Still fighting with this. Although I had already switched the IAC out with one from another running truck I decided to replace it anyway since after all my digging it still seemed like the most obvious solution. New AC Delco IAC made no difference. I also unplugged the EGR valve when the truck was warmed up but still idling properly (just warmed up). This made no difference either. The issue still showed up when the truck got hot and went away when it cooled off a little. I removed the EVAP line from the intake and plugged it since according to my scanner the purge solenoid isn't working properly; no difference to the surging issue.
I tried to rule out a transmission issue by jacking the truck up and putting it in gear. The idea was to see if it would idle in reverse with no load (wheels spinning) but this didn't work. With no load at all the engine never 'idled', it ran at like 1,200-1,500 in forward or reverse. I can say that according to the MAP sensor the truck is seeing the same load in forward and reverse.
Still stumped here. I have purchased a new MAP as well because it's fairly inexpensive. I'll try that out tomorrow.
Seems like there are two likely scenarios here. Either the IAC counts are getting messed up by a voltage change when the truck is hot, or the truck is getting extra air somewhere. I checked the voltage at the battery and it showed no drop when the truck is shifted into reverse (14.1v solid). Also made no difference when the lights were turned on. I couldn't figure out a way to get a read on voltage at the IAC, maybe someone can suggest something? Is one IAC pin supposed to be a steady voltage or is the voltage only on the pulsed pin?
As for the extra air, I've capped off whatever I can think of. Still a possibility with the CCV system. I re-read east302's troubleshooting section on it but it wasn't clear. Maybe I'll figure out how to check that tomorrow too.
I tried to rule out a transmission issue by jacking the truck up and putting it in gear. The idea was to see if it would idle in reverse with no load (wheels spinning) but this didn't work. With no load at all the engine never 'idled', it ran at like 1,200-1,500 in forward or reverse. I can say that according to the MAP sensor the truck is seeing the same load in forward and reverse.
Still stumped here. I have purchased a new MAP as well because it's fairly inexpensive. I'll try that out tomorrow.
Seems like there are two likely scenarios here. Either the IAC counts are getting messed up by a voltage change when the truck is hot, or the truck is getting extra air somewhere. I checked the voltage at the battery and it showed no drop when the truck is shifted into reverse (14.1v solid). Also made no difference when the lights were turned on. I couldn't figure out a way to get a read on voltage at the IAC, maybe someone can suggest something? Is one IAC pin supposed to be a steady voltage or is the voltage only on the pulsed pin?
As for the extra air, I've capped off whatever I can think of. Still a possibility with the CCV system. I re-read east302's troubleshooting section on it but it wasn't clear. Maybe I'll figure out how to check that tomorrow too.
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