I’m stuck. When my truck gets up to operating temp it’s idle surges. A few hundred RPM high and low, raised and drops about once per second. Everything I’ve done and replaced is listed here:

IAC
TPS
PCM (bad internal voltage regulator)
cleaned throttle body
checked vacuum, no leaks (unless around intake manifold, haven’t checked)
alternator was replaced
power steering pump (PSF pouring out weep hole)
fuel pump (sending unit was seized and MPG was horrendous, 180 miles a tank, now getting about 500)
New Cat and y pipe
new o2 sensors

when I bought the truck, the idle screw was maxed out, and it was idling around 1200rpm. It surged far worse, but it still does, even after replacing everything idle related, aside from touching the distributor and the ignition pick up, as the screws to the distributor are completely stripped and I can’t replace cap, rotor or pickup (new cap and rotor was in the truck when purchased so I suspected there was a possible issue, but aside from the idle surge it’s been fine). No engine codes or chassis codes. Any ideas? It’s been ongoing for months and I’m not sure where else to look, honestly.

 

What brand O2 sensors? If they are bosch, you don't want 'em. These trucks don't like 'em.

Does the problem go away if you disconnect the O2 sensors?

 

it happened for months before I replaced them, and then with the o2 sensor ports capped off for a few weeks, and ran nothing. Ran super rich but still no change in surge. And no they’re not Bosch, I was told by a few people to avoid them, seems I made the right call to listen to em

 

Good deal.

You can download the service manual for your truck from here. I think I would start with the procedure to set base idle, as it isn't supposed to be adjustable, at all..... If the throttles are cracked to far open, that is going to increase TPS voltage, and the PCM won't know that you aren't actually at idle.......Probably going to be something along the lines of disabling IAC, when it is fully closed, and setting min idle to around 500 RPM or so. Exact procedure should be in the manual. (it was for GM vehicles.....)

 

The idle screw for the throttle blades is capped off from the factory. So somebody remove the cap and changed the setting. I doubt there is anything in the manual for adjusting it. Really no need to adjust it with a stock motor the iac should adjust idle speed. I suppose you could see what the voltage reading on the tps is and get it into the ball park. I suspect you have another issue that's why somebody changed the setting. Cap off the vacuum ports for the PCV and HVAC/cruse, If there is a leak it will raise the idle speed. Any oil in the plenum? You may want to do a load test on the battery, Weak batteries can cause unstable idle.

 

Batteries only a year old is still near 100% health, 12.55v no load, 14.1v charging. And like I said I checked for vacuum leaks everywhere but around the intake manifold, which I heard is a fairly common issue with this gen. I’ll do it again though and check it out. I also backprobed the IAC and it was within normal voltage range from idle to WOT.