Mirage GT miss at high RPM's

[georgiaboy]

The problem I am having is on my Mirage; I have been working on it to try to solve this problem, but my lack of available time lately has taken its toll and I would like to get this problem corrected so that I may use my time to drive instead of troubleshoot this car. The problem is a miss at high RPM's. If I am free-revving it, the miss will occur at about 5000 RPM if I quickly push the pedal down; it will miss at about 6000 RPM if I gradually run it up to the rev-limiter. If I am driving and am in boost the miss will occur at about 3800-4200 RPM. If I driving and am in boost but gradually apply pedal (get boost later in the rev-range) the miss won't occur until about 4200-4500 RPM.
This problem was noticed when I was doing a re-tune (from E-85 to gasoline) at DynoProven with Ngoc and Thuoi (Dr. 2.0). I spent many hours there and they were gracious enough to charge me next to nothing. Thanks, guys!
What has been done so far:

1. Two new sets of spark plugs. They have been gapped all the way down to .024, but are now at .026 (NGK BPR7ES).
2. Three sets of spark plug wires. One set was a cheapie new set, one was the original set, and one was a set of used Magnecore's.
3. Compression check is *okay*. Cylinder one is a little low, but it had the same reading when the car was making 505hp on the E-85.
4. The car has been tuned on gasoline. I have an excellent AFR (up to the miss that is, it is confused once that hits) through the rev range.
5. I am using a MAF-T and an AFC to tune. I have wideband in the car.
6. The tank is super clean, the filter is new (changed it after being at the dyno). I have a 255lph in the tank. I took it out and it pushed to >100psi (guage-limited), then the overpressure valve relieved on the pump itself.
7. The fuel does reference boost and vacuum at the Aeromotive FPR; fuel does not dive in the higher rev's. I don't think this is a fuel issue.......
8. I pulled the valve cover yesterday. The car has BC springs in it with big cams; nothing was visually broken or bent (springs and rocker arms).
9. I did find that the car was a tooth off on the exhaust side (was advanced one tooth). I reset it and the miss was STILL there although it seemed to miss a *little* higher in the rev-range.
10. I have tried 3 CAS's. Two were known good ones. One was the original one on the car.
11. I have changed coil packs.
12. I have changed ignitors.
13. I have checked for boost leaks.
14. I have changed MAF's. I had a couple of 3" MAF's on the shelf so I changed one with no success. The MAF-T has not been changed.
15. The TPS is at .9VDC at idle and is at 4.6VDC at WOT.

I have a new JayRacing 342hp pump to go in it, but I don't think this is my problem. I feel that the problem is cyclic; meaning that one cylinder is dropping out on every power cycle. This would leave me to believe that the problem is cylinder-specific, like A fuel injector or A valve spring, or A spark plug........dunno. But I am pulling my hair out!

Anyone have an idea?

-Les

 

[Brandon Mau]

Have you checked the ECU for leaking capacitors. I recently bought a car that had a slight miss in the RPM range and after a month the car did not start. Trouble shot everything checked my ECU and the caps were toast.

 

[georgiaboy]

Yes, I checked the caps in the ECU and I tried another stock ECU with the same results. I should have put that into my explanation......

-Les

 

[Brandon Mau]

Do you have anything to log with? Another question is what size injectors are you using right now? You said you have a wideband in the car and it's reading good throughout the RPM band but if you are running bigger than 650's it gets really hard to tune your car with the SAFC. I just got rid of mine and am running link now. Could be the problem not saying it's not possible just really hard to tune with.

 

[architechnik]

It sounds to me that you're having some instability in the wiring. If you're running the original wiring to your injectors, inspect those lines for any torn, melted, or worn insulators. Likewise, check along the wire, especially at the connector, for sharp bends where the wires may have broken inside.

Automotive wiring is not meant to last as long as we have been running these cars and often the connectors are starting to fail on the inside. There are a number of places to get new connectors and modern wiring has actually improved since these cars were made. Consider replacing all wiring that can easily be reached.

The same goes for your CAS. If you're still running the one with the pigtail, you can actually disassemble the cover and pull out where that wiring is attached. It's held in by a number of screws under a plastic cover. It's fairly common under higher rpm's for these to miss, as it's not really a very accurate device.

Aside from wiring and what you have noted, consider pulling your intake pipe and check for excessive oil in the charge pipes. Your turbo may be shot and under the conditions you described are those most likely to put a bit of oil in your combustion chambers.

 

[georgiaboy]

I was running a set of 780cc injectors, and then I swapped to the 650cc injectors because they were in my other Colt and I knew they were good. When on E85, the car was running 1000cc injectors and controlling them (with success) with the same MAF-T and AFC. Obviously the problem has surfaced since running the 1000cc injectors, so I will eliminate the the MAF-T and AFC to run some tests. The car cannot rev positively to redline, so the problem can be detected sitting still (without the threat of leaning the car out due to not having the proper fuel-enrichment mechanisms in place).
I have thought of a bad continuity senario, architechnik. The harness in my other Colt was so bad I ended up replacing the whole thing, but I don't have another good GT harness (although I have 3 or 4 good 1G harnesses on the wall right now).
Yes, as far as the CAS goes, I made two good ones from three. One had the conductors showing through the insulator right out of the case like it had been wiggled at that point too many times. This was a good CAS so I used the pigtail off of another that was questionable to repair. Continuity check was good; CAS works. I also picked up another CAS at the pull-a-part on Sunday. I haven't tried it yet, but will this weekend.
The wiring in the car is a little suspect. I will pull the harness back and inspect. I have prothane inserts in all of my motor mounts so the engine/trans doesn't rock *too* much under throttle, but do you think that little bit of movement could cause a continuity break? Could be. Hopefully a good check will solve my problems.
My turbo is a ball-bearing T-66. It is fairly new. I did check for oil in the intake tract (also in the hot side of the I/C). It is dry and I haven't been losing any measurable amount of oil from the crankcase.
Thanks for all of your suggestions!

-Les