98 Silver Bullet

iTrader: (28)

My tuner is having hard time stopping the surging at idle. He can reset the LTFT's to zero with his tech 2 and it idles fine. After about 30 seconds they start creaping up in the +15 - +20 range indle the surging starts again. He feels I have a vacuum leak. I have searched everywhere I can't find anything. I removed every hose going to the intake and plugged them and it did not help. The intake seems the sealing good on the heads. I even replaced the gaskets. There was some other issues. The PCM was not seeing the PNP switch when I shifted from park to drive and a couple times while driving it would let him command it to shift while other times it did. I was wondering if a loose ground can cause all this. I checked the PNP input at the PCM and was fine. I sent my PCM to him so he cound check it in another car. Everything was fine, he could see the input from the PNP switch. I did find the ground on the passenger side a little loose and groung strap on drivers side was missing. Could these ground issues cause these idle problems and the problem with the PCM not seeing the PNP switch. Could loose grounds effect the LTFT's and cause them to creep up due the fact that the O2 sensors not having good grounds?

jimmyblue

iTrader: (11)

The 4-wire O2s should return ground to the
PCM, not the block. However I bet there are
other sensors that are block-referenced and
might get corrupted by ground offsets.

It soulds like the car wants to be lean to idle
right, too rich probably saturates O2s and makes
things "soggy", putting the engine outside the
PCM idle loop's stable region.

Restoring the ground is a good thing to do
regardless. If it was the problem, your trims
and your idle will go back to right.

A propane or carb-spray vacuum leak check
would be good to do, it will find leaks you
can't see.