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Can idle be adjusted on 87 xj?

chris y

NAXJA Forum User
Location
oklahoma
My xj seems to be idleing a little low because chargeing seems fine when rpms are up a little but at idle which seems low the guage almost goes to the negative side till engine rpms go up any ideas?thanks
 
The IAC should automatically hold the RPM's correctly. I do believe that there is a screw stop for the throttle body that CAN be adjusted. I suppose that this should be done with the IAC unplugged.
 
On renix era XJ's the TPS can be adjusted, not 100% on the IAC. I do believe there is a write up or link to a page which shows how this is done.
 
my 90 had a low idle when i bought it, the iac seemed ok but it idled at about 4-500rpm. there is a screw on the throttle body that can be adjusted, and that is what i did to correct it temporarily. keep in mind that this method is not recommended as a solution, just something to get you going.....the problem will lie elsewhere and all this does work around it instead of fixing it.

nonetheless i ran it like that for almost a year with no issues. i decided to pop the valve cover and clean out all the grime, at the same time i cleaned out all the vacuum hoses. once i put it back together the idle jumped up about 400rpm, i set that screw back to the factory position and it idles right around 800rpm now. so you might want to check your vacuum system.
 
A broken vacuum line results in a higher idle. If it once idled correctly, a low idle indicates something is wrong. It's not really likely that someone intentionally misadjusted the idle to be low; therefore, just adjusting the idle to be higher isn't really correcting the problem. You should probably start by removing and cleaning the throttle body. You should give you main attention to cleaning the two idle passages. One is controlled by the IAC on the front of the TB and the other is a screw adjusted passage on the street side of the TB. You can leave the TPS in place but should remove the IAC. Don't physically monkey with the "plunger" on the IAC or you can ruin it.
 
Not sure if it used to idle good or not i got it in oct last year and it had been sitting for over a year fired it up and put it on trailer have done stuff to it but more it seems to get drove seems like idle is lower now but i also havent changer air or fuel filter yet..
 
There is no idle screw on the TB of an XJ. There is, however, a "keep the throttle plate from slamming shut and damaging the throttle body screw", and that screw is sometimes called a "base idle screw" but that isn't what it does.

The IAC is a stepper motor and it will handle the idle fine. You should, however, check the "base idle screw" and make sure it is set properly. Remove the intake tube. Use a 3/32 allen wrench to back out the "base idle screw" until the throttle plate is fully closed, then turn the allen screw back in until you can just perceive the throttle plate moving AND THEN STOP. Put the intake tube back on, and let the IAC do its job.

If the IAC isn't doing its job, then pull it and clean it with throttle body safe cleaner--it will state that on the can. Clean the throttle body cavity too. They both get pretty nasty. While you are doing all this you should test and adjust the TPS--if it is telling the ECU the throttle is off-idle then the IAC won't do its job.
 
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