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Old 12-26-2008, 01:23 PM   #1
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'00 TJ stalls after idling at lights/during warmup

Hi Guys,

Newbie here with an 00 TJ 4.0 auto, 96K miles. A week ago, after waiting in traffic maybe 1 minute, it stumbled off idle, almost stalled. Few minute later it did stall but restarted right away. Couple of days later, warming up in the driveway (20F) started fine but stalled after a minute or so. Restared, idle fine, stalled again after about a minute. restarted and drove OK a minute or so, waited at a light, stalled after starting off from the light. restarted hard this time, coughing and sputtering, but started and drove ok for about five miles, then stalled again. Turned around an headed home hoping not to get stranded. Stalled once more but made it home. Now on cold start, all ok, but stalls after a minute or so, sometime restarts sometimes not. Had it towed to a trusted mechanic. He reports that he has tested the crank position sensor, fuel pressure, and probably a few other items I do not remember from the phone converstation. He can reproduce the problem easily and said that he can keep it running spraying starter fluid in the intake so does not believe it is ignition. He said that the signals from the ECM to the fuel injectors goes away at the time of failure. He is fairly certain it is the ECM, so I authorized the replacement, $500 for a remanufactured unit. Anyone had a similar experience that was resolved with a new ECM?

Unrelated, windshield wipers stop working in cold (20-30F) conditions, bring it in the garage to debug and they start working again. Could a bad motor act this way? Is there a relay somewhere? Took the cowl off, peeled back the rubber sock from the motor, moisture underneath and lots of rust on the motor shell. Cleaned up, recovered, same issue.

I've had this jeep for 3 weeks. Are these things reliable or am I in for some more misery?

Discouraged in mass,
Dave

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Old 12-26-2008, 04:19 PM   #2
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Don't spend the $$$ on a new ECM (yet). Odds are it's nothing more than a dirty IAC (idle air controller) which is what controls/passes all of the engine's air when at idle and your foot is not on the accelerator pedal. They get dirty and cause exactly those symptoms you are having.

You can first try cleaning the interior of the throttle body with a good aerosol throttle body or carburetor cleaner, concentrating the spray at the IAC Passage Inlet per the below illustration. That inlet is controlled by the IAC's solenoid actuated plunger that opens/closes the air hole (passage inlet) under control of the computer when at idle. The spray may get inside enough to clean it and cure the idle problem.

If not, it's only a few more minutes to remove the IAC controller (it's held on with two smal torx bolts) and clean the plunger and the hole it fits into. Be gentle with the plunger, don't push/pull/twist it.

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Old 12-26-2008, 10:17 PM   #3
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"""""He can reproduce the problem easily and said that he can keep it running spraying starter fluid in the intake"""""

If the IAC is sticking wide open, or at least open too far it would be lean when cold, before closed loop. If it was that lean, when he added starter fluid (fuel) wouldn't it rev like crazy? Stopping at stop signs it should rev up too, once it's in closed loop.

Sounds to me like it's getting the proper amount of air, just not enough fuel - sometimes.) If it was too rich the added fuel would kill it.

My suspicion - he said he recently bought it - the PO or car lot steamed or pressure washed it to make it "pretty." Water got in the TPS and now the TPS is "scratchy." (They aren't sealed around the shaft.)

It may even got inside the PCM plug (why did they put it in such a stupid place? Easy to get to, but easy to get water and dirt in too.)

TPS - CAREFULLY remove it, blow it out and spray with a good electrical cleaner (not WD-40 - it ATTRACTS WATER.)

Same with the PCM plugs. Remove, clean, replace.

Hopefully 8 minutes of time saves that $500.
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Old 12-27-2008, 01:10 AM   #4
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Rich, the engine is running when he sprays starter fluid into the throttle body because he also must open the butterfly valve which is letting air in the engine at the same time. That he's spraying starter fluid in and it's running is coincidental. It's running because he's opening the butterfly valve to let air in, which the IAC isn't... at least in my opinion.
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Old 12-27-2008, 05:43 AM   #5
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Well, that could be also. We really don't know how or what he was doing to duplicate the problem, or under what circumstances he was using the starting fluid. Holding it open or letting it alone.

Plus he said: """""He said that the signals from the ECM to the fuel injectors goes away at the time of failure."""""
We don't know how he determined that either - a scope? A Noid? Listening for the click? Digital voltmeter? Physical observation of no fuel? Or a guess?

That's what makes it really tough to diagnose with very little to go on over the Internet.

I didn't mean to infer it "can't be" the IAC or passage, but it wouldn't be high on my list. It can't hurt cleaning the passage anyway, and only takes a moment.

Actual PCM failures aren't common at all, but they get replaced often. Unfortunately you can't get PCM's on a "try this and see" basis. It's not just a plug and play unit either, it takes a bit of programming the anti-theft portion, incurring expenses just trying one. Once ya plunks yer money down, you own it, right or wrong.

Unfortunately since today's vehicles are so "computerized" we have a tendency to blame the oft misunderstood computer system and forget the basics - compression, air, fuel, and spark.
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Old 12-29-2008, 05:47 PM   #6
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crank angle sensor after all...

Guys, thanks for the help. My mechanic was uncomfortable with the ECM diagnosis so kept poking around today. While restarting, the OBD reported a code that sent him back down the crank angle sensor path. He said there are 2 tests for the sensor, one passed one failed. Replaced the sensor and reports all is well. (and a whole lot cheaper, too) Now I get my Jeep back sooner, just before the next snow!

BTW, after reading Jerry's post I did a little research on the IAC. I had noticed the idle hunts a bit - so as soon as I get it back I'll clean that up per Jerry's suggestions.

Thanks again. In this economy I can no longer afford to have this sort of work done for me. I appreciate you folks that take the time to share your experiences.

Dave

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