The old 'it won't idle' problem

Hi there, frequent reader and new member here with a question (a cry for help, really). My wife and I just moved back to the U.S. from Mexico. We lived in a small village north of Cabo for five years and covered almost every square mile of southern Baja in our '99 XJ. It's gone off cliffs and into ditches many times and once (almost) washed out to sea but it never stopped running, bless its' little heart. Ironically, as soon as we made it back to Arizona it started acting up. Here's what's going on:

1. Most days it will turn over strong and catch initially but will not idle. Only after staying on the accelerator, keeping the revs up, and getting under way will the idle kick in to the point where you can let go of the gas without stalling, normally by the time you get to the first stop sign.

2. Subsequent starts will usually be trouble free but occasionally the engine will struggle to turn over, at least initially, like the battery is depleted. It will always catch eventually, and sometimes it will idle fine while other times it won't, struggling again as described above.

Thinking (well, guessing) that I'm dealing with an electric or electronic problem, but without the benefit of having actual knowledge or expertise, here are the steps I have stumbled through so far:

1. I checked all the wires as best I could but in most places the wiring harness is covered in a black sleeve making a visual inspection difficult. I didn't test any individual wires but from what I can see there are no obvious issues with the wiring.

2. I did whip out the multi meter to check out the battery and the alternator. The battery is brand new and showing 12.4 Volts at rest. With the engine idling and no electrical load, the alternator is putting out 13.8 Volts to the battery. So I'm thinking (perhaps erroneously) that the issue is not with the battery or the alternator.

3. I replaced the Idle Air Control valve (I cleaned up the old one but decided to just go ahead and replace it anyway. I mean it's, like, twelve bucks). No improvement.

4. I was going to replace the Throttle Position Sensor next but holy crap that thing is on tight. So far I've twisted two Torxs bits on it and I'm no closer to getting it off. I will keep on trying though.

After that, I'm running out of ideas. Can the problem be in the PCM? I mean, it's like it keeps forgetting what the idle position is. I know this problem (or problems much like it) have been covered before on this forum -in fact, that's what has guided my actions to this point, but I haven't had any success. Provided I somehow get the TPS off and replaced, and it doesn't solve the problem, how should I proceed?

Thanks for any and all input you might have!

Have you tested the fuel pump pressure and determined if the check valve 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 relearn the idle settings after a short period of driving and the engine will sometimes 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 PCM, the ground wires at the coil, and the ground wires at the oil dipstick tube. You must remove, wire brush, 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.

Definitely check the battery cables. With the slow crank symptom, it sounds like you're dropping battery voltage somewhere and freaking the pcm out.

Mine would do the same thing with a dead battery, when itd get jumped it wouldn't idle for a few minutes. figured that out when someone at work thought itd be cute to turn on my light bar after I went in. but like everyone said, double check all wires, connections etc. look at the wire for your battery cables to see if they corroded. Mine were and it gave me some issues.

When you replaced the IAC did you clean the sludge out of the seat inside the TB? I'd be a little skeptical of a $12 IAC, if it turns out to be something else, swap the original back in and keep the new one as a spare.

Dumb question, but what does your air cleaner look like? And/or is your Cat making noises like a tin can full of pebbles. I've done a lot of driving in the south west. Cats are fragile, dust gets into everything.

If you can get the battery load tested, the only way to really know for sure. And any sort of battery issues often messes with the IAC first. Often the first sign of battery or charging issues on mine is the IAC gets sluggish.

Not long ago, on another thread, a guy had the main cable going to the back of the alternator cook off on him. Volts only tell part of the story, you can get good voltage through a single strand of wire, you can't get many amps. Worth a look if it isn't any of the normal stuff.

Had another thought, oil fouled plugs will not fire right when cold, but after awhile when the oil crusts up or burns off they will, somewhat.

Hey thanks everybody for your replies! I've been at work for the past couple of days and didn't have time to reply -nor did I have time to work on the Jeep again.

Tim_MN, based on your description I have a PCM reboot issue, for sure. I will go over all the wires and clean or replace as needed. I'll take it for a load test, too.

8Mud, while the IAC was all sludgy and black, the TB was (strangely) clean when I checked it. The old thing was in the shop recently and I suspect that the guys there took pity on the TB and cleaned it out. The air cleaner, too, is squeaky clean and new.

As far as the catalytic converter goes, there is indeed a tin can full of pebbles under the car when I drive and the cat seems to be original to the vehicle (and thus 17 years old). But when I shake it/tap on it, it doesn't sound like there's anything loose in there. I will probably have it tested as well, though, to make sure. I kind of thought that the rattling sound was coming from the transfer case but it totally could be the cat.