base fuel pressure was set at both 50 and 60psi - didnt change the dropoff
fuel pressure according to the autometer gauge does not fall off or flutcuate
pumps were swapped, running either pump as primary did not impact the prob
coils were all replaced with another set of GM coils
injectors were sent out for testing, all tested within spec
3bar OS, w/GM 3bar MAP
PCM was replaced in Nov 2010 to troubleshoot the same problem (we suspected injector drivers to be the issue)
AEM truboost controller
This problem is eating up my brain and I can't stop thinking about it.
As you can see in the graph, the car will not pull past 5k. You can also see that boost creeps up after the HP/TQ curves flatline at 5k, which I would guess to indicate load of some sort? Changing boost level does not change the 5k drop off point.
At one point in time, when the PCM was replaced, pulsewidth was 16.1ms every time it would begin to fall over. Now it seems that the 16.1ms isn't consistent after scaling to get resolution in the timing tables, we're seeing pulsewidth in the 12-13ms regions when the curve flatlines.
The harness was ohmed out to the injectors and coil packs, all checks passed.
MSD wires were all ohmed out at 11-12ohms. New ones were installed JUST IN CASE. No impact.
The plugs were replaced as the car was put on the dyno. NGK BR7EF (PN 3346) were installed, first at .22 gap. We later suspected blow out, and gapped them down to .17 to ensure that wasn't happening. Changing the gap had no impact on the 5k flatline.
We suspected valve float. I had 918s in the car when it was strapped down. Everyone on hand agreed it could be float, so we installed 928s in the car as it sat on the dyno. No impact on the 5k stop.
I'm strongly considering chopping the ground wires that run to the back of the heads, and splicing in new ground wires right at the PCM and running them back to the battery to ensure the PCM is being properly grounded. I guess the PCM has 3 grounds, all run to the back of the cylinder heads. The grounds are responsible for the injector and coil circuits as well.
I'm at a loss for what to try next. I'm tired of throwing money at it. I really need good ideas, I'd love to hear fresh ideas.
My opinion would be if it were a grounding issue it would be more sparatic and not happen at the same exact rpm point every time, thats just my guess though. With all those mods i didnt see any mention of the tune, since youve ruled out what would be some of the issues i would guess, its possibly something in the tune. From here its hard to tell but it looks like it goes lean when it starts to break up. Im assuming who ever has this dyno is tuning it? I would also think that if it were a PCM problem the feedback from the PCM would be all garbled and not give good feedback.
The AFR on the dyno graph is not accurate as we did not log AFR to the dyno, rather just to HPTuners while the car was on the rollers. We tried several different points ranging from 10.8 to 11.6, no fueling changes impacted the 5k fall off.
Bump for the morning crowd. The reason I suspect a ground problem is because its at the point where boost comes in fully and is first fully loading the injector and coil circuits. It's a stretch, granted. But at this point, I'm really running out of ideas.
You have too much airflow and boost trying to keep the valves open at higher rpm. Your 928 springs are way overmatched for a PT88, even with a stock cam. Get some quality dual valve springs and your problem should go away.
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It wouldn't hurt to go a step colder on the plugs (TR8). One thing you need to check is the connection tightness on the plug boots. If there is play in the boot to plug connection they will drop out at a certain load. This is what happened to me on the dyno and at the same rpm as well.
You can look at the last page of my build thread. There's a video on there with a pull then the dyno graph. Looks similar but worse than yours. Same symptoms as you listed above. Put your car in neutral then rev to redline. If it does this fine then put it in drive and try first gear slowly with partial throttle to redline. If no issues here than it's most likely an electrical related issue when under full load.