96GS#007

Still troubleshooting a miss that occurs at high (>6000) rpm. The miss is intermitent and generally happens about 70% of the time.

To date, the following items have been replaced with an eye towards upgrading in the process...

- Ignition System (switched to the Delteq setup....includes new coils, wires, and ignition module)
- Spark Plugs (Denso Iridium IT20 and NGKs). Also tried various plug gaps
- Opti (so that the Delteq was being triggered by a new optical sensor)
- Fuel Injectors (was seeing 90%+ duty cycle so went to 30# flowmatched Ford/Bosch units and programmed accordingly)
- All the usual filters & valves are new (air, fuel, PCV, etc)
- Swapped MAF sensors with another LT4
- Swapped MAP sensors with another known good sensor
- Cylinder heads are fresh (new valve springs, seals, etc)
- Cranking compression varies 5 psi between low and high cylinders
- Added an additional ground strap between the engine and the frame
- New CompCams "R" high rpm lifters set to 0 lash
- Fuel pressure is good at idle and WOT (checked while on the dyno)
- Fuel Pressure Regulator is good
- No knock retard per my scanning software (AutoTap & OBD-2)
- All other parameters look "good" in my data logs
- I've wiggled wires, misted the ignition system with water, checked for arcing in a dark garage, tapped on parts, etc in the hopes something would help pinpoint the cause

Turned 363rwhp on the dyno.

I have a Harlan shift light, and although I've never heard of problems with it, I'm going to disconnect it from the PCM feed to see if it's introducing noise to the line. After that I'll do a continuity check individual wires from the PCM to the optical sensor and injectors. I may also reflash my PCM with the stock program and try the PCM on another LT4 car to see if there are any odd results.

Any other ideas are welcome.

SPD DMN

I had one I could not find for the longest time and it turned out to be one of the pins on the coil connector had spread just enough to cause it to skip at high RPM's. Probably not the cause of your specific problem, but may be something to review all the pins on the connectors to see if they are nice and tight. I had spread mine using a ohm meter checking for something else.

MSR

jim, can you post a dyno graph?

-michael

Dr. Evil

Were you able to get a tech 2 and watch the misfire data any? That could at least narrow it down to a cylinder.

STL94LT1

Jim, sorry to hear the injectors didn't solve your problem.

VR'92

I'm thinkin you got some kinda defective product that you have newly installed. I know thats no help but you have done everything. :nonod:

ericcer

Have you checked all your PCM grounds and power leads? A mechanic friend told me to check them by inducing load on the wiring. So in addition to the DVM checks he would light up a headlight using the PCM wiring.

GusBustamanteJr

I had that once, but it was on my L98, it was the ESC wire not seated right. Dont know how that applies to the LT4 world.

jfb

Pull the plug wires one at a time and measure the resistance. They should measure 5000 ohms per foot and you should wiggle the ends with the ohmeter connected to see if you have an intermittent connection.

96GS#007

The miss predates any of the mods made to the car

96GS#007

OBD II doesn't log misfire data above 3000 rpm. Below 3000rpm, the logs are clean with less than a handful of misfire counts during the course of a 30 minute drive. This is well within normal limits.

96GS#007

Wires have all been checked and are well within spec.

Dan Parker '96

Jim,

I've got your LT1 Edit tunes from the first dyno session. I can flash my PCM and mail it to you for testing. A week or two without it won't bother me.

At least we can eliminate a PCM driver failure.

EDIT: Or send me your latest program and I'll flash it with that.





[Modified by Dan Parker '96, 7:39 PM 3/23/2004]

FostersPerformance

I would hook up a f.pressure gauge at WOT and see what happens.....You could be barking up the wrong tree as far as ignition goes..Start looking at fuel issues....Have you ran maybe better gas??? It could be that simple dude...Good luck!

Aaron71771

Have you checked the O2 sensors? Also what does the service manual say?


[Modified by Aaron71771, 8:42 PM 3/23/2004]

superlund

I wonder if some how it is related to the old rev limiter settings in the PCM. The factory limiter cuts the injectors @ 6400? Turns back on @ 6390?
Since 2-3 people had the same dip at 6300 my thought is if somehow the binary code being output by LT1 Edit or even Tunercat is in slight error and possibly still cutting an injector for a split second? Have you posted the problem over on LT1 Edit forum or Tunercat forum?

Seems like the ’95 – ’96 cars had sequential FI and the code for cutting the injector(s) @ the limiter would need to be specific to sequential FI??
Are you still using the OBD-2 PCM in your’96?

When recording Wideband o2 data does the car go lean or rich at the miss point?

TRWSD90

Tell me about your distributor cap. Factory? aftermarket? It is possible that at wot the spark is occasionally jumping to the wrong post.

Steve85

Fuel, fire, compression. How about a compression test at 6000RPM? :mad

I have been somewhat following this story and I seem to remember someone referring to a Sun diagnostic machine. This machine has a swing arm loaded with data collection sensors. One of the many pieces of data it can record is spark energy and duration. From what I have read, it seems you are getting fuel and spark at the correct time. But what about the quality of spark. Certainly not the first thing one would look at, but reading the title, I guess you're open to suggestions.

It could very well be that at high RPM, a relatively weak spark is unable to fully ignite the fast moving high pressure charge. There are enough variables in any engine to make this intermittent.

I don't know if you have tried this, but, maybe play with plug gap. Make it smaller so less energy is required to jump the gap. The spark won't be as hot but should be harder to "put out". If that doesn't work, make it even smaller. :confused:

96GS#007

Here's one of my dyno graphs from a couple weeks ago. It's representative of all of them. Sometimes the miss occurs around 6100, sometimes around 6600, and any rpm in between :(...

96GS#007

Had a fuel pressure gauge hooked up to the fuel rail for multiple dyno runs. Fuel pressure was perfect. I run 93 octane and have 0 knock retard.