2001 Grand Cherokee idling high

Here we go off on another High idling story. thanks in advance for sticking around and reading through, i want to be sure to get all the issues down.

First a bit of backstory. I have a 2001 WJ with 91000mi on it, that has an I6 4.0L Selec-Trac. This jeep sits in a parking lot for weeks at a time as i’m a long haul truck driver. It is lifted, and well lit, as i tend to do a bit of offroading at night. I have noted a number of things before all this took place. MY Cruise control when active, going up a hill it would lose speed but not downshift to get back up to speed, it would stay in 4th gear. Soon, i noted that the Cruise would not even turn on. When stopped and my foot is pressing the brake, i’ve noticed a bit of a squeak, not all the time. But it incicates the brake booster diaphram may have a small hole in it.

In october 2013 i had to replace my starter battery, that’s when things seemed to go horribly wrong. I drove the jeep from seattle to tacoma to go to work, shut off the jeep, and went truck driving. when i got home for xmas, I started the jeep and it revved very high, about 2500rpm. I let it sit and warm up in the hopes that it would come down, which it did, finally settle down to about 1400rpm. I noticed that the AC was on, so i turned it off and the RPMs settled down to about 1000-1200 rpms. and it stayed there. I noticed that once i put into gear rather roughly, it idled down to 600rpm, i had to put my foot quite hard down on the brake to keep it from moving. Once i was moving it seemed ok, but when i pulled up to a stop light, i’d kick it in neutral and it’d rev up to about 1600rpm and then settle down to about 1200rpm.

After peeking under the hood, I noticed that a few rubber boots -that attach some vacuum lines to the intake manifold- was rotted out, so i replaced them. that did nothing. I then started poking around the forums. some suggested that my TPS (Throttle Position Sensor) or IAC (Idle Air Control) was bad and that i should replace. While i’m not totally mechanically inclined, i decided to call my local mechanic and get his input. He suggested that i try pumping on the brake, and pinching the vacuum line from the booster to the intake. I pumped the brake and noticed that the RPMs would go up about 4-500RPM. so i was set on it being the brake booster, beacuse i would pinch the line and the pedal would seize up and the rpms would drop back down to 1000, not back to a normal 600, but it would drop down…

Took it up to my Mechaic, hinting that the brake booster had gone bad, and that it was causing a vacuum leak. He pumped smoke into the vacuum lines and determied there was no vacuum leak, next he tested the brake booster and determined it was fine as well. next he hooked into the OBDII plug to check the MIL code that was present in the system, i looked it up, it was something to do with the evaporative system and i can’t remember the code, but i believe it was just at symptom, not the cause. Anyway, he Hooked into the OBDII plug and found the TPS was throwing some weird voltages. He replaced that and put the jeep into a quick learning mode. He discovered the engine idle speed came down a bit. Around 800. Next he tore apart the Idle air control motor and greased it, cleaned it up a bit and put it back together and on the jeep. That did it! He said it was idling comfortably at 600-700rpms depending on if the AC was on or not.

After reading around on the forums i found a worrying amount of posts online -about having replaced the TPS/IAC and it working for a day, then the next day it going back to idling high- So i had my dad drive it around town running errands for the next few days and he reported no high idling at all. No MIL lights, nothing wrong.

On his drive back down to tacoma, he tried activating the cruise control, and said it still wouldn’t come on, and also noticed that as he was driving he’d press any cruise control related button, the RPMs would jump as he was driving along by 200, and then settle down within a couple seconds, and then when you released the button it would jump and settle again. This happens if your foot is on the accelerator. when your foot isn’t on the accelerator, then my engine rpms feels like it’s being forcfully slowed, kinda like a Jacobs Engine brake. he also noticed at the end of the journey that it went back to idling high at a stoplight, and a new MIL code was present in the system. this code was a P0123 Indicating fault with the TPS…

Neat huh? Thanks for reading, any input is appreciated. Sorry for making such a long post, but i wanted to be detailed.

'for high idle problems, it’s always a good idea to check the engine coolant temperature is correct, and that the computer reads it as correct. Otherwise the computer may think the engine is cold and boost the idle speed. Ask your mechanic to check the thermostat operation, and test the engine coolant temp sensor for proper calibration.

That vacuum leak in the brake booster could be the source of your problems as well. The engine feels that as a vacuum leak.

well the mechanic said there was nothing wrong with the brake booster, but based on the description above i thought there was. UPDATE! went and talked with mechanic today, and he thought innitially it was the clock spring in the steering wheel, which is related to the cruise buttons, however he said that when he unplugged the wiring harness for it, the idle speed went even higher, and so he thinks it probably isn’t the clock spring. he also discovered that if he holds the cruise buttons down, the idle speed will drop to normal. and then when he releases it, the idle speed goes up again. anyway any and all input is welcome. thanks!

There is an Electronic Speed Control Servo.

Electronic sppeed control servo for the cruise control? anyway, more updates… we think that the clock spring in the steering wheel isn’t the cause of the problem as the button signals seem to be getting through. and i have Stereo controls on the steering wheel as well, and those aren’t causing any problems. He shaved off some of the contacts on the TPS, and that brought it down to a normal idle speed. while the cruise control buttons still cause a 200rpm jump while driving, it no longer causes a high idle or triggers an MIL. while we seem to have put a bandaid on the issue, there is still an open wound underneath. Thusly, i think my next step is to go to a jeep junk pull-a-part and get a cruise control module. What do you think?

Are you talking about the Electronic Speed Control Servo. The ESCS, the switches, clock spring and the EMC are all that’s in the circuit.

I got off track here, can you be more specific?