutral, the rpm’s rise but take extra long to come back down to the 2k mark. Not sure if this is relevant to the problem.

 

[TotalyHucked]

That could be or it could be a separate issue. I think you likely have a few small issues that are adding up to the larger issue

 

[Elliot W]

That could be or it could be a separate issue. I think you likely have a few small issues that are adding up to the larger issue

Have you used off-brand injectors before or used GM injectors only?

 

[TotalyHucked]

We have a place here in Ga, Fuel Injector Connection, that's all we use. They will (at least used to) flow test your injectors or they'll sell you new ones. The new ones are very well priced (I think I paid ~180-200 for my new set of 52lb/hrs last fall), are new machined bodies in house with orings and everything to install and the flow data is included on a flash drive.

 

[Elliot W]

Quick update on this. I brought the truck to a tuner thinking they could find the problem on the computer side. Unfortunately they said they couldn't do anything until the idle was fixed and suggested I replace the IAC. Went the cheap route, got the Amazon special TPS and IAC, cleaned the TB at the IAC seat, put them on, and it seems better but not fully fixed (yet). I've heard it takes time for the pcm to relearn the idle, and I forgot to clear the CEL which would problem help the process. The truck does still rev to 3k rpm at start up and did smoke initially. When I took the TB off, there was a good amount of moisture in the intake which probably explains the smoke as it was likely running rich. The rpm hang mostly went away but the idle didnt come all the way down to 1k (it wasn't fully warmed up yet and had smoked me out of the garage), so no road test yet either.

Anyways, fingers crossed that it was a bad IAC and will relearn itself with clearing codes and driving. If I find a good TB with sensors I'll replace the whole thing.

 

[Elliot W]

FIXED!!

Brought the truck to a friend who had access to HP Tuners and a lot of experience with LS swaps. Trying to lower the idle RPM in HP Tuners was not possible as the IAC position was at 0%, so the truck is still acting like there is a vacuum leak when there is none. There is an orifice hole in the throttle body that seemed to be messing with the idle. The air was accounted for by the MAF, but the IAC couldn't control it, so we taped that hole shut. We also adjusted the idle set screw, which allowed the idle RPM to drop and let the PCM control idle RPM with the IAC. When we had to TB off to clean/adjust the set screw, my friend noticed the moisture in the intake manifold, which I couldn't determine if it was normal or not. This brought him to ask questions about the PCV valve. Apparently this engine didn't have one installed? The vacuum line on the drivers side was cracked, so when I was hunting vacuum leaks, I sealed it up. When we pulled it this weekend, there was no PCV valve on it. Added a PCV valve, and the truck runs perfect!

In summary, these three things fixed the high idle and RPM hang (once confirmed no vacuum leaks):
1. Plug hole in throttle body butterfly valve.
2. Adjust idle set screw as necessary.
3. Check on PCV valve, replace/install if necessary.