running like crap..

91, 4.0,ax-15 .

My idle/rpm have been bouncing between 1000-2000 for about 2 weeks with an occasional journey to where its supposed to be.
Yesterday the RPMs were going crazy, revving and such with the clutch in.
This morning,while taking the spawn to school it was running really rough, and almost died a few times. Once I went a few miles and it warmed up, it ran ok but the rpm's were still up there.

I did the junkyard TPS when it first started acting up with no change and squirted some carb cleaner into the IAC port also with no change.

This afternoon it was running rough again when cold so I jiggled every connector & vac line and still nothing..


any ideas where to go next?

*Search comprehensively for a "vacuum leak". Vacuum leaks are easy to miss. Try spraying around all things vacuum with a light mist of carb cleaner or a light mist of water works too.

*Through cleaning of both the idle air control AND the throttle body is never a bad idea. They get carboned up over time and miles.

thanks, I sprayed the obvious culprits and nothing, I'll try everything again this afternoon.

rysam said:

thanks, I sprayed the obvious culprits and nothing, I'll try everything again this afternoon.

Click to expand...

IAC is sticking or has a vacume leak. spray with carb clean at all intake areas and see if idle changes. unplug iac and see if speed changes, if not replace iac.. Before all of this stuff try a pcm reset, unplug battery for 15 minutes and reset everything. Also check fuel pressure, If its running lean it could cause all kinds of problems for ya.. You also want to check the map sensor for proper readings, remove it and check for crud blocking the port. if not take it to autozone and borrow there scanner and check vacum signal from the map sensor make sure its not reading crazy signals.



PCM IS BEING PROVIDED IMPROPER INFORMATION- If the PCM does not receive proper sensor readings it will not deliver the proper injector pulse width for conditions. The following is a list of sensor problems that could cause short and long term adaptives to be highly negative: COOLANT SENSOR CALIBRATION (sensor is reading much colder than the actual engine temperature). OXYGEN SENSOR CIRCUIT PROBLEM- (this includes O2 sensor heater, grounds, signal wire problems, heater operation. Once the O2 sensor reaches operating temperature the voltage should never exceed 1.02v) MAP SENSOR CALIBRATION--One of the most important readings for testing a MAP sensor is the voltage reading obtained with the DRBIII with the ignition on and the engine off. This voltage reading is important because the PCM uses this for calculating barometric pressure. If the PCM does not "know" the correct air density from the MAP sensor (baro. reading) it will have to drastically change the fuel curve through the adaptives to get the proper response from the oxygen sensor. Below is a chart of key on MAP sensor readings for a given elevation. You should get to know what is normal for your area. Elevation (feet) Baro Reading (in.-hg.) MAP voltage 0 (sea level) 29.92 4.6v 29.67 500 29.42 29.17 1000 28.92 28.67 4.4v 1500 28.42 28.17 2000 27.92 27.67 2500 27.42 4.2v 27.17 3000 26.92 26.67 3500 26.42 26.17 4.0v 4000 25.92 25.67 4500 25.42 25.17 5000 24.92 3.8v VOLTAGE SENSE-- The PCM compensates the injector pulsewidth based upon charging system voltage. The higher the voltage => the stronger the magnetic field opening the pintle valve in the injector. The PCM compensates for this condition. If the PCM sense a lower voltage than is applied to the injectors the Adaptives will become negative.

I think Ive got it down to the IAC. I sprayed it out good with carb cleaner again and the idle has returned to where it should be. Im going to douche it again this afternoon and cross my fingers.

dont do the ecm battery disconect, it wont change anything. oh and dont bother going to auto zone as they will not have a scanner for a pre 96 obd1 vehicle. the drbIII doesnt work on these jeeps (i know i use one every day). only your local jeep dealer will be able to communicate with your jeep. do check for any obstructions in the intake/throttle body. your best diagnostic weapon is going to be a digital multi meter to check voltages of all possible issue sensors. or you can just take it to a mechanic and let them fix it.

Try this...

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The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality

Thats what I needed! Thanks for that. Its still acting up a bit but much better than its been so I'll pull the IAC tomorrow morning and clean 'er real good.

final diagnosis was a multiple fold issue.

the plastic TPS "catch" was broken allowing the plug to work out and run rough by breaking the connection.. fixed with a ziptie. easy.

the second issue was the gasket on the intake had gone south. It wasnt entirely unexpected as I re-used it after being on another rig for a couple months, but that was 2 years ago.

both the problems mixed together had me stumped looking for a single issue.
fixed them both and now it runs smooth as can be.