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I have been dealing with this problem for a year now on my 98 LS1, and nothing I do seems to fix it. The problem started when I swapped to a FAST intake/90MM TB/36lb Injectors, but I have tried swapping the parts back to my original intake/TB/injectors, and the same thing happens.
When I rev the engine, or come back down to idle when coming to a stop light etc. the engine stumbles and almost dies. Usually it catches itself, but sometimes it dies. If the engine does catch itself, it will then stable itself out. If I let it idle without reving it up, it will idle all day without a problem. It's just returning to idle where I have the problem. It also seems like the problem gets worse the higher I rev the engine.
Looking at the log, there aren't any obvious signs as to what is causing the problem. The IAC position is returning to the normal spot. I would suspect timing because I noticed the timing drops a few degrees when returning to idle, but if I manually lock the timing with the HP Tuners scanner, it doesn’t help the issue. (Maybe the timing is still changing when locked by HP Tuners but it isn’t displayed on the scanner??). The 9-10-17 Idle log shows 2 revs without the timing locked and 2 revs with the timing locked.
Take a look at the log and tune. Tthe MAF is switched off because I just made some engine changes, but I was having the same problems when I had the MAF turned on.
I’ve tried using the idle configuration process with no success.
MODS: Stock LS1 bottom end. LG G5X3 Cam. PRC Stage 2.5 5.3L Heads. FAST 90 Intake/90mm TB. Kooks Headers.
I have not played with the actual throttle blade to allow more airflow. The IAC position is usually around 65 at idle, so I'm not sure if I should open it up anymore. I can try though.
When your engine is spinning down to idle, it's consuming air. When you shut the throttle, you see your intake manifold pressure drop... the engine is still taking air from the manifold (the higher the revs, the more it will draw down the air pressure in the manifold). If you let the revs come down too fast, you won't have enough air left in the manifold to maintain idle... watch your MAP readings as the engine speed comes back down towards idle and you'll see if you have enough air left in your manifold to catch the idle.
Actually the 67KPA that I posted was when the car was mid-stumble and down to 650RPM (the idle is set to 900RPM). It looks like when the idle is coming back down to 900RPM, the MAP is about the same as it was before reving the engine.
Okay, I've had a look at your log and your airflow isn't all that bad. There is something strange happening with your spark... it shouldn't be dropping when the revs go below target idle speed. You might have a corrupted operating system.
And would that mean the spark is moving even after I locked it in place with the VCM scanner? I ask because the RPMs still dip below desired idle speed even if I lock the IAC and spark position with the scanner.
Go back to your original tune file (the first read you ever did with HPTuners)... and do a full flash with that file. Then flash your current calibration back in.
It might be worth starting a new thread at this point, but I think I am a little closer to figuring out what is going on!! I found that both my injector pulse width and Dynamic air are different when idling smoothly and returning to idle. About 95lb/hr when idling normally and 80lb/hr when returning to idle. The injector PW is about 2.6ms-2.7ms when idling and around 2.1ms when returning to idle.
I think I'm close to figuring out what is going on, but I don't know what would be causing this difference.
I tried adding fuel to the cells that go lean when returning to idle in my primary and secondary VE tables, but it didn't seem to make a difference. Is there a table that overrides the VE table when returning to idle?
And yes I know my LTFT's are off again, but I was using new injector data given to by someone on the HPTuner forum. I had my LTFT's within 5% before the injector data change and it still didn't help.
One last thing to notice is that when I barely crack the throttle (.4% TPS) when returning to idle I don't seem to have the problem.
All 3 pictures below show my scenario with the IAC position locked.
One thing I noticed - your transient min fuel mg is 0.000. With your injectors, you really should have SOME minimum value in there. I use .010 with 42 lb injectors. .04 is stock injectors. A value like 0.015-.020 should be a good spot.
Also, in your fuel tab, I noticed your injector timing is still stock. I find on cammed engines a bit of a delay helps with idle. try raising the boundary to 7 from 6.5 under fuel-general also.
Some of what you described sounds like DFCO is on and holding on too long. Consider disabling DFCO under fueling by setting the enable temp really high.
You have some room to make your adaptive spark a more aggressive. you're idling at 20 degrees and your underspeed spark is only 4 degrees. make your max underspeed spark 12 degrees at -150 and 10 degrees at -100. interpolate to 0 at 0. Then, either carry 12 degrees to the end of the table or else carry 12 degrees out to about -600 and then in the 400 column of your idle spark, put in 31. Don't do both. one or the other.
Lastly - if you're at a dead stop and free revving, your cracker routines won't kick in. I'm not even sure the follower routines kick in. So if you're having the problem just sitting in the garage, You might need to get a little creative.
First of all, thank you for your input. I appreciate the help.
Transient Fuel MG: I haven’t touched this. So I will try this and let you know.
Injector Timing: I will try changing these as well. However, if I put stock injectors back in, I still have the stumbling problem. So I have my doubts that this will cure my problem.
DFCO: I have tried disabling DFCO during VE tuning and there was no effect on the stumble.
Adaptive Spark: I have tried adjusting adaptive spark as you described with no luck
Throttle Cracker/Follower: You are correct. I have eliminated these as being the issue. Even when I lock the IAC position in, it still stubbles. Actually the throttle cracker helps it catch the engine by bringing the idle back down slower. But it’s just a band-aid on the real problem.
Is it normal for my dynamic air to be different at 900RPM when coming back down to idle than the dynamic air when idling smoothly at 900RPM? I can imagine the ~20% less dynamic air is causing the ~20% shorter injector pulse, which is causing the stumble? But I don’t have enough experience to know whether that is normal or not.
First of all, thank you for your input. I appreciate the help.
No problem. I'll try to answer your points below one by one, hoping I don't mess up the formatting...
Transient Fuel MG: I haven’t touched this. So I will try this and let you know.
That change should at least help keep the injector min pulse width from dropping too low on return to idle. Won't necessarily be a magic bullet but should help prevent going way lean.
Injector Timing: I will try changing these as well. However, if I put stock injectors back in, I still have the stumbling problem. So I have my doubts that this will cure my problem.
I hear you there. It won't cure the problem. But the change actually has nothing to do with the injector swap. It's to compensate for the overlap in your cam. it'll help keep fuel from short-circuiting out the exhaust valve on return to idle, which is when it's most likely to do it. Might even help a smidge on your mpg
DFCO: I have tried disabling DFCO during VE tuning and there was no effect on the stumble.
OK. Good to know.
Adaptive Spark: I have tried adjusting adaptive spark as you described with no luck
Gotcha. but keep in mind, it's very often a combination of things. Every little bit can sometimes help, so I'd leave the adaptive spark in to help it along. Really, same with DFCO. I typically turn DFCO off until I get it stable then see if I can enable it for fuel savings later on.
Throttle Cracker/Follower: You are correct. I have eliminated these as being the issue. Even when I lock the IAC position in, it still stubbles. Actually the throttle cracker helps it catch the engine by bringing the idle back down slower. But it’s just a band-aid on the real problem.
That's sort of what I expected you'd say.
Is it normal for my dynamic air to be different at 900RPM when coming back down to idle than the dynamic air when idling smoothly at 900RPM? I can imagine the ~20% less dynamic air is causing the ~20% shorter injector pulse, which is causing the stumble? But I don’t have enough experience to know whether that is normal or not.
Yes it is normal. On the way back down, the intake vacuum is much higher (map lower) vs steady state idling. The cracker and follower are supposed to help "break" that vacuum to prevent stalling, which is why it's dangerous to disable, but guys do it. We can work to prevent that map dipping though.
Let me know how these changes do, and we'll go from there. I bet we fix it pretty quickly
I may have mentioned this before, but the problem didn’t exist before my intake and throttle body swap. The car ran fine with the same heads and cam with an LS6 intake.
Ill play with the suggestions above and see if it gets better.
First of all I replaced the PCM with an 0411 PCM from a 02 Camaro. The new PCM seemed to react much faster and helped a lot.
The big difference was that I finally figured out how to dial in the IAC effective area so that desired idle air matched dynamic air. This didn't solve the problem in itself, but with the new throttle body the PCM didn't know how much airflow was being added by the IAC valve.
Once I had the IAC effective area dialed in, I simply increased the throttle follower and cracker airflow and delay to set the engine back down to idle softly. It took a lot of tweaking, but I ended up tripling the values in the throttle follower to prevent a stumble. The idle comes down a little slower than I would like, but that's probably just something I will have to deal with considering I have a large cam, FAST intake, and a lightweight 8" clutch.
Big shout out to Darth_V8R for walking me through getting this thing fixed via email. The guys in the HPTuners booth at SEMA helped out a lot as well. They spent a fair amount of time walking me through potential issues and thinking of solutions. Also appreciate all of the help on this forum!
The weather is crappy here in Seattle so I still haven't taken the car out to really make sure I haven't caused any driveability issues, so I'll post back if it comes back
09-11-2017, 09:20 AM
Engine not returning to idle smoothly in 69 Camaro LS1 Swap 
