2001 xj stalling at idle

2001 XJ, 4.0, auto. Power almost everything. Right up to about a week ago it was running fine. Now after it warms up, and occasionally from a cold start, it stalls out when you come to a stop. Everything is fine when you give it gas and cruising along. If you put it in park it usually will stay running but idles a little low and slightly erratic. The voltage gauge on the dash will bounce towards 9 volts on the low end of idling and read towards 14 on the high end. She starts strong even on the 20 degree mornings. Runs fine while driving, and accelerating. Any suggestions? We just bought it about 7 or 8 months ago with about 109,000 on it now, so we don't know the history.

90NHXJ said:

The voltage gauge on the dash will bounce towards 9 volts on the low end of idling and read towards 14 on the high end.

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Does this happen mostly with electrical accessories (don't forget the heater, defroster, and wipers) running?

I'm thinking you've got either a shot alternator / voltage regulator, battery, or both; the swinging volt gauge is what's making me suspect these items first. Check that the serpentine belt has proper tension and, if it does, have both the alternator and battery tested. Replace as necessary.

There are some other possibilities involving certain sensors, but this doesn't fit the usual pattern of failure behaviour for them that I'm aware of.

I thought about the alternator and battery but everything seems to be charging fine. The jeep starts strong. No problems. I would think (with past experience of driving with a blown alternator until the battery died too) that it would run ok off the battery at idle until you step on the gas again but that's limited past experience telling me that. It's due for inspection so the garage will be called tommorow to check that as well. I was hoping to figure it out before then. Like I said it runs fine while driving down the road (foot on gas and coasting down hills) but coming to almost a complete stop and she wants to shut down (low, erratic idle). Pop it in neutral and put slight pressure on the gas pedal and it stays running. Pop it in drive and go has been the trick. The dealership said they've been selling a bit of throttle position sensors for those. I can start it in the morning to warm it up and it is fine. Drive it to down the road and come to a stop and it acts up for the rest of the drive.

90NHXJ said:

I thought about the alternator and battery but everything seems to be charging fine.

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Understood; it's just that the wavering gauge has me a bit concerned. Of course, it's always possible that the gauge isn't reading correctly, but given the age and mileage of your Jeep I'd be surprised if it weren't.

Oh, stupid left-field idea: how's the condition of your battery terminals (including tightness) and grounding straps? I had a car years ago that displayed some similar behaviour and it turned out to be a rotten ground connection.

Like I said it runs fine while driving down the road (foot on gas and coasting down hills) but coming to almost a complete stop and she wants to shut down (low, erratic idle). Pop it in neutral and put slight pressure on the gas pedal and it stays running. Pop it in drive and go has been the trick.

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Something to try: pull the Idle Air Sensor and clean it. Looks like a spark plug set into the top of the intake manifold; should be on the right-hand side as you look at it from the top over the driver's side fender. Get some good penetrating oil (not WD-40 or PB Blaster; I tend to use gun oil) into it: keep spraying it in, letting it settle, then pouring it out until it's running out cleanly. DO NOT pull on the little plunger inside of it. Reinstall and see if the problem recurs.

You may also want to grab a can of SeaFoam at the same time and hit the intake and throttle body; you'd be surprised at what a good intake cleaning on these things can do for the quality of their running. More:

The dealership said they've been selling a bit of throttle position sensors for those. I can start it in the morning to warm it up and it is fine. Drive it to down the road and come to a stop and it acts up for the rest of the drive.

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I've blown two TPSes in water crossings, but each time the physical manifestation of the problem has felt more like a blown transmission - late upshifts and refusal to downshift. Granted, that's only my circumstances and 104,000 miles seems low for a TPS to fail, but it's possible.