993944S2

There are zero dumb suggestions at this point. We exchanged out all the injectors this morning. No help. Anyone want to buy a race car?

Droops83

What is happening to the air/fuel ratio when the miss/stumble occurs?

Have you verified that the fuel pressure AND volume are OK?

The suggestion about the cylinder head temp sensor is not a bad one.

scarceller

Battery and Alternator charging system? Has that been double checked? Try a different battery? I'm just looking for things that may not have been tested yet, you seem to have looked at all the obvious stuff already.

Does the tach act up at the time the failure occurs? If the DME re-boots for power loss or issues that tach will drop to 0RPMs for a moment.

scarceller

He mentioned that it goes lean and exhaust temps go up.

993944S2

Update;
Tach does not move unusually during miss. Yesterday morning we changed the speed and reference sensors, again.... It seemed much better. Almost gone? However we only had one 17 minute session before the day ended. Next time on track is Friday. Fingers crossed.

Thanks again for all the input.

scarceller

Did you check the air gap on these sensors (0.8mm)? A dime is about 1.2mm thick, sand it down till it's 0.8mm or find a small washer that's 0.8mm. Now take the old sensor and epoxy (glue) the dime/spacer to it to set the gap by adjusting the bracket. You use just the speed sensor for this adjustment (Lower sensor on the bracket). Then leave the bracket where it is and remove the sensors and install the new ones. Some folks say it's OK if you place them real close (<.8mm air gap) but I don't agree, you should follow the specs, air gap of 0.8mm

scarceller

One other thought here:
What is the ignition system being used? The DME has a drive transistor for the coil that is current limited at about 12AMPs and if more than this current is seen it shuts down the current.

I've also seen some CDI units cause a lot of electrical noise that interferes with the DME. The worse I've seen is the MSD product line, not sure why but those boxes seem to induce a lot of noise via the input trigger line.

What's the ignition system?

scarceller

I re-read what you wrote in post #1:

'-Seems to appear soon after a shift, or after getting off the throttle.'

One possibility is a idle switch that is sticking closed? But you said it also happened with the other motor so I'm not sure this makes sense. But if that switch did stick closed from a moment it would cause hesitation. You could try unplugging the switch and take a lap or two, but doing this will cause the engine to idle high and also never enter fuel cutoff on decel, neither really matters much on the track anyway.

Just sharing my thoughts on the internals of the DME and possible causes.

Steve W

Do you have a data acquisition system drawing an RPM signal off the tach or DME? Are you using an original 3.2 tach, or a converted SC or earlier tach?

993944S2

Data acq. We did, but we isolated it. The disconnected it.

Steve W

We traced one 3.2 track car with high rpm cut-outs to a loose steering rack coupler that somehow pinched the fuel line depending on the angle of the steering wheel.

Droops83

Good point. A fellow POC racer had a similar issue that was traced to a 911SC tach in a 3.2 engine race car.