jchat

Jeep will start up and drive fine. However when I stop and put it in park and let off the brake I can feel the Jeep trying to die and the tach drops down to almost nothing. Sometimes it will catch itself and idle back up but other times it will just die. I can tap on the gas if I catch it and it will idle out fine. What is making it do this? Also while driving if I accelerate my ac changes from the vents to defrost then back when I let off of the gas. Where would that vacuum leak be? O2 sensors are fairly new, new exhaust from the header back including cat and muffler. Header is not cracked or leaking that I can tell. Any help would be great. Thanks everyone

cruiser54

First:

How long since a full tune-up with plugs, wires, cap and rotor?

Ever cleaned the throttle body?

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



Second:

You have a leak in your vacuum storage system causing the air to go out the defroster vents under load.

jchat

Firstly, thank you for your response. It has been a little over a year since a full tune up and throttle body cleaning hasn't been that long ago. The IAC was replaced a little under 2 years ago. In my 96, I don't have the vacuum ball. I have followed the vacuum lines the best I could and I haven't found a leak (have tried spraying starting fluid to find it) with no avail. Any other ideas, or am I missing something? Thanks again.


edit: Where the picture you posted that says "Vacuum supply line" that is where my line ends.

cruiser54

You're missing the vacuum ball............

Turbo X_J

Junkyard or

&

jchat

That's the issue though, I've never had that vacuum ball since I bought this Jeep back in 2011. I have never had this issue up until the last couple of weeks.

Turbo X_J

Lets approach this differently:
There is probably no "vacuum leak" per se. When you accelerate the engine (ported) vacuum drops and without a check valve & source of vacuum (stored in the ball) the door motor switches to its normal (defrost) position. When you let off the gas the engine vacuum once again builds and is applied to the motor causing the switch back to the vent position. So without the ball and check valve in place, the system could have never functioned normally.