I'm new to XJs, and this has me flummoxed.

99 XJ Sport, 4.0, 113k miles.

Runs great, but once every month or so, when I go to start it, it will just die. Doesn't matter if its 10 degrees or 60 degrees. Starts up but won't idle, just stalls right out. Battery is good, alt is good, starter is good. Seems to me like a fuel issue?

So I had it tuned up, and it still does it. When it does this, I can get it to start by holding the gas pedal down for a couple minutes, FORCING it to idle. After a while it holds the idle and runs like a champ.

Ideas????

 

Maybe a bad IAC? Idle air control. Maybe clean it up if you haven't cleaned the throttle body?

 

Thanks GeeohDee. I have tomorrow off so I will try to clean the throttle body first - can't hurt I guess. I will have to look up how to test the IAC. Because this happens so sporadically I don't know how I will know when it's "fixed"..

 

That's why I think its the IAC hanging up on gunk lining that little channel. I doubt its bad, just needs to be cleaned.

 

I know there's a write-up on that floating around here somewhere... I'll get to it this weekend and report back...

 

Sounds like the ecm is loosing power enough to erase itself. You idiling it while giving it throttle is letting it relearn itself.

 

You don't need to hold the gas in relearn...
Clean the iac.

 

1. Marginal battery is high on the list for stalling. And yes, you can have a battery that is strong enough to start the engine, but marginal enough to give the engine management system fits. With a marginal battery, the engine can be symptomatic until the alternator puts enough juice back into it where it behaves. LOAD TEST your battery. Any parts store will do this for you for free. It is the only accurate way of determining battery condition.

2. Clean throttle body and idle air control. How-to is shown below

3. Test your "throttle position sensor". How-to is shown below.
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) 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
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

• Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted acordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

 

Lol

What????

 

Whats so hard to understand about it? Yes it has to do with the IAC but it doesnt mean it needs to be cleaned. If the ecm looses power or doesnt have enough voltage from the battery it looses memory. The computer is relearning the adjustment for the iac when its being idled like i described.

If he cleans it and it happens again its the battery.

 

I doubt the PCM would reset hoked to a live system. Does bring up a question though. What if a guy had some nasty short, the kind that melts things and shoots sparks. Maybe like the hood hitting the POS post for an instant. Wonder what the brain might make of that.

 

Whats so hard?

Your suggestion makes no sense.

At all.

 

I have a 99 as well... Here is what I did.

I took the throttle body off and completely cleaned it, took apart and cleaned the IAC, and replaced the Throttle Position Sensor (TPS). The TPS needed replaced anyway. After doing this my Jeep ran noticeably better. Be sure to replace any gaskets, and use NEW BOLTS/SCREWS for reassembly. I tried to re-use the same bolts that hold the Throttle Body on, while putting it back on I snapped one of the Bolts off. Needless to say but I invented a whole new group of cuss words while getting it out.

I have also recently replaced the Crankshaft Position Sensor. It had gone bad and the Jeep would not start at all. Once replaced it fired right up.

I would also recommend that you put a K&N Air Filter in place of the stock paper filter. Im not sure if they really save you gas, but they do make a vast difference in the Jeep's performance.

 

For my 98, all I needed was a good tune up. (distributor cap/wires/rotor)

 

Hello from down under! Although and old post this helps a lot!
There is also another option -leaking head gasket? On start up
after seeping overnight causes 3 cylinder firing but if you hold
accelerator down you can clear the miss fires and it will run ok
This of course is only a weeping amount -just enough to wet
plugs --it will unfortunately increase and require a gasket replacement!
How do I know this ? 7 years ago I was told to retighten the head
and did not!!!
If XJs had power rack and pinion steering to allow for a sensible radiator ,they would have been far more popular from the beginning and maybe a stainless heater core???