427 L92 a4 GTO issue

[Stealth97]

You checked compression and leakdown in all 8 holes?

Compression tells you if a ring is fried. Leakdown tells you if valves are fried or not opening/closing.

This is a new motor, right? Is it possible a valvespring broke? I'd pull the valve covers just to give it a look see. Easy thing to do (well, on my truck anyway).

I'm wondering why it layed down at half track. Layed down on a shift too I assume? Overrevved? Did you hit 2nd instead of fourth and hit 8000 RPM?

Is it possible that a valve came in contact with a piston at high RPM (valve float) and now all your valves are bent, leading to no compression? You may not float the valves normally but you could float them on a mis-shift. I have also seen this happen. Your setup is extreme (427) so I'd imagine the least room for error.

Just trying to be of assistance. May not be EFI related, but hard parts related.

Please let us know what you find! I am very very interested to know.

Car has no catalytic converters right? Maybe a high EGT melted the substrate inside the cat into a ball. The ball is now blocking your exhaust. Happened to me before.

Eric / Stealth

[joecar]

If it was good (1st pass: 11.60@ 120) and then in a short time it was no good (2nd pass: 12.55 @105, "layed down at 1/2 track"), then something broke/bent...

Have you considered any of these:
- timing chain jumping,
- pushrods, rockers, springs, lifters, cam, valve (as Eric/Stealth said),
- cam walking,
- CKP reluctor hitting/bending,
- CKP and/or CMP sensor failure,
- other sensor failure,
- air leak in intake/exhaust,
- blockage in exhaust.

Are you running MAF or MAF-less (your MAF pid is all zero)...?

And set C0805 to an in-range value (altho I don't think that's it).

[10secvette]

While looking the car over in the pits, I discovered a hanging ground wire off the drv. side head. I figured I had found the problem. Put the ground back on and no improvement. At this point the car would hardly idle, and was backfiring throught the intake.

I think this may have fried the ecm.

The front of the motor came off, no timing chain issue. CPS was changed along with all of the other engine sensors. Microscope was used to check the reluctor, ok. No vac. leaks. No cats on the car at the track. No maf.

[Stealth97]

If it was good (1st pass: 11.60@ 120) and then in a short time it was no good (2nd pass: 12.55 @105, "layed down at 1/2 track"), then something broke/bent...

Have you considered any of these:
- timing chain jumping,
- pushrods, rockers, springs, lifters, cam, valve (as Eric/Stealth said),
- cam walking,
- CKP reluctor hitting/bending,
- CKP and/or CMP sensor failure,
- other sensor failure,
- air leak in intake/exhaust,
- blockage in exhaust.

Are you running MAF or MAF-less (your MAF pid is all zero)...?

And set C0805 to an in-range value (altho I don't think that's it).

Agreed. I was just coming back to say is it possible the timing chain jumped a tooth or something?

No DTC's? Do you have a MIL malfunction indicator light? I'd check for DTC's also.

Quite possibly is the intake manifold leaking somehow (you say it backfired, did it break. Is it a sheet metal intake? One N20 explosion and it breaks at the welds. Repair it?). I know you aren't nitrous but any backfire of pressure could possibly break it.

Throwing out ideas trying to help. Lets get this thing fixed!

[Stealth97]

I think this may have fried the ecm.

The front of the motor came off, no timing chain issue. CPS was changed along with all of the other engine sensors. Microscope was used to check the reluctor, ok. No vac. leaks. No cats on the car at the track. No maf.

What state are you in? If you lived near me you could borrow my spare ECM. I live near joecar in Los Angeles, CA.

[10secvette]

Agreed. I was just coming back to say is it possible the timing chain jumped a tooth or something?

No DTC's? Do you have a MIL malfunction indicator light? I'd check for DTC's also.

Quite possibly is the intake manifold leaking somehow (you say it backfired, did it break. Is it a sheet metal intake? One N20 explosion and it breaks at the welds. Repair it?). I know you aren't nitrous but any backfire of pressure could possibly break it.

Throwing out ideas trying to help. Lets get this thing fixed!

No dtc's.
I borrowed a tech 2 and scanned everything, nothing tripped. I swapped intakes, no change. I appreciate the idea's. I think I've gone through or swapped about everything I could think of besides the major control modules. It's frustrating because I've been messing around with LSx motors for 10 years now and this one has me stumped.

[10secvette]

What state are you in? If you lived near me you could borrow my spare ECM. I live near joecar in Los Angeles, CA.

Damn, I'm in Murrieta, near Temecula. I'm in and out of LA every week. I appreciate the offer, I would have taken you up on it but my new one should be hear Monday or Tuesday.

I will say this, if the ecm is not the issue, I have plenty of beer in the fridge about 20 inches from the nose of the car for whoever wants to come over and troubleshoot with me.

[joecar]

Murrieta... I'll have to ask my Mrs for permission...

[10secvette]

Murrieta... I'll have to ask my Mrs for permission...

Bring her along, she can go wine tasting or shopping w/my wife.

[joecar]

She'll want to go to the casino...

I'll check my GM service manual for 2005 GTO.