Mavaveric

I have an 06 GTO experiencing an intermittent misfire while engine braking. It is picked up by my scanner and causes a pop in the exhaust when it happens. It happens every time i let off the throttle, happens to every cylinder in nonregular intervals. No CEL.
The vehicle has headers installed, has aftermarket cats, and factory rear O2s behind them. As far as I know, everything else is stock, and the ECU tune is not modified from factory (i think).
I have done quite a bit of diag on the car myself and even enlisted help from a few other mechanics, to no prevail. I have replaced several sensors as listed: Coolant temp sensor, Crank position sensor (recalibrated with new sensor), Camshaft position sensor, new maf (tested with new, saw no change and put the old one back on).
All the sensors appear to be reading within factory specs, so I tested several possible mechanical failure points. Combed for vacuum leaks, nothing major found. Tested for exhaust leaks, there are some small leaks around the headers, I will be replacing the gaskets soon regardless. I'm sure theres more that I'm forgetting to mention.

I need ideas from people with more LS experience.
This car is beginning to frustrate me. I don't understand how something can be so far off that the engine can misfire, yet, the computer doesn't see an issue with it.
I can provide some datalogging files and graphs if needed. Thank you all in advance.

Rich-L79

Are you sure it is misfiring? The stock GTO tune is set with a slightly delayed fuel cutoff so that when you let up on the throttle it will pop a bit in the exhaust, this is normal even with a stock tune. This can be tuned out or tuned in such a way to increase this occurance.

Mavaveric

I have pretty solid evidence. It started with a failed emissions test. On the HC graph, it spikes to the moon any time the car is decelerating. So i checked it with my cheap BT scanner, and it shows misfires popping up occasionally on every cylinder. (rather intermittently). I then had a coworker look at it with his Snap-On scanner, and it showed misfires too. He initially thought the crank position sensor readings looked off, so I replaced it and had him recalibrate it with the Snap-On scanner. It didn't solve the issue, so he recommended the temp sensor next, since its lived next to the unwrapped headers for a while, he thought it might have cooked it or something. The new temp sensor doesn't seem to have made a difference. He also said the knock sensors could have been damaged too, i'll look into those after I replace the exhaust gaskets and wrap the headers next week.

gametech

iTrader: (1)

If it has headers and aftermarket cats there is a high likelihood that the factory tune has also been modified. Has it had this issue the entire time you have owned it, or is this a new development? If this issue is new, it is mechanical. If it has always done it, it is likely in the tune. No way to be sure from an internet post.

Mavaveric

It had the long tubes on it when I bought the car just a few months ago, the gentleman I bought it from said he had installed the headers but hadn't tuned the car for with them. I put the cats on right after I bought it.
It crackled and popped when I first got it, before it had cats, and it pops (slightly less) after I put cats on too.
I do have a local tuner that I've thought about consulting, I would like to save myself the financial ruin of entering a tuning shop though.

gametech

iTrader: (1)

I have no idea if the "local tuner" is any good or not, but anyone with tuning software should know how to easily check to see if your tune has been modified from stock. If it has it is an easy matter for a tuner or dealership to flash it back to stock. If it truly is a stock tune, then you know you have other issues.

GMRL

What do the LTFT look like? Reset the fuel trims with a scanner or just unplug the battery for a few minutes then drive it again.
​​​​​​One possibility could be dirty injectors dripping fuel during decel and high vacuum situations.
Long shot but you never know

Mavaveric

The injectors seal well, and the fuel trims are normal while driving. they spike to +10% while at idle or while engine braking. Fuel pump is pegged at about 60 PSI as it should be

Mavaveric

I can have him look at it, I don't have any reason to think its not a stock tune though. I would suspect a hardware issue of some sort.

On a side note, the car has exhibited some very strange behavior that could be linked to an electrical fault. For instance, I took an additional ground cable, and ran it from the battery negative to the AC condenser... yeah... and it changed the idle vibrations a bit, so I ran my data logger, and I watched the MAF readings standard deviation quadruple when I touched the ground wire to the condenser. I haven't the slightest clue why on earth the electrical continuity to the condenser would have any effect whatsoever on the mass airflow sensor, but it did. I then tested all the normal grounds, and I couldn't find anything out of the ordinary. The block and both heads are nearly 0 ohms. The body is about 7 ohms. I checked some sensor ground terminals and got near 0 ohms. The valve covers are not grounded though, which I believe is normal.

GMRL

You may be onto something with the ground thing. Im guessing you already thoroughly checked all the grounds, including the negative side on the battery. Do you live in a rust prone part of the country?
Im gonna rattle of some random things Ive ran into over the years.
Worn out or shorted o2 sensors sometimes cause weird part throttle/off throttle bucking and misfires.
Leaky injector to manifold o-rings do weird stuff like that as well, usually dont go bad unless someone removed the rail and pinched/cracked the o-rings.
I know GTOs dont have AIR pumps, but I have seen that same exact symptom your describing when the diverter valves that go into the exhaust manifolda go bad. Air goes into the exhaust and pops during decel.
Small chance exhaust leaks may be causing a similar condition.
Just some ideas,its late, my brain is fried from work lol