ElMartillo

Purchased days ago. Bone stock. 4.0 4WD. 107K well maintained miles. Planned on doing plugs/dist cap, first oil change this weekend, so I have done nothing to it yet.

It’s been running great, first time starting/driving after dark last night. It was a bit cooler last night than it’s been lately, possibly into the 40s so not crazy cold. Started fine, didn’t let it warm up. Hestation off of idle entire 20 min drive home. Temp gauge has been consistently reading just under 210, but last night it never quite got there. Hesitation seemed to diminish as temp rose.

I think it may be time for a new thermostat, but I feel it shouldn’t hesitate even when cold.

Thanks in advance for helpful suggestions.

Cheers, all!

ElMartillo

After starting this am I’m leaning toward dirty/sticky IAC/TB. I’m going to clean the IAC first and see if there is a difference.

tjwalker

1. Full tuneup, not just plugs and cap. Champion plugs, quality plug wires, cap, rotor. Don't buy cheap hardware; plenty out there.

2. Clean idle air control and throttle body. See below.

3. Evaluate your cooling system. Might be time for at least a new Mopar 195 degree stat and Stant radiator cap, but those hoses can rot from the inside out. For a new to you vehicle, I often recommend wholesale replacement of all hoses. For a DIY, it's not expensive. Drain and fill with 50-50 of green coolant and distilled water.

4. Enjoy your XJ!
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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

jordan96xj

Cleaning IAC worth doing, but typically it fails in the opposite way, where RPMs are strange when entering idle, not leaving it. Because as soon as you use the gas pedal, you are basically controlling the air entry manually and overriding the IAC.

During the hesitating, does the engine RPM remain low, or does it rise without a corresponding forward motion of the vehicle? I ask because low transmission fluid level can also have a "hesitation" effect when leaving idle, and will also tend to get better as things warm up. But the difference is, that the engine RPM will rise normally with the gas pedal, and then there will be short delay before the transmission engages and starts moving the vehicle forward.

A motor that is running rich can also cause hesitation that gets better as RPMs increase. This will also be more pronounced when the engine is cold/cool and get a little better as things get up to operating temperature. It can also present as a slightly lower operating temperature. So don't rule out the possibility that you have an upstream O2 sensor that is starting to reach its end of life.

ElMartillo

Good advice, thanks for the help folks.

I did just pull the IAC and clean it. It needed it. Unfortunately my IAC is on the valve cover side of the TB, and the throttle position sensor is on the back, which I removed first thinking it was the IAC as according to the above instructions. I hope just removing/replacing the TPS doesn’t cause any issues. I did start it up after and everything seems to be normal.

The hesitation is/was a near stall that occurs off idle just as the throttle is applied, if I give it a little more throttle it accelerates fine. It seemed to be somewhat intermittent, occurred after cold startup this am, but after shutting it down, upon restarting it did not occur. I’ll see if it reoccurs after I let the engine cool down again.

UPDATE: Several start ups today, with varying levels of engine warmness, starts up better than it did. It was idling high on startup before, longer than what seemed normal. Now idle reaches normal speed sooner. Seems to run generally smoother/better.