My '98 K2500 with the 5.7 Vortec surges very badly at idle. Park idle is 700 rpm. In gear idle is 650 when it stabilizes, but drops as far as 350 in a regular hunt that oscillates every 3-4 seconds. Sometimes after 10 or 20 seconds the idle will stabilize. Sometimes when you put it in drive or reverse (especially reverse) it will stall out. The truck only does it when warmed up.
I've checked the usual. No vacuum leaks I can hear or find with propane, vacuum booster has been tested. IAC was removed and cleaned, and it does move when cycled. TB was also cleaned at that time. TPS was checked, no flat spots and it seems to read properly at idle. Truck has no codes. Coolant temp sensor is reporting the correct temp.
I've put a scanner on it and everything seems ok. The IAC actual and commanded positions match, short and long term fuel trim is decent (around -1.6%), and both banks are equal. O2 sensors are around 0.5-1.0v and both match also. IAC counts do seem a bit high during in-gear idle. They get up to ~130, some places I've read say max count should only reach 80-90? What we also found was with a good scanner we could manually set the IAC positions. An IAC count of 0 was equal to ~525rpm in-gear idle and the truck had no problem idling when we were manually adjusting it with the scanner. Pull the scanner off and put the truck in gear and it's back to hunting.
Any suggestions? I wanted to bump the throttle blad up to a min idle of over 550 rpms at 0-count IAC so the truck couldn't possibly idle low enough to hunt, but when we open the TB blade up that far it's past 0.67v on the TPS and the truck stops using the IAC's because it doesn't think it's at idle anymore. The TPS isn't adjustable, although I'm tempted to slot the holes and bump it up to see what I can accomplish. Beyond that I don't know what else to do. The truck is capable of idling at 550 in gear, but the computer makes it hunt badly.
I've checked the usual. No vacuum leaks I can hear or find with propane, vacuum booster has been tested. IAC was removed and cleaned, and it does move when cycled. TB was also cleaned at that time. TPS was checked, no flat spots and it seems to read properly at idle. Truck has no codes. Coolant temp sensor is reporting the correct temp.
I've put a scanner on it and everything seems ok. The IAC actual and commanded positions match, short and long term fuel trim is decent (around -1.6%), and both banks are equal. O2 sensors are around 0.5-1.0v and both match also. IAC counts do seem a bit high during in-gear idle. They get up to ~130, some places I've read say max count should only reach 80-90? What we also found was with a good scanner we could manually set the IAC positions. An IAC count of 0 was equal to ~525rpm in-gear idle and the truck had no problem idling when we were manually adjusting it with the scanner. Pull the scanner off and put the truck in gear and it's back to hunting.
Any suggestions? I wanted to bump the throttle blad up to a min idle of over 550 rpms at 0-count IAC so the truck couldn't possibly idle low enough to hunt, but when we open the TB blade up that far it's past 0.67v on the TPS and the truck stops using the IAC's because it doesn't think it's at idle anymore. The TPS isn't adjustable, although I'm tempted to slot the holes and bump it up to see what I can accomplish. Beyond that I don't know what else to do. The truck is capable of idling at 550 in gear, but the computer makes it hunt badly.