high idle problems

i have a 88 xj 4.0 and i recently changed the alternator. and since then the idle has been above 2-2500 rpm,,,,, this is not not my first time doing automotive work but this has got me stumped.....i have done usual checks to vaccum lines and find nothing.... anyone had this problem before ?????



thanks
kris

I just went through something similar today, I washed my motor and was careful to avoid spraying the TPS. I wasn't careful enough, every time I'd start the motor it would idle anywhere between 1000-1200 and 2000. Depending on which mood the motor was in.
Replaced and adjusted the TPS and it got down to 700-800 and after driving it for an hour or so it settled back in to the normal 500-600 RPM range.
I plug off one vacuum line at a time, including the brake booster line and see if that changes the idle. The only vacuum line you actually need connected to make the motor run is the MAP vacuum line.
if the IAC sticks open it will idle high. I clean my IAC a couple of times a year and let a drop or two of light oil run behind the piston. This seems to help limit random high idle events.
Check the large vacuum line to the vacuum reservoir behind the right front bumper. The alternator is near there and the line sometimes rots, from battery acid dripping on it.

GREAT_WHITE said:

i have a 88 xj 4.0 and i recently changed the alternator. and since then the idle has been above 2-2500 rpm,,,,, this is not not my first time doing automotive work but this has got me stumped.....i have done usual checks to vaccum lines and find nothing.... anyone had this problem before ?????



thanks
kris

Click to expand...

Since you recently did electrical work (alternator) I would clean each and every ground wire from the battery, to the stack of multiple sensor grounds near the oil dip stick connection on the passenger side of the engine and the two grounds from the engine to the firewall. They are notious for screwing up the TPS sensor reading the ECU gets and forcing the ECU to run it at a high idle (the poor ground makes the ECU think you have your foot on the peddle).

Once you have cleaned all the grounds, then recalibrate (simple adjustment of the possition before you tighten the bolts holding it in place) the TPS to read 0.80 to 0.82 volts at the idle possition, using pin B abd C as the test points. More details are in some threads here if you search Ecomike as a poster and use "throttle Position sensor ground" as the search words.

I almost made a career of solving Renix high idle problems on my 87.

Oh, and 8Mud is right, keep loking for vacuum leaks. I found out that if the valve cover and all its hoses are not absolutely leak proof, air tight seals they all add up (multiple tiny leaks) to a big enough leak to cause high idles too!

thanks for the replies ........ i am gonna continue my search...... its not a daily driver , so no big deal if i cant get it "right" at the moment..... my other issues are trying to find a tranny for it... reverse is nowhere to be found . btw its an auto


thanks
kris