Have Jeep Questions? Ask a Jeep Repair Expert for Answers
Hello and welcome to Just Answer.
I am sorry to hear about your problems with this vehicle. If you like, we can try to do a few tests to isolate the cause of the problem.
Your truck uses a device called an IAC valve to control idle speed. Base idle is determined by the amount of air that gets around the closed throttle plates. The computer opens the iac valve to admit more air as necessary to raise the engine rpm.
The iac valve is located on the side of the throttle body.
I'd like to see if it's working at all. Here's what we are going to do to test it. When the engine is shutdown, the IAC valve should be parked in a position that will allow a large volume of air to enter the engine. I WANT YOU TO TURN THE IGNITION OFF AND UNPLUG THE IAC VALVE. When we restart the engine , the engine should be revving at a very fast idle. This will tell us if the iac is working or if it's stuck in the closed position for some reason.
Get back to me with the results of this test and we'll go on from there as necessary.
Thanks for using JustAnswer.
UNPLUGGING THE IAC MADE NO DIFFERENCE, ENGINE RUNS AT CURB IDLE, NO FAST IDLE INCREASE.
Ok, that's good news, it looks like we've found a problem.
I want you to back out the screws for the iac vavle and pull the valve out partially and start the engine. As you pull back on the valve does the rpm go up ?
YEP, WITH THE ELECTRICAL CONNECTOR STILL DIS-CONNECTED, THE IDLE INCREASES WHEN I PULL OUT THE IAC VALVE.
Ok Russ, that tells us that the idle passage in the throttle body is ok.
Ok, do you have a digital voltmeter? I can upload and image and a diagram and we can test the circuit for the IAC valve. If not, you may want to try a new valve. Let me know which path you would like to take.
YEP, GOT A DIGITAL METER, NO PROBLEM WITH THAT, SEND IMAGE FOR TESTING.
Hello again, sorry for the delay, I was away from my desk.
I have uploaded a two page document . Click HERE to download it. You should be able to print it.
I want you to leave the valve disconnected.
I want you to start the engine
I want you to set your DVOM set to DCV 20 (or close to that)
I want you to connect the black lead of your DVOM to the Violet/Black wire at the IAC harness and the red lead to the GREY/RED wire.
I want you to move the throttle slightly to raise the rpm and release it ..do this several times. RECORD THE VOLTAGES YOU OBSERVE.
I want you to repeat these steps with the black lead of the DVOM connected to the BROWN/WHITE wire and the red lead to the YELLOW/BLACK wire and to record the voltages you observed.
Get back to me with the results.
Ok, this is important. Did they stay at 12-14 volts, or did they fluctuate from +12 to - 12?
Did the polarity reverse as you BLIPPED the throttle ? Make sure to push the iac valve back in to lower the RPM .
Question.. I meant to ask this earlier. Can you get a good, stable low idle speed (normal idle speed) by moving the iac valve, or does it stall as soon as you slow the engine down? What is the lowest Idle speed you can maintain ?
YEP, FROM + TO -, AS I CHANGED THE IDLE SPEED.
IAC IS BACK IN AND SCREWS TIGHTENED.
IDLE SPEED IS STEADY, CHANGES (SPEEDS UP) WHEN I MOVED THE IAC OUT, IDLED BACK DOWN WHEN I RELEASE AND PUSH BACK IN.
Ok. You have completed testing the IAC circuit.
If the voltage polarity toggled from plus to minus on both circuits, the PCM (powertrain control module) is trying to operate the valve.
Ok, as long as the terminals on the plug are firmly gripping the terminals on the valve, the valve is faulty and needs to be replaced.
There is a IAC relearn procedure in the factory scantool, but we're going to go about this a little differently.
Replace the valve. Disconnect the battery and leave it that way for a couple of hours. Restart the engine and see how the idle is. It may stall the first time. If it does, turn the key off for 10 seconds and retry. It should reset. If it does not, you are going to have to take it to a shop that has a scantool that will perform an IAC RESET.
(oh, if it doesn't start after reconnecting the battery, take the key and lock and unlock the driver's door.. it might go into antitheft fuel cutout)
Let me know if you have any questions here . Let me know how you make out.
THANKS FOR THE HELP. I'LL SEE WHAT HAPPENS AFTER I CHANGE THE IAC.
Bookmark this page in case you have to ask followup questions.
Thanks for the ACCEPT.
Thanks for using Just Answer.
THIS IS RUSS AGAIN, WITH THE JEEP PROBLEM. REPLACED THE IAC VALVE TUESDAY LATE. WAITED UNTIL WEDNESDAY MORNING TO DRIVE. WANTED TO DRIVE UNDER SAME CONDITIONS AS WHEN HAVING THE PROBLEM.
WEDNESDAY MORNING DROVE TO DAY CARE AND HOME, HOUR LATER WENT OUT AGAIN, VEHICLE RAN GREAT (NO STALLS) FOR 10-15 MILES OF STOP & GO DRIVING, BACK HOME. TWO HOURS LATER WENT OUT FOR A TRIP TO GROCERY STORE, 3 BLOCKS FROM HOME, STALLED AT STOP SIGN, WAS DIFFICULT TO START AGAIN. STALLED TWO MORE TIMES BEFORE GETTING TO THE GROCERY STORE. PARKED, BOUGHT GROCERIES, TRIED TO START, JEEP STARTED, BUT STALLED AGAIN BEFORE PUT IN GEAR. FINALLY GOT STARTED AND DROVE HOME. 4-5 HOURS LATER, TRIED STARTING AGAIN IN DRIVEWAY, STALLED IN PARK SEVERAL TIMES, STALLED WHEN PUT IN GEAR SEVERAL TIMES, AND WAS MORE DIFFICULT TO START. DROVE APPROX. 3 MILES, RAN FINE, SHUT OFF FOR 1 HOUR, STARTED AGAIN AND DROVE BACK 3 MILES HOME.
THIS MORNING, THURSDAY, STARTED AND DROVE GRANDSON TO DAYCARE, RAN FINE THERE AND BACK.
I'm glad to hear it's running ok at least part of the time now .
Russ, are you sure the terminals in the connector are tightly mating with the IAC valve pins? These pins can get loose on the plug.
I use the blunt end of a "numbered drill bit" that is the same size as the pins to check the female terminals for "pin grip". If inspection reveals that pin grip is an issue, you might want to see if you can purchase a used plug and install it if you cannot tighten the pins.
Russ, I want you to check something for me. Remove the air intake tubing on top of the throttle body . Start the engine. Find the small hole above the iac valve that provides air to the IAC. BLOCK IT. Did the engine stall?
If yes, the base idle is set too low. It should idle (though very slowly) when this hole is blocked.
OK, I'LL GIVE THAT A TRY, IT WILL TAKE ME A FEW MINUTES. IT'S COLD OUTSIDE, AND MY PC IS INSIDE, SO I RUN BACK AND FORTH DOING THIS.
Ok, I'll be around. Let me know.
Russ, one question I didn't ask before ... how many miles on this Jeep?
WHEN I BLOCK THE PASSAGE, RPM'S DROP TO 500 OR A LITTLE LESS, WHEN I UN-BLOCK THE PASSAGE RPM'S GO WAY HIGH AND THEN IDLE BACK DOWN.
IT HAS 196,271 MILES.
Ok, the valve is working properly and the base idle is ok.
Does this ever cut out on you while you're foot is on the throttle. ? I'm wondering if we have more than one problem here ?
Did anyone ever replace your crankshaft position sensor ? I'm just pondering what other issues may exist... I'm not suggesting you replace it..
Does your DVOM have a min/max record feature ?
YES, IT WILL CUT OUT WITH FOOT ON THROTTLE, WHENEVER IT FEELS LIKE IT.
TO MY KNOWLEDGE, NO ONE HAS REPLACED THE CPS.
NO, MY DVOM DOES NOT HAVE.
Ok, I understand.
We are now entering into one of those situations that make me unhappy. If I had the truck in my service bay, I would connect a multiple trace labscope and roadtest this truck until it stalled to try to determine if the CRANKSHAFT POSITION SENSOR was faulty.
Without access to equipment, my job here becomes difficult and you are reduced to throwing parts at the vehicle. I don't like doing that because sometimes new parts that get installed are faulty,and then you really end up in the sewer...
Ok, I'll stop complaining and try to make a useful comment.
After re-reading this entire thread, I'd like to try something simple.
I'd like you to try tapping on the cranksensor. Try heating it with a hair drier. Try moving the wiring harness at the back of the engine, try moving the harness plugs at the computer... did any of these cause a change?
I'd also like you to disconnect your exhaust oxygen sensors. BOTH OF THEM. Make sure you tie the harness plugs back so they don't hit the hot exhaust and melt. The heater circuits on these share a power feed with other components and if the heaters are starting to "short out" (water intrusion over time is a problem) they may drop the voltage on A142 circuit (feeds ignition coils, injectors, etc) momentarilty and cause the engine to stall.
Drive it some more and let me know.
( 9 out of 10 mechanics asked about this would say "throw a cranksensor at it, it's usually the problem". I just hate to see you investing more money on a guess. )
IMPORTANT. If you decide to try a cranksensor, only buy it from a dealer. The non-oem units have been nothing but trouble (in my and others experience).
OK, THE CRANK SENSOR SEEMS TO BE OKAY. ON TO DIS-CONNECTING THE OXY SENSORS. I'LL LET YOU KNOW WHAT HAPPENS.
THIS IS RUSS AGAIN WITH THE JEEP PROBLEM.
AS INSTRUCTED DIS-CONNECTED BOTH OXY. SENSORS, DROVE THE JEEP THE WEEKEND. ONLY CHANGE WAS 'CHECK ENGINE' LITE ON NOW, FROM THE OXY. SENSORS I ASSUME.
OTHERWISE, IT STALLS SAME AS BEFORE.
I have 2 suggestions today.
1/ replace the crankshaft position sensor
2/ get the truck connected to a scantool that can reset the IDLE LEARN.
Please let me know how you make out with this.