Stumped

[charliepeterson]

OK I'm looking for a new idea.

'95 3500 series, 6.5TD runaway engine. This truck has only 60K. on the clock. No smoke, no oil consumption, no antifreeze consumption, a little blow bye. The truck runs good except if you bring the rpm's up to 1500 in neutral, don't move the throttle, the rpm's will climb by itself to 3500 quickly. Take your foot out of it and they drop down to idle. Now the PMD was just replaced and the engine has NO codes. The CDR valve was swapped out as well.
The Tech II shows NO increase in fuel delivery either while this happens.
I've had this happen before and the PMD swap took care of the problem. Not that this matters but the truck runs on dyed fuel 100% of it's life.
This one has me stumped. Any ideas where to go?

[rjschoolcraft]

Does the Tech II show any change in the Accelerator Pedal Position sensor output when this happens?

[Barry Nave]

My 95 has always done this. Seams at that point the turbo spools up and takes off.

[gmctd]

That was one of the symptoms requiring the Optic Sensor noise filter harness, right?

Revving the engine in neutral often rsults in Speedo indication, as harness connections oxidize, become noiser, more susceptible to 400hz generator noise.
And PCM functional 'noise'.

Cruise increases\decreases rpm, with no input from APP module.

Oops...sorry - looks like I done went and exceeded your one idea (guess) limit ;)

[Kennedy]

I think this will come down to the pump and the encoder inside.

[charliepeterson]

John I tend to agree with the pump swap. Everything is still under warranty and we do our own warranty to GM.
It just seems so strange how this one panned out. All readings were normal, (APP, fuel rate,voltages, TDC,OP,Coolant levels).