• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

4.0L Idle Issue

xjklmnop69

NAXJA Forum User
Location
MI
Yello mate'

I have a 99' XJ with about 267k miles. 4.0L AW4 -no check engine codes

Just recently started having a cold start problem. I have to crack the throttle open just the slightest bit to get her to start. It'll crank and fire but will not idle unless I give her a little bit of throttle on cold starts (10-40 deg F outside air). Sometimes on warm starts with the same outside temps but much more rarely.

To me it seems like an Idle air control valve issue. I put a new one in but got the same symptoms. For the most part she has always started with the cracked throttle method.

Any ideas what is wrong with my girl??
 
Alternator is putting out mid 14 volts and fuel pressure is about 50 psi at idle.; Once it finally will idle without the throttle input.

New idle air control valve came from the zone. AC327. If I unplug the iacv while its running it throws the P505- Idle air control valve mulfuntion. Plug the TPS while running, she dies and throws TPS faults.

Could the MAP be going bad? It runs good at part throttle and WOT. She even even spin her 31's with 3.08s

This one has me stumped!:banghead:
 
I'm guessing it doesn't like the AZ iac.

Is the throttle body clean?

Map isn't a bad bet, it will cause poor idle and hard starts, but you've got no CEL. Vacuum leaks can do this to. Run the gauntlet of small tests for these parts and you'll find your culprit. dont go throw money into the map unless you've got an easy jy part or a friends to try first. As many say the jeeps like Mopar sensors, I would agree after all those I've tried.
 
Last edited:
Just recently started having a cold start problem. I have to crack the throttle open just the slightest bit to get her to start. It'll crank and fire but will not idle unless I give her a little bit of throttle on cold starts …

These are typical symptoms of low battery voltage rebooting the PCM and deleting the idle settings. I would suspect that one cell in the battery has failed.


For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging. Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem.

A faulty Coolant Temperature Sensor may also cause similar symptoms. Testing of the CTS is recommended as part of the diagnostics. A poor ground at the ignition coil can also cause similar symptoms.

The reboot symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will usually relearn the idle settings after a short period of driving and the engine will start and idle normally.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

If the symptoms reoccur, you need to have your battery and/or the alternator Load Tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Handheld testers are very inaccurate and will usually pass marginal alternators. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance for smooth starts.
 
TPS and tightening the serp belt fixed her. Replaced TPS with a newly used unit

I wanted to drive it around for a few weeks to confirm
 
Back
Top