Every other cylinder firing LS6 2005 CTS-V

[colo_evo]

Hello,

Trying to diagnose a very odd issue on a friends V.

Car ran great no issues. Took it apart for a heads/cam swap and had the ECU sent out to be bench flashed for new modifications.

First crank the car fires right up on all 8. All is good. Went to take the car around the block for the first time and that's when the issue first came about, seems like it lost all power when just stepping into throttle. Could barely get the car to move on its own after that because its misfiring so bad. Later found out for some reason the car doesn't fire four cylinders. Would let the car sit and it would fire up and do the same thing. This same issue kept happening more and more frequently to where it wont even run on all 8 anymore.

Tried a different PCM and new cam position sensor so far. Verified fuel pressure is constant during issue. Double and triple checked grounds and wiring connections throughout. Threw HP tuner on there to play with the tune datalog the issue when it happens. Even flashed the vehicle back to stock using a file from the repository. Was able to replicate issue and capture a datalog(attached). The vehicle appears to be just fine until everything goes haywire and we're not sure whats causing it. Initially noticed timing drops to ABDC. By locking timing at ~10-15 advance via "vehicle controls and special functions" we can get it to run at ~1500rpms to actually try and figure out whats happening but its still only firing on 4.

Cylinder firing order on this vehicle is 1-8-7-2-6-5-4-3.

If we let it run long enough with timing locked cylinders 1 7 6 and 4 get very hot(glowing). 8 2 5 and 3 are still cold. The odd thing here is every other 1-8-7-2-6-5-4-3 cyl is getting hot. There's gotta be a significance to this, I just cannot figure out what it is.

Attached is a log of the event happening. At the beginning of the log everything appears just fine, closed loop fuel control, all sensors appear happy. At 12 minutes into the log goes from closed loop to open loop accel / decel, timing drops, O2 sensors( B1, B2 and aftermarket AEM) all go super lean(misfire), trims drop to zero and everything for lack of better terms goes to shit. Oddly commanded fuel is still stoic at this point and remains stoic until the vehicle shuts off.

In the log we've got cam position HI LO, LO HI and crank sensor current. I've never logged these before so I can only guess what they mean. It appears the cam HI/LO signals are counters each time the signal goes HI and LO. Crank sensor current appears to be a function of the inverse RPM. Maybe I'm missing something here, but I don't see any smoking guns. My next guess would be the crank sensor but instead of throwing more parts at it we'd like to see if anybody here has seen similar issues and what the root cause was.

All help and/or suggestions are welcome and appreciated!

[5FDP]

Post the tune file.

Try a crank learn too.

[GHuggins]

Did you make sure to tighten the grounds on the back of the heads and intake manifold really tight and all installed back?

Here's the thing - this particular engine/setup uses odd and even bank controls and yours is misfiring on two odd and two even cylinders, so everything traces back to either an ecm tune issue, possibly loose ground causing weird problems, or some sort of mis-sync problem?

You said you tried a different ECM - was it tuned by the same other bench tuner? You tried an OE tune - was it indeed the original read from this vehicle? Final far far reach - is the cam actually for the motor? In other words was the firing order in any way shape or form changed - I know - it's a really far reach...

[colo_evo]

Post the tune file.

Try a crank learn too.

Current tune was created by copying over all differences on the attached cal to the bench flashed ecu. That attached cal was pulled off the repository and was supposed to be a factory cal.

Haven't tried the crank learn. How's that done? I see the button in HP tuners but haven't tried using it. I'm hesitant to just start mashing buttons.

Did you make sure to tighten the grounds on the back of the heads and intake manifold really tight and all installed back?

Here's the thing - this particular engine/setup uses odd and even bank controls and yours is misfiring on two odd and two even cylinders, so everything traces back to either an ecm tune issue, possibly loose ground causing weird problems, or some sort of mis-sync problem?

You said you tried a different ECM - was it tuned by the same other bench tuner? You tried an OE tune - was it indeed the original read from this vehicle? Final far far reach - is the cam actually for the motor? In other words was the firing order in any way shape or form changed - I know - it's a really far reach...

Going to look at the wiring behind the heads again. Does anybody have a wiring diagram from this particular configuration? Couldn't find anything online. I would like to see whats common with the problem cylinders. I agree with the cylinder behavior, its very odd!

Both PCM's were bench flashed by the same tuner and exhibit the exact same behavior. When starting out with HP we pulled the bin that was bench flashed and modified it back to stock(or so i think). Since we don't have the factory original tune pulled from the vehicle I believe copying over everything from the cal attached is as close as we could get to a factory tune without actually flashing with a GM tool at the dealer. Note changing cals doesn't seem to have any effect.

The cam is for the engine, or so we're told. The reluctor wheel for the crank wasn't touched. What strikes me as odd is the vehicle suddenly acts up out of nowhere. I would think if we had a mis-matched component or mechanical issue it would always behave the same. The inconsistency is whats really annoying and makes troubleshooting difficult.

Thanks for the help guys!

1254875138.hpt

[GHuggins]

With it killing 4 cylinders - it acts like it's going into one version or another of reduced engine power. This is why I was asking about who tuned both ecms. Makes me wonder if the throttle maps have been altered. See if you can get the stock file emailed to you. Only other thing I could possibly think of is possibly you've got 2 problems. Cylinders 3 and 5 connectors swapped and then an issue with cylinders 1 and 7,but this is only if there's a problem with 5 and 3 all the time.

[colo_evo]

With it killing 4 cylinders - it acts like it's going into one version or another of reduced engine power. This is why I was asking about who tuned both ecms. Makes me wonder if the throttle maps have been altered. See if you can get the stock file emailed to you. Only other thing I could possibly think of is possibly you've got 2 problems. Cylinders 3 and 5 connectors swapped and then an issue with cylinders 1 and 7,but this is only if there's a problem with 5 and 3 all the time.

Update: You were correct. Thanks for the help! We actually eliminated those codes a while back thinking it would cure it, but it had no effect.

Dialed up the P0514 table about 20% and significantly helped with the reduced power... went and dialed another 20% and it's gone now.

Now us amateurs have to correct the rest of the "professional" tune.