5.3 C10 Swap Idle Issue

[shortboxin]

Thanks for all of the help with the A/C pressures. Now with that behind me, I need to resolve the second major issue on this truck, the idle.

It's more of an issue when the engine is first started, doesn't have to be a cold engine. When it's idling without stalling, the truck's lucky to be at 570RPMs.

Things I've checked and/or tried:
Replaced IAC - went back to OEM since problem wasn't resolved
Cleaned MAF sensor
Replaced O2 sensors with Bosch specific fit
Sprayed around intake with starting fluid to determine if there might be a vacuum leak - no change at all in RPM and I doused it with 2/3 of a can.
Double checked PCM C1 blue pins 20 and 57 to constant +12V
Verified coil pack side and plug side are all fully seated for all 8 wires
Checked fuel pressure: KOEO = 54PSI, KOER = 44PSI at idle and 54PSI when throttled moderately. Also disconnected vacuum line while running and it came up to 53PSI from from 44PSI.

I realize that the fuel pressure is low, but I don't know if my gauge is accurate. Tried to rent one from Autozone, but all 4 stores near me were out of stock.

BTW, no codes in the computer per my OBD II scanner, but the tuner also just told me that it won't throw P0300 misfire codes, he believes because the fuel tank level isn't being sent to the PCM.

Youtube videos of how it sounds:

https://www.youtube.com/watch?v=Z9LVq95IQTA
https://youtu.be/BPxKjJetD0Q

[skyphix]

A little bit more info is needed;

What PCM are you using and has it been tuned/modified at all?

Edited because I realized the DBC stuff doesn't have an easily adjustable idle speed screw and its controlled by the IAC. There is a TORX screw below the TB but you also have to do a TPS reset. If you didn't change that during the swap, its probably more related to build up than adjustment. Clearly I have more experience with DBW.

Another stupid question is, did you verify that all of the spark plug wires are firmly snapped into the coil packs? Two clicks total for factory wires.


Do you have a check engine light or OBD2 reader that you could scan for any codes?


I am finding a ton of info on dirty DBC Throttle bodies causing the throttle blades to stick. Idle speed between 500-600 is very normal, but below that isn't. Have you tried using brake cleaner to clean out any gunk in and around the throttle blade, including pushing it open with your finger and cleaning any residue around the blade and hinge areas?

[shortboxin]

Thanks for the reply!

Originally Posted by skyphix

What PCM are you using and has it been tuned/modified at all?

The PCM is from a 2002 Chevy Tahoe with DBC. I don't know specifics as to model number if that's what your asking. It has been modified, but pretty much only to remove anti-theft and rear O2s. I had wondered if the guy that tweaked it may have made a mistake?

Originally Posted by skyphix

There is a TORX screw below the TB but you also have to do a TPS reset. If you didn't change that during the swap, its probably more related to build up than adjustment. Clearly I have more experience with DBW.

Everything stayed stock.

Originally Posted by skyphix

Another stupid question is, did you verify that all of the spark plug wires are firmly snapped into the coil packs? Two clicks total for factory wires.

I can double check this, but I'm pretty confident on this.

Originally Posted by skyphix

Do you have a check engine light or OBD2 reader that you could scan for any codes?

No CEL on the truck yet, but no codes in the scanner. Sorry, should've mentioned this up front.

Originally Posted by skyphix

I am finding a ton of info on dirty DBC Throttle bodies causing the throttle blades to stick. Idle speed between 500-600 is very normal, but below that isn't. Have you tried using brake cleaner to clean out any gunk in and around the throttle blade, including pushing it open with your finger and cleaning any residue around the blade and hinge areas?

I have not done anything more than a quick clean. Thing is that it never feels like it's sticking when I press the gas. Additionally, the truck sounds like it's on the verge of stalling when it's at 550-600RPM.

[skyphix]

Originally Posted by shortboxin

Thanks for the reply!

The PCM is from a 2002 Chevy Tahoe with DBC. I don't know specifics as to model number if that's what your asking. It has been modified, but pretty much only to remove anti-theft and rear O2s. I had wondered if the guy that tweaked it may have made a mistake?


Everything stayed stock.



I can double check this, but I'm pretty confident on this.


No CEL on the truck yet, but no codes in the scanner. Sorry, should've mentioned this up front.


I have not done anything more than a quick clean. Thing is that it never feels like it's sticking when I press the gas. Additionally, the truck sounds like it's on the verge of stalling when it's at 550-600RPM.

Its a little weird that it sounds on the verge of stalling at that RPM with a totally stock engine. Does it ever high idle when its cold? Or does it keep trying to find low idle?

Have you checked your fuel pressure at all? Whats the fuel system setup?

I'd give the TB a more thorough cleaning. Not all I found felt sticking - but they all seemed to benefit from a good cleaning. Both of the 5.3's I have here are DBW so I dont have a DBC throttle body here to look at.


Removing VATs and the rear O2's (disabling secondary O2 bank) really shouldn't cause this issue - even without O2's hooked up it should idle (mine did, just really rough.)

Do you have any exhaust leaks BEFORE the O2 sensors? What is the exhaust setup? Its hard to tell from the video but it does sound like theres quite a bit of exhaust leak. Might just be the video.

What is your intake tract setup? Are there any modifications to the engine? Specifically, cam?

In this video, this is my truck at "cold" idle (air temp was around 80) - around 750-800rpm.
https://www.youtube.com/watch?v=kUoSWj7fvXo


At the beginning of this video, you can hear what hot idle is, fluctuating a bit between 525 and 550. I still need to do some fine tuning to this tune (this is a 6.0 tune thats been adjusted for my displacement and injectors, done by PSI, but I'll be picking up HPTuners and fine tuning it.)

https://www.youtube.com/watch?v=nWyB34SdS9U


Yours sounds like its running rough through the whole RPM range that was in the video you posted... I know you checked for vacuum leaks, but if the cam it stock, it does sound like a vacuum leak. All the ports are plugged off on the TOP of the intake, correct?

[shortboxin]

Originally Posted by skyphix

Its a little weird that it sounds on the verge of stalling at that RPM with a totally stock engine. Does it ever high idle when its cold? Or does it keep trying to find low idle?

As for wanting to stall, if it's at 550RPM, it's okay except it will drop lower to the 350-400RPM range and then stall from there. If driving, then coming to a stop or slowing for a turn, it may just stall under the additional load. When you say cold, I will assume you mean engine cold. To that, I don't believe it ever high idles at all.

I should probably also mention that I assume it is a stock cam. I bought the engine used with 125K and just put new gaskets, seals, and externals like sensors and water pump on it.

Originally Posted by skyphix

Have you checked your fuel pressure at all? Whats the fuel system setup?

I have not checked fuel pressure. I have a hard time believing that the pressure could be low enough to cause an idle problem, yet accelerate with no issues. I stomp on it and it responds very well. Am I wrong in that assumption?

Running a stock pickup and tank from an 87 R10 and using a 96-97 Chevy P/U pump.

Originally Posted by skyphix

I'd give the TB a more thorough cleaning. Not all I found felt sticking - but they all seemed to benefit from a good cleaning.

Will do.

Originally Posted by skyphix

Removing VATs and the rear O2's (disabling secondary O2 bank) really shouldn't cause this issue - even without O2's hooked up it should idle (mine did, just really rough.)

This is what he was supposed to do. I'm not 100% sure that's what was done... he asked a number of things, all answers below. He's been doing these for years, so I anticipate that he's just got files he downloads quickly and that's it. Maybe he did the wrong one?

PCM Part #12200411
Drive by cable TB
5.3 from 2004 Silverado (based on dished piston)
2532_ _ _ _ fuel injectors (VIN Z)
2002 return style fuel rail
Case grounded O2s

Originally Posted by skyphix

Do you have any exhaust leaks BEFORE the O2 sensors? What is the exhaust setup? Its hard to tell from the video but it does sound like theres quite a bit of exhaust leak. Might just be the video.

I can get better video maybe Sunday from outside the truck so it'll be easier to hear. I'm running S10 swap shorty headers with true dual exhaust, no crossover pipe, with side exit. I'm not running cats and I have Flowmaster "glass packs". In my opinion, I don't think I have any exhaust leaks, but I had to drill out 5 of the header bolts and one of them had some minor thread damage, so I didn't want to torque it a lot for fear of stripping it out.

Originally Posted by skyphix

What is your intake tract setup? Are there any modifications to the engine? Specifically, cam?

Intake and throttle body is stock 2002 Tahoe. As far as I know, the engine was stock. I bought it used from a friend of the guy that owned the truck (he went to prison). As for cam, I can't be 100% sure.

Originally Posted by skyphix

Yours sounds like its running rough through the whole RPM range that was in the video you posted... I know you checked for vacuum leaks, but if the cam it stock, it does sound like a vacuum leak. All the ports are plugged off on the TOP of the intake, correct?

In my opinion, I don't believe I'm having any issues with how the engine runs under acceleration or even at a steady speed. As for ports on the intake, the only ones I'm aware of are the port for the fuel pressure regulator, the port in back for the power brake booster and then the evap canister purge valve, which I'm still running.

[skyphix]

Originally Posted by shortboxin

As for wanting to stall, if it's at 550RPM, it's okay except it will drop lower to the 350-400RPM range and then stall from there. If driving, then coming to a stop or slowing for a turn, it may just stall under the additional load. When you say cold, I will assume you mean engine cold. To that, I don't believe it ever high idles at all.

I should probably also mention that I assume it is a stock cam. I bought the engine used with 125K and just put new gaskets, seals, and externals like sensors and water pump on it.


I have not checked fuel pressure. I have a hard time believing that the pressure could be low enough to cause an idle problem, yet accelerate with no issues. I stomp on it and it responds very well. Am I wrong in that assumption?

Running a stock pickup and tank from an 87 R10 and using a 96-97 Chevy P/U pump.

Will do.

This is what he was supposed to do. I'm not 100% sure that's what was done... he asked a number of things, all answers below. He's been doing these for years, so I anticipate that he's just got files he downloads quickly and that's it. Maybe he did the wrong one?

PCM Part #12200411
Drive by cable TB
5.3 from 2004 Silverado (based on dished piston)
2532_ _ _ _ fuel injectors (VIN Z)
2002 return style fuel rail
Case grounded O2s

I can get better video maybe Sunday from outside the truck so it'll be easier to hear. I'm running S10 swap shorty headers with true dual exhaust, no crossover pipe, with side exit. I'm not running cats and I have Flowmaster "glass packs". In my opinion, I don't think I have any exhaust leaks, but I had to drill out 5 of the header bolts and one of them had some minor thread damage, so I didn't want to torque it a lot for fear of stripping it out.

Intake and throttle body is stock 2002 Tahoe. As far as I know, the engine was stock. I bought it used from a friend of the guy that owned the truck (he went to prison). As for cam, I can't be 100% sure.

In my opinion, I don't believe I'm having any issues with how the engine runs under acceleration or even at a steady speed. As for ports on the intake, the only ones I'm aware of are the port for the fuel pressure regulator, the port in back for the power brake booster and then the evap canister purge valve, which I'm still running.

So, the PCM is from the same Z code vehicle right? Z code means flex fuel injectors which I assume you probably already know, and are different than the normal 24lb injectors in the non flex vehicles. If the injector parameters are off, it would cause this issue.

I agree though on checking fuel pressure. I’m running the same tank/pickup/EP381 pump, with a corvette fuel regulator/filter and -6an lines.

I do mean cold engine temp - you can hear the difference in my two videos between cold idle and hot idle. Cold idle is the revving video, hot idle is the driving video. My exhaust IS leaky (I need to reweld a couple of spots) but all leaks are after the o2s and it runs great.) Open and closed circuit fueling within the PCM is what I’m really referencing at a technical level.


EVAP purge at the front of the intake is what I was most concerned about. I blocked mine off. And the big brake booster 3/8 barbed fitting was the one that got me - totally forgot to plug that and it did what you describe.

Me saying it sounds leaky or like it’s running rough, could just be your video. It’s hard to tell without standing right in front of it. The exhaust manifolds only call for 13-15 lbs so should be ok there.

My suggestion would be fuel pressure (could even be too high if you’re not running a return system and don’t have any regulator online,) clean the throttle body thoroughly. If neither of those address it, a conversation with the tuner to verify your injector settings are correct.

[layinrocker65]

seems like a fuel pressure issue to me. Throw a gauge on it and see where it’s at.

[shortboxin]

Originally Posted by layinrocker65

seems like a fuel pressure issue to me. Throw a gauge on it and see where it’s at.

I will make this a priority Sunday morning when I'm off work. I get to work at 5:30AM and head home around 6:30PM. Then the 30-45 minute drive so it doesn't leave much time on work days.

[layinrocker65]

Definitely has a pressure problem that needs to be fixed. I’d fix that first to rule that out from anything else.

[skyphix]

Yup. You’ve got a fuel pressure issue. Needs to be up around 58psi consistently.

[shortboxin]

Originally Posted by skyphix

Yup. You’ve got a fuel pressure issue. Needs to be up around 58psi consistently.

Originally Posted by layinrocker65

Definitely has a pressure problem that needs to be fixed. I’d fix that first to rule that out from anything else.

I need to verify that my pressure gauge is accurate as well. I'll follow up with that in the next few days and post results.

[Rich84]

When checking for a vacuum leak be sure to check the throttlebody itself. Mine was leaking at and around where the linkage passes through. It was only realized by doing a smoke test. It had very similar symptoms to what you describe.

[shortboxin]

Originally Posted by skyphix

Yup. You’ve got a fuel pressure issue. Needs to be up around 58psi consistently.

Originally Posted by layinrocker65

Definitely has a pressure problem that needs to be fixed. I’d fix that first to rule that out from anything else.

Okay, replaced my fuel pressure regulator last night and the pressure was identical. Figured I'd locate a pressure tester from a friend to make sure mine was accurate. In the meantime, I did a search for "fuel pressure testing ls engine" and immediately came across Bad Luck Garage's "LS ENGINE FUEL PRESSURE FACTS AND MISCONCEPTIONS". Very informative piece.

He breaks down the three different fuel systems of that era and the different expected pressures for each 1. returnless, 2. return style, and 3. return style - flex fuel. The returnless system should operate in the 58PSI range. The return style - flex fuel (my engine) should operate in the 42-50PSI range with vacuum connected.

He also explained that by removing the vacuum line, the pressure will come up to maximum (which I did early in the week and got 54PSI). On this system with vacuum disconnected, he states that the proper pressure is between 48-54PSI at idle. This would mean my fuel pressure is NORMAL. If you're interested, this is covered at 12:56 in the video.

https://www.youtube.com/watch?v=wA6mmc8fvQo

This guy performs his own tunes and sounds like he has a very strong grasp on the different systems. At this point, I'm going with the fact that I'm good on fuel pressure. I think my next best bet is to check for vacuum leaks around the throttle shaft on the throttle body per Rich84.

[layinrocker65]

I’d still wonder if the tune is not correct for the fuel pressure. You don’t have a standard regulator you can throw on?

[shortboxin]

Originally Posted by layinrocker65

I’d still wonder if the tune is not correct for the fuel pressure. You don’t have a standard regulator you can throw on?

This is more along my lines of thinking. According to the tuner, it's correct, but I'm with you.

What do you mean by standard? Do you mean adjustable?

[layinrocker65]

If I’m reading the thread right, your using the stock regulator in the stock early year LS truck rails. The regulator for flex fuel is different than the standard(non flex fuel) ones per online info stating the flex fuel runs at a lower pressure. So changing the regulator or rails to a regular(non flex fuel) setup will test if my theory is right. It will(or should) put the fuel pressure to the standard LS 58psi and if the truck runs normal, it’s an issue with the tune.

[shortboxin]

Originally Posted by layinrocker65

If I’m reading the thread right, your using the stock regulator in the stock early year LS truck rails. The regulator for flex fuel is different than the standard(non flex fuel) ones per online info stating the flex fuel runs at a lower pressure. So changing the regulator or rails to a regular(non flex fuel) setup will test if my theory is right. It will(or should) put the fuel pressure to the standard LS 58psi and if the truck runs normal, it’s an issue with the tune.

Per Bad Luck Garage on youtube, the return style systems are only around 42-50PSI at idle. Granted, the returnless non-flex runs higher than the flex systems, but at idle should still max out around 50PSI. The only way I could test your theory would be to have a returnless system altogether. Assuming that the in tank pump can drive to 58PSI, it would require a swap to a returnless fuel rail and a return filter inline.

Thanks for the idea, but I think there's too much required to test out. I'm thinking of having someone else reprogram a second computer to the flex specs and see how it runs.

Another question for anyone interested... Could there be a problem with the computer not getting any feedback about the type of fuel in the system thus causing an issue like this?

[layinrocker65]

Do you have a set of non flex fuel rails with a regulator that you can put on?

[layinrocker65]

The early LS truck motors were return style with a 58psi regulator at the rail.

[shortboxin]

Originally Posted by layinrocker65

The early LS truck motors were return style with a 58psi regulator at the rail.

I have no experience with the Chevy LS stuff. This is my first. I'm a 30 year Ford guy, so I only have feedback online to work with. You say the return style non-flex is 58PSI and this guy on Bad Luck says only the returnless hit that high.

When I disconnect the vacuum line, I get 54PSI at idle. This should be close enough to resolve the issue, but it still idles very low until the computer bumps up to fast idle after 20-30 seconds. Maybe that's an issue with computer needing to learn? Don't know.

[layinrocker65]

I have no experience with stock flex fuel LSs but I can tell you that the early style LS truck rails with the regulator in them sets the fuel pressure to 58psi. I’ve done more than 10 swaps with most of them being off the stock truck return rails. Even the corvette fuel filter regulators set the pressure to 58psi.

It’s a super easy thing to swap in another regulator or complete rails and check to see if that’s the issue. Hard to tell if the tune is the issue without seeing it.

[shortboxin]

Originally Posted by layinrocker65

I have no experience with stock flex fuel LSs but I can tell you that the early style LS truck rails with the regulator in them sets the fuel pressure to 58psi. I’ve done more than 10 swaps with most of them being off the stock truck return rails. Even the corvette fuel filter regulators set the pressure to 58psi.

This is where it's a problem for me because some guys say one thing and others say another. I haven't personally experienced any of this, so I don't know who's right. The Corvette would be the "returnless" system from my understanding, so I would expect 58PSI.

Originally Posted by layinrocker65

It’s a super easy thing to swap in another regulator or complete rails and check to see if that’s the issue. Hard to tell if the tune is the issue without seeing it.

I did already test another flex fuel regulator, so I am aware that's easy. It's just that I can't see spending a bunch on one just to test when I won't use it after that. Is a generic $10 unit reasonable to test with?

[shortboxin]

Originally Posted by layinrocker65

I have no experience with stock flex fuel LSs but I can tell you that the early style LS truck rails with the regulator in them sets the fuel pressure to 58psi.

Swapped in a 12 bolt with lower gears on my days off, so I didn't get back to this idling issue. Found some comments straight from the Ford repair manual at the links below that reinforces your statement, "Non flex fuel engines, fuel pressure spec is 55 to 62 psi with vacuum line disconnected, fuel pump running." That would put it about 58PSI mid range with vacuum line disconnected on return style, non-flex systems.

https://ls1tech.com/forums/fueling-i...injectors.html
http://www.lt1swap.com/identify_vortec_engine.htm

Also found that the flex injectors are rated at 8PSI less than non-flex injectors, "25326903 - 5.3L L59 Vin Z, flex fuel 5.3L - flows 4.164 gm/sec or 33lb/hr @ 50 psi".

Thing is that the low end of acceptable range for non-flex systems is 55PSI and my flex is running 54PSI with vacuum disconnected. Assuming my gauge is 100% spot on, I doubt 1PSI is going to make a lot of difference with the idle. I'm still thinking tune or something outside of the fuel system altogether.

I will look further into the throttle body for vacuum leaks this weekend. Thanks for your continued help!

[shortboxin]

Sprayed brake cleaner all over the throttle body, no change in idle at all. Sent a message to a tuner via ebay the other day. He asked also about the cam and told me to plug in a good scanner to verify if the IAC counts were between 50-70. If they were out, adjust the idle screw to dial it in (didn't know there was an idle adjustment).

Neither of my scanners check counts, so I looked into a scanner software package for my bluetooth OBD scanner that would read them and decided money might be better spent on another PCM with the idle bumped up.

In the meantime, I decided to just rotate the idle screw 1/2 turn. Started engine, still almost died and then improved significantly after 15-20 seconds. Put in gear and it still was iffy. Turned screw another 1/2 turn (1 turn total). Started up and it seemed pretty good, but idle was still fairly low and unstable.

Turned screw another full turn (2 turns total). Idle sounded great. Took the truck for a drive on surface streets, on freeway, and back to surface streets. That was when I found out I'd gone too far. Idle took a little too long to drop back down and was around 725RPM in park/neutral. Also found that I now had a P0507 code, IAC count high.

Backed out 1/2 turn (1 1/2 turns total). This brought the idle down to 650RPM. Took the truck for two drives, one yesterday and one today. Starts great, idles great, drives great and idle isn't too high even when warm. Just checked codes and nothing. I'd say for now, I'm good.

[skyphix]

Good news! IAC Stepping can definitely be a pain, I just don't have as much experience with it (all my stuff is DBW.)

Glad you got it worked out - though I'd still keep an eye on that fuel pressure but as long as its working for you at this point, enjoy driving!