Idling issues

[Brian in Ontario]

Good-Day Knowledgeable Jeep People,

I need some input and was hoping someone could help.
Jeep: 1990 Cherokee Laredo 4x4, 4.0L

A couple of time over the last year my Jeep would idle very high upon startup (~3000 rpm) and wouldn't settle back to the appropriate ~1000 rpm. To fix the problem I'd shut the engine down and start it again (sometimes it would take more than one attempt).

These days my Jeep rairly reeves so high initially but instead settles into a 1500 rpm idle after the engine has reached operating temperature. Tapping the throttle or working the throttle linkage under the hood does nothing to change the situation.

I've considered that my Throttle Position Sensor (TPS) may need replacing so I metered it's voltage output (as Chilton's recommended) and the output was appropriate (200 mV-closed throttle, < 4.8 V-throttle wide open). Next I was planning to meter the Coolant Temperature Sensor but wasn't able to find it. According to Chilton's it should be right at the front of the engine but on my year it's nowhere near there. If you think that this is something worth looking into can you suggest where I should look to find it.

Other details:
-my fuel economy is in the toilet (~400 Km / tank)
-my vacuum system may be a culprit (this system seems more audible than normal)

So, is the initial high idling situation separate from the sustained high idling situation?
What are the possible causes of each?

Thanks,
Anxious to fix my Jeep

[scuba4321]

try cleaning your IAC, valve, this is known to cause idle problems,

[ops5]

I don't know the answer to the idle problem, but the temperature sensor should be right on the thermostat housing; at least mine is

[runuover]

try your vac line to your map sensor

[flyboy1997]

Originally Posted by ops5

I don't know the answer to the idle problem, but the temperature sensor should be right on the thermostat housing; at least mine is

On the 1990 Cherokee 4.0 motors the coolant temp sensor is in the block on the forward left side. Don't confuse it with the Knock Sensor which is on the same side but mounted more towards the bottom of the engine block.

I've also had the same idle problem. Removing the Idle Stepper Motor from the Throttle Bottle assembly and them cleaning it with throttle bottle cleaner has ALWAYS fixed it for me. It always seems to get gummed up in the passages for the stepper motor.

[kmerian]

I was having the exact same problem with my 89, I replaced the MAP sensor and that fixed the problem. Try disconnecting the vacuum line from the MAP sensor, if it idles down, there's your problem.

[mrbill3]

It might be your O2 sensor also. I had trouble wiyh the idle on mine and had to replace the IAC first. That solved most of the problem, but it still had high idle at start-up at times. I checked the O2 sensor and it was bad so I changed it and now 1200 miles and no problems!