View Full Version : Over Revving on start up

11-20-2010, 12:17 AM
I have a 88 XJ 4.0 Ltr I had it parked for 2 weeks and started it up and ran fine stopped for about 45 minutes and re fired it and the engine over revved and would not Idle down I suspect that the throttle position sensor needs to be replaced but any info would help

11-20-2010, 01:01 AM
its the iac(idle air control) motor .

11-21-2010, 09:14 PM
i'd check any sensors on the intake side. and make sure you dont have a newly busted vacuum line etc. Agood bet it has to do with the intake/vacuum system.

11-21-2010, 10:54 PM
Well the dam thing fired up the next morning and had no idle issues and I cant find any Vacum leaks so I think it's time to do a rebuild on the Throttle body...... Thanks for all the help[cheers]

11-21-2010, 11:24 PM
its the iac(idle air control) motor .

11-21-2010, 11:31 PM
yeah sounds like you may have had a stuck iac

may even want to try a can of carb clean and see how that works for ya

11-28-2010, 08:56 AM
Mine was doing the exact same thing last week, Replaced the IAC and tps while I was in there I also cleaned the Throttle body and adjusted the kick down cable for the tranny. Jeep idles and drives like a whole new machine.

11-28-2010, 10:35 AM
yeah sounds like you may have had a stuck iac

may even want to try a can of carb clean and see how that works for ya
make shure any cleaners u use are sensor safe just saying

11-28-2010, 12:06 PM
Revving on startup on the renix 4.0s (87-90) is almost always due to faulty TPS or the wiring for the TPS. For a proper diagnosis you need a very good digital graphic multi meter or a diagnostic scanner, so you can read the voltage on the TPS in real time. Minor fluctuations in the TPS voltage will cause the revving issue. The TPS wiring will cause the same issue due to loose or corroded contacts. There is a large harness connector at the top of the fire wall on the drivers side that is a known cause for said faulty contacts. A technical service bulletin from Jeep outlines this problem and thier cure for this is to find the wires for the TPS in the connector, cut them out of the connector and solder them directly together.