Is there a link between the transmission being in a drive gear and engine idle speed? Does the trans send a signal to the PCM that raises the idle? My truck is an S10 but this forum gets much more traffic than the S10 room does so I'm posting here. Sorry. Truck has 137k miles, I've kept up on maintenance to include replacing the F.I. spider a couple months ago which fixed a "hard to start" issue. I cant find any vac leaks, TPS and IAC valve are new too. The truck knows when it's in drive or reverse and the idle speed rises to about 1400rpm. Sometimes, when I'm driving, I'll pull it into neutral, the rpm's will sort of cycle, 1000, 1500, 2000, 1500, 1000. Strange.
Yes, I've searched here and on the net in general. Everyone says throw a TPS or IAC at it.
I bought one of those bluetooth dongles on ebay but to be honest, I dont know much about the diagnostic data it gives me.
I'm wondering if it's a transmission or PCM issue and not a fuel system issue at all. Really need a little help, any input would be appreciated.