Hey, all...I just bought my 98 D 5.2L a week ago; when I test-drove it, I mentioned to the seller about the high idle (abt 1200RPM); he said he had been having "problems" at lower idle & had his mechanic set it higher. I've already had a cursory look under the hat, and the set-screw plug is still in place - what other ways could this guy have stepped it up, and how can I get it back to spec (550?) Thanks!

 

Well the set screw supposed to factory set, so I would most likely go get another TB from a bone yard, clean it out REALLY good with 2 cans of TB cleaner and a tooth brush and install, then reset the PCM and see what happens.

I would bet he did something the set screw. Now if your rpm's fall back into range then your fine, if not replace the IAC on the new TB and monitor the TPS voltage to see if anything is coming out of it.

 

Thanks to all the info here & elsewhere on the site, I was able to solve the problem; one recommendation was to check for vacuum leaks, so I pulled the air hat while the engine was running, and "whaddayaknow"? There's a secondary vacuum port below the PCV port on the passenger side of the intake, and the rubber cap that normally covers unused ports was missing, so the open port was sucking air like nobody's business! Plugged that baby up, and she settled down to a respectable 750RPM (I did pull & clean the IAC for good measure prior to this). I just hope that some of the other "issues" with my new girl are as simple...

(Oh, and the "problem" the seller was trying to cover up is a squeaky fan clutch - I'll deal with that at a later date )