XJfan96

Hi everyone. I own a 1996 Jeep Cherokee with the 4.0, automatic, 4x4. I’ve been trying to solve my high idle issue for a long time now. Recently I replaced the IAC which did not solve my high idle, I took the throttle body off, cleaned it and replaced the gasket. Still nothing. I’ve checked for vacuum leaks and I have found nothing, I also tightened the upper manifold bolts which dropped it down a little bit, but I didn’t tighten the lower ones. When first started, the car idles where it’s supposed to. Then around 120 degrees it goes up to about 1000 RPM, then fully warmed up is around 1200 RPM, another thing to note is that if I turn off the engine, and restart it When warmed up, it immediately goes down to about 800 RPM and stays there for probably 10 seconds, then goes back up to 1200 RPM. When the engine is warmed up or cold, when it’s in gear the RPMs are about 800-900 RPM. I’ve replaced the coolant temp sensor too, and have cleaned every connector in the engine bay. Only stored code is a P0420, catalytic converter system bank 1. I’ve also cleaned up every ground all around the vehicle. If anyone has ANY ideas on how to solve my issue please tell me below.Thanks for any tips!!

Noah911

It sounds like the issue may be with your Throttle Position Sensor (TPS). A faulty TPS can send the wrong electrical signal of the throttle position when it is idling.

Do you know how to test the TPS?

XJfan96

Yes, I tested the one currently on it and I remember it giving the wrong reading at idle, something like .9 volts or ohms when it’s supposed to be .2 volts or ohms. I tried replacing the TPS but every single sensor I got from the auto parts store came broken in the box, so I eventually just gave up.

greythunder3

what you described is EXACTLY what my cherokee is doing. i took my throttle body off and backed the throttle plate stop screw our about a full turn. it helped until i drove it and the engine warmed up. back to 1000 rpms. i just posted about a sucking sound i cannot pinpoint..that i think is related to the high idle
https://www.cherokeeforum.com/f2/bef...ifolds-256810/

XJfan96

The 96 has a throttle plate screw? Where at?

Noah911

I was going to say.. it might also make a difference in the test readings if you were to test the TPS on a fully warmed/hot engine compartment versus the same readings done on the TPS from a cold engine compartment?

The TPS is one of the more important sensors. If it tests bad or needs replaced, it is only usually recommended to replace it with another Mopar sensor.

XJfan96

First of all, I want to thank everyone for replying, I appreciate it. I finally solved my high idle. Someone installed a piece of plastic around where the idle screw was, which extended the idle to around 1200 RPM. Obviously one of the previous owners did this, and I thought it was factory. I removed this plastic piece and now my idle is around 600 which is perfect. Again, big thanks to anyone who replied to this thread.