Okay... after much debate, I decided to post this. As I think I know what the issue is, but not sure what lines to start checking first on the XJ....

Here's the deal...
My 99 XJ with the 4.0, and auto trans.... She starts and runs fine, but... it seems that if I take off before the motor is completely up to normal operating temp before I do, then she stumbles like it's going to die. The motor doesn't actually shut off.. but completely drops like you would think it was going to...
If I pull off and just let her idle and come all the way up to normal operating temp, the all is well with the world.
I first started thinking of the IAC, but when up to temp, if I test by turning the a/c on, it compensates the idle just fine. And after it's up to temp, the issue goes away, and doesn't seem to have a problem while at normal idle either. I've also checked my OBDII port with a scanner, and not one code has been logged on it.
So... any ideas? Anyone had this issue before? I was thinking it might be a air/vacuum leak of some kind that when the lines are cold, that it's letting some by where it shouldn't, but once warmed up plastic/rubber expands and the leak stops. I'm not sure.
Thanks!

 

Really? NO one's had this issue before? I find that hard to believe.

 

How old are your spark plugs , or it could be your coil starting to get weak or your coolant temp sensor is bad . last a weak fuel pump .these are the things I would check .

 

It reads like the issue is only during closed loop, once in open loop mode it runs good. Fuel trims are different during closed loop. Coolant temp sensor has a lot to do with fuel trim.
I had temp sensor (on t-stat housing) that had corroded connection and was causing a no-start unless holding throttle to floor. All i did was clean the connection at sensor and it ran great ever since.

 

1. Test coolant temp sensor (located on thermostat housing). Any manual will have the resistances. The recommended way to do it is to remove the sensor, place in a beaker of water, etc., heat water and take temps but I have found that I can get a reasonable test of the sensor by testing resistance with the sensor still installed both COLD and HOT and compare to resistance table.

2. Test your MAP sensor. It has a lot to do with fuel trim as well. Be sure that the vacuum line leading to this sensor is in good shape and pulling good vacuum.

3. Test your fuel pressure. It is not that unusual to have a fuel pump that is marginal upon startup and once it has run for a while, the fuel pressure is solid enough so that symptoms go away. For a 99, the fuel pressure spec. is 49 psi, plus or minus 5 psi.

4. Clean your idle air control and throttle body. This is just never a bad idea either periodically or especially whenever any symptoms relating to idle come up. Have you ever done this?
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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

 

I will check into the sensor. I don't have a no start issue...nor does it completely shut off when this issue is present. It didn't seem corroded when I re-installed the new t-stat housing a few months back. But I can't swear to it....but I usually clean things well before I put them back in. Just how I am.
The one thing I pray it's NOT is the fuel pump. I don't notice any issues with the delivery, though I haven't tested the psi. But if it was, that might be the end of the XJ for me. as it's currently my only vehicle and can not afford to have it out of commission. I don't currently have the garage that I used to have access to anymore, so I can't pull things apart like I normally do/would.

Thanks for the responses... I will check into what I can first.
I may clean the IAC just for good measure after I do the others, but even if it's a bit dirty, it does seem to be currently functioning the way it should.
I think you got it right though, it does seem to be only happening when the engine is cool enough to be in closed loop.

 

haven't had the chance to diagnose the root cause yet... but the more i think about it... I'm leaning towards it being the temp sensor on the t-stat. So when I can after work this week, going to check that one out first. Fuel trims being slightly different while in closed loop and the symptoms mine is displaying... really seems to fit the profile there. Hope that's the beginning and end. but at least it feels like the right place to start. I'll still clean out the IAC just for good measure, as I'm fairly certain it's original to the Jeep.

Thanks for the tips.

 

Before replacing that sensor remove it and clean the end. I used a 3M pad on it until the end was a bright brass color. Eliminated the code 22 on mine that was coming up.

 

Way too easy a job to not try this first. I’d be surprised if this isn’t the solution.