Engine dies when letting off throttle

If i dont baby the gas when I let off it stalls out on me... seems to get worse when its warm. Is it my IAC being crappy?

The battery is always suspect for dying. Have it load tested. Voltage test isn't conclusive here. You can have a battery that is strong enough to start the engine but give you the symptoms you describe.

Idle air control can be cleaned and I'd try that once you verify battery condition.

I'd clean the IAC.

birchlakeXJ said:

The battery is always suspect for dying.

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It has a sexy optima, I'll clean the IAC thats what I figured thanks guys.

Cleaning the IAC is a good place to start, along with the rest of the throttle body.

IAC is most probable suspect although it's out of the loop above ~45 mph so if you have the same problem at highway speeds I'd probably suspect something else

It's only when I'm going slow mainly when I'm crawling I have to hold the brakes and the throttle

not sure if I just missed year and engine but the IAC, like above is a good place to start. If its later than a 91 don't forget about heatsoak. And If before a 91 on a Renix, check fuel pressure, the computer iirc cuts the fuel at the injectors on decceleration if above 1200 rpms

The IAC is mounted on the back of the throttle body. 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.

CLEANING THE IAC

1. Remove the IAC with a torx 15 driver (2 bolts)
2. Gently wiggle out the IAC from the throttle body
3. Clean the IAC with throttle body cleaner (not carburetor cleaner ) Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle as it is fragile and you can damage it pretty easily
4. Also clean where the IAC seats in the throttle body with the same throttle body cleaner
5. Consider cleaning the entire throttle body itself as long as you’re at it
6. Reinstall IAC and check idle quality

I have the same problem with a 100% cleaned throttle body and a new IAC.

It's most prominent when cold, too.

Basically if it's idling and I rev it up to 2200+ for a bit, then let off, it'll drop to sub-500 RPM and stumble and sometimes die. Also if I let the clutch out and let the RPM's drop below idle, it stumbles and hicoughs like crazy. Hm.

I'm working on a friend's zj and his battery is the culprit for the same symptoms. Batteries do produce less amps when it's colder. . .

Step back from the problem a bit. The IAC is a computer-controlled servo to manage airflow into the engine at low-speed idle when the throttle plates are normally closed. If the IAC is not jammed up then the computer is either getting bad info or it is purposefully trying to compensate for some other problem. It could be something simple like bad TPS that is not telling the computer when the throttle plates are closed (most likely) or it could be something like O2 sensor (not likely at cold start).

I have a simalar problem with my Renix 88XJ except that it seems to die with the A/C on.

The XJ run great. Idles fine, etc. But when I have the A/C on and I come to a stop light, put in the clutch, the engine will idle down to the point where it dies. This doesnt happen everytime I get to a stop but enough to be a PIA.

Ideas? The XJ has 272k on it

ehall said:

Step back from the problem a bit. The IAC is a computer-controlled servo to manage airflow into the engine at low-speed idle when the throttle plates are normally closed. If the IAC is not jammed up then the computer is either getting bad info or it is purposefully trying to compensate for some other problem. It could be something simple like bad TPS that is not telling the computer when the throttle plates are closed (most likely) or it could be something like O2 sensor (not likely at cold start).

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O2 was replaced when I smogged it (california) I'll order a TPS

MpSlayer said:

O2 was replaced when I smogged it (california) I'll order a TPS

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Consider testing it. A multimeter is only about $10 and you need to learn how to use one anyway. You probably have a 10% chance the TPS is bad and about a 95% chance you could figure out what is really wrong with a meter. And usually the fix is a wiring connection or ground so that costs you $0.

I have a Multimeter... hmm I read the guide to testing on lunghd.com.
Is there a way to test it when its off the motor?
My jeep is in California. If I can just drop in the TB I have with me when I go to pick it up for Moab it would be perfect.

The TPS is not the only way the computer knows when the throttle is closed.

Think of the TPS as an accelerator pump on a carb - it's useful for injecting a quick amount of fuel when you step on the gas so the engine doesn't run super lean for a split second and cause a backfire.

The sensor that really lets the computer know how much air is flowing is the MAP or Manifold Absolute Pressure sensor. It's located on the firewall right behind the valve cover. I think this might be a culprit. That, or a intake manifold vacuum leak.

MpSlayer said:

It's only when I'm going slow mainly when I'm crawling I have to hold the brakes and the throttle

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How slow? Is engine near idle at the same time, or not?

I'd say pretty close to idle. Its been a while since shes under the knife.

Hard to test a remote jeep several states away. I agree with the TPS and MAP sensor culprit suggestions. From what I have read here, it does not (yet) sound like the IAC or the O2 sensor in this case. If I had to buy a part and make a guess I would say the TPS is the highest probability on this so far, especially if it is not new. But a loose, noisy ground on the TPS could act the same way.

Unless there are other symptoms of problems of any kind, TPS is bad, or ground wire is funky on the ECU side of the tps, or both.

Could the TC be locked up, and not unlocking the way you drive it, or unlocking to late?