Hey guys,
First time poster but long time reader.
I've got a 99 cherokee xj 4.0, pretty much stock for this moment.
My problem is my xj will start, idle fine and stay started... untill you give it gas, which sounds like the norm question here.
The engine will sluggishly rev then die out unless my foot is on the gas pedal.
I've checked all my connectors and the such, but to no avail.
Heres the part i don't understand, when i take off the hose to the brake booster to the intake manifold, it runs fine. No stalling,no hesitation, (no brakes) and the idle is just above normal.
I've looked thru post after post for this and nothing includes the brake/intake line.
It ran great the night before. It just happened after sitting overnight.

Any help would be greatly appreciated guys! Thanks in advance!

 

Check your idle air control valve. I bet once it heats up, it is sticking and not letting air into your intake. When you take off the line to your brake booster, it is allowing air into the manifold that your idle air control valve normally let's in. By doing this your are letting in dirty air and that is never good to do! That is where you should start, take it off and clean it really well.

You should also check your TPS as I believe it tells your IAC to open or close but not positive on that.

 

This is considered normal maintenance:

Courtesy of TJWalker:
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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 retracted, the air control passage flows more air which raises the idle speed. When the valve plunger is extended, 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. This should be part of a normal tune-up procedure and whenever idle/stalling issues are present.

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). On 91 and later, it may be easier to just remove the whole throttle body. Be sure to use a new throttle body to manifold gasket when reinstalling.

“Gently” wiggle out the IAC from the throttle body. Gasket/O-ring 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 oxygen sensors. 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 bore itself, the butterfly valve inside of the throttle body and it’s edges, and all associated linkage as long as you have things disassembled

Revised 10-6-2014

 

hey guys, thanks for the input.
i took apart the throttle body today and cleaned it, the iac and the map sensor.
after i put it all back together, it still had the same issue. i went and got a new iac, installed it, and it ran the same.
i also checked the pressure in the fuel line, which was fine, my plugs, my wires, the rotor, cap, and even put on a new coil that i have(gonna upgrade the wiring and stuff). it all ran the same.
does it now come down to the ecm?

 

Not likely.