Random Misfire Help

[ajmuller]

Hey guys,

New here to the group. Just received my HPTuners suite to use on my 06 GTO LS2 M6 I have some experience on older cars, the last tune I did was on an 89 Firebird Formula 350 with TPI (obviously, that tune was a bit simpler than what is in the newer cars).

I am having what I consider a pretty weird situation. Car ran great and had no issues, started getting a lot of vibration and found a bad engine mount. I changed the mounts for polyurethane engine and trans mount. I also changed the starter while working on the car to solve a hot start issue with weak starter. Shortly after getting it back on the road, I started getting random misfires and CEL with code P0300. All 8 cylinders see misfires and none seem to have a statistically higher count than the others. It only seems to happen after highway cruises. If I drive the 60 miles to work, CEL will come on halfway home. Sometimes it may take it a day or two, but it will reliably come on. If I stick to round town driving, it will sometimes go back out after a few on/off cycles.

Admittedly, I threw some parts at it assuming it was something simple. Here is the list of changed parts/procedures:

Plugs/wires
Upstream O2 Sensors
Cleaned fuel injection system (OTC tool with Pro Inject-R Kleen)
Checked fuel pressure
Smoke tested intake tract looking for vacuum leaks, none found
Changed Crank sensor and did a relearn (banged against it a number of times taking the starter in and out)
Cleaned MAF
Had dealer perform top End clean as per TSB

I have not data logged the car yet, but will get a few logs starting tomorrow to work and back.

Anybody have any ideas? I cannot see why Poly engine mounts would cause a misfire but unless the timing was pure coincidence, that is all that changed.

Thanks in advance for any advice,

Drew.

[espinjt]

What have you done so far on the tune? I know on cammed vehicles you need to mess with the Engine Diag, Misfire tab. What has been changed?

[ajmuller]

What have you done so far on the tune? I know on cammed vehicles you need to mess with the Engine Diag, Misfire tab. What has been changed?

The only mods to the car is a CAI, Long tube headers, cat back exhaust. I have not done anything to the tune yet other than eliminating errors from the rear O2 sensors.

It had a street tune from a local tuner on it, but the dealership put it back to stock when they did the top end cleaning. The tune is stock right now.

Both the tune from the tuner and the stock tune see the same random misfires.

[espinjt]

Do a MAF fail on it to put it into SD. See if it runs any better.

[Earwax]

Does it actually miss? Or is it just setting the code?
If you can feel it missing, then you need to see if it loses spark, fuel or compression.
Post the tune and a log of it missing.
Check the GNDs/wiring since the motor was moving due to the broken mount.
Make sure that nothing is Grounding out causing a vibration.
A crank learn will usually clear up false P0300`s. Try using a Snap on scanner to do the crank learn.
The cam and crank sensors are used to determine misfires based on crankshaft acceleration during the power stroke.

[ajmuller]

Got a log today on my way to work. I am still learning the SW, but most of the misfires logged seemed to be during periods of cruise on the highway. Interestingly, they are not as evenly distributed as they were the last time history was pulled. Cylinder 1 and 7 are the worst, with very few relatively speaking on cylinder 5.

I did not notice any misfires logged during acceleration, and I don't really notice much at cruise? Would I feel it as a vibration or something at 80 mph? I have some minor vibrations but attributed that to poly engine mounts and bushings, one piece driveshaft, etc...

Here is the log and tune, any ideas/help appreciated.



I can disable the MAF and log the ride home in SD mode.

8-29-AMGTO.hpl 2006GTOAMv1.0.hpt

[Area47]

put a new computer in it. aas weird as it sounds, this is the third or 4th gto i have heard of with random misfires for no reason

[espinjt]

Put your high octane table back to stock and copy them to low octane. Your spark may be too high but instead of showing KR, it switches to low octane table. There is a huge swing between table that would cause the crank sensor to see the jerking triggering false misfires.

[ajmuller]

Interesting idea. I have been chasing my tail on this for a while and nobody has been able to find anything wrong.

At $250 for a new computer it may just be peace of mind even if it does not fix it.

[ajmuller]

Put your high octane table back to stock and copy them to low octane. Your spark may be too high but instead of showing KR, it switches to low octane table. There is a huge swing between table that would cause the crank sensor to see the jerking triggering false misfires.

I tried this last night but did not get a good log. I will need to try again. The problem only seems to be happening at highway speeds (70+ to about 85) in 6th gear.

[espinjt]

Push comes to shove, you can always max out your misfire data. I personally do that on cam swaps. I know for a fact it is not misfiring and if it did misfire I would be able to feel it.

[Motoxrrar]

I have been chasing a similar issue. Random Misfire Cylinder 1. I can feel the misfire. Some Key cycling on off will clear it. However it will eventually come back. Newbie to tuning. Ill attach my tune as rudementary as it may be. Changed plug wires. But honestly I haven't a clue where to start as I have a unique setup.

72 GMC Longbed Truck
4.8L Gen IV from 2007 Silverado
2012 4L80E trans
3.53 rear Gears

[ajmuller]

Do a MAF fail on it to put it into SD. See if it runs any better.

Sorry for the space between replies. It took me a while to get a few logs of the runs to and from work with vacations and stuff. This seems to be the answer; set the MAF High Freq fail to zero and drove it. Misfires essentially disappeared across the last two runs.

The last log of the hour+ long drive had zero misfires recorded on the Current Misfire per cylinder 1-8 PIDS compared to thousands logged with the MAF enabled.

Considering this is basically a stock tune, I would assume it needs a new MAF. Is there anything else that could cause it to log misfires with the MAF enabled vs SD other than a weak MAF sensor?