GaiaXJ

My XJ, my first car, broke down because of the infamous head crack on that year when I was 16, has been sitting since then, finally getting the money now to be able to repair it, but I've learned that it won't start up or stay on without holding my foot on the gas, could this be caused by the bad head or do I have more problems?

tjwalker

The #1 cause of the 4.0 engine dying at idle is a weak battery. 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. And you mentioned that your XJ has been sitting for a while; that isn't good for batteries either unless you have a maintainer on the battery. Did you fully charge the battery before driving again? If not, you need to do so.

Here are my recommendations.

1. Any check engine light? If so, codes must be retrieved and post the exact codes here for comment.

2. LOAD TEST your battery. Any parts store will do this for you for free.

3. Clean the throttle body and idle air sensor. That is another common problem area with these symptoms Here is how...
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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

EZEARL

How many years it been sitting? Wonder what the gas in the tank and fuel system is like?

GaiaXJ

it's been sitting about a year and a half, the gas should be fine, I know the fuel filter was a bit old before I parked it though so that might need to be replaced

GaiaXJ

I replaced the IAC a few months before I parked it, and I've got the battery charging while I'm at work, I'll start it up tonight and see if it's got any improvement

tjwalker

Did you have STABIL or equivalent in the gas before you parked it? Gas can start to deteriorate in a couple of months.

The fuel filter on your XJ is part of the fuel pump assembly, located inside the gas tank. It is not like the older ones where the filter was along the frame rail.

GaiaXJ

My 99 Miata sat for two years before I bought it and the gas was fine, I didn't think gas could deteriorate so quickly.
And the placement if the fuel filter is the only thing that stopped me from doing it a long time ago