'96 XJ Won't Idle At All

Woke up this morning to find that my '96 XJ will not idle at all. My wife woke me up to tell me that it wouldn't start, so I got up to check it out. Up until this morning, it has always started on the first crank. Today, it cranked a few times, then started right up. It revs up just fine, but will die as soon as you let off the gas. I can hold it at 1000 RPM with my foot on the gas, and it'll sit there and purr. The moment I let off the pedal, it dies immediately.

It has the 4.0L engine and 170,000 miles. Other than a water pump, radiator, and starter over the 75,000 miles I've put on it, I've never had any problems out of it. It is the most dependable vehicle I've ever owned. Any ideas?

Sounds like one of the sensors in the throttle body. You drive through any really dusty fields lately. Throttle position sensor maybe?

EDIT: Idle Air Control maybe?

check all the vacuum lines and sensor plugs. Make sure the MAP sensor on the firewall is plugged in.

A marginal battery is a primary cause of stalling when you let off the gas. Quite common. Have battery load tested at any parts store for free.

The battery is a red-top Optima and should be in very good shape. This XJ is my daily driver -- I drive it on the highway to work five days a week. It drove home from work yesterday afternoon just fine, and hasn't been acting the least bit funny. I'm thankful to God that it did this in the driveway and not out on the road somewhere.

I checked all of the throttle body sensors and all appear to be plugged in just fine. When I get it started, the check engine light is not on or anything. The check engine light lights up at start-up (like all the other indicator lights do) and then goes off (as it should), so I know that the bulb is good. Between the original owner and myself, the check engine light has NEVER come on while the Jeep is running.

Thanks for the suggestions so far, guys, and I'll keep checking on things later today. I have to have it running right by Monday morning.

Here are a couple of pics to let you guys know what I've got. The header panel is red due to a deer collision two years ago next month.

May very well be something else, but don't get tunnel vision that your battery is not in play here. New(er) batteries fail more frequently than you think. A load test takes about 1 minute....It is the first thing I do when working on an XJ with that symptom.

I pulled the throttle body off, and took the whole assembly to AutoZone. The only sensor on it that they could check was the TPS, and it checked out good. I went ahead and bought a new IAC, and when I went to reassemble everything, the IAC won't fit in my housing. These have a plastic "shaft" that the o-ring slides down on. On mine, that "shaft" is the same size all the way down, and the o-ring slides down against the flange. On the new one, The "shaft" has two different diameters (stepped), and the o-ring just slides down until it meets the fatter part of the "shaft", which is about halfway down. The wider part is too fat to fit into my housing. I called Autozone and I have the right part number on the box, but she said that the part may have been missboxed. They're supposed to order a new one overnight.

BirchlakeXJ, why would a battery suddenly fail to let the Jeep idle, yet still start it and the charging system show okay on the gauge? No doubting inferred, just trying to learn. Thanks.

There are several theories about why a dying battery causes idle problems. The most competent(and complex) idea I've heard is the system is slightly over-charging due to the dying battery. The resulting over-voltage causes all the inputs to the computer from the sensors to have a slightly higher voltage then expected. One of the results is that the IAC is closed down too much to allow proper idle.
Weather or not this is true, I have NO idea.
I do know that my batt. died a couple years ago, and would start and run fine. The only symptoms were a slightly slow crank, and a low idle, which got progressively worse until the jeep would die at idle. Interestingly, my buddy(a dealership mechanic) called the batt. problem 6 months before it started causing serious problems, just listening to it start once.

How old is the red-top? I seem to recall there was a production problem with Optimas when they moved their factory for about 6-10 months a while back.

It is quite common for a marginal battery to be strong enough to "start" the engine, but not strong enough (without tipping in throttle) to keep the engine management system happy. Stalling is the result. I see it all the time. Google search and you'll find many threads.

Again, not saying that is what it is, but am saying that is always the first thing I verify.

Just trying to prevent you from chasing your tail in the event that is the root cause. Simple things first. Always.

Just checked, and the battery is a little older than I thought. I bought it in 10/07, which makes it exactly 4 years old. So much for the 3-year free replacement. LOL!

What strikes me as odd is that there have been no symptoms whatsoever before today. It has ran fine, idled fine, and started right up as soon as the key was turned.

I had an Autozone in another town pull a IAC with the same part number as mine off the shelf over the phone, and it is not like the one I bought today. That confirms that the part was missboxed. Tomorrow when I pick up the replacement, I'll take the battery in and have it checked.

One thing worth mentioning: For quite a long time now, if we have a rain overnight, or if the air is really heavy with mist/fog in the morning, the dash lights, headlights, etc. are really dim upon starting the XJ for the first time. Revving it up a couple of seconds results in the belt squealing for just a split second, then everything brightens up to normal. This never occurs unless there is really a lot of dampness in the air. It rained overnight last night, and has rained sporadically all day today. Driving in such conditions has never had any negative results -- only sitting overnight.

Well, I took the Optima battery to Autozone for testing when I picked up the new IAC. Guess what? The battery is shot. Who would have guessed? The thing starts the Jeep just fine everyday, so that came as a shock. Tomorrow evening I'll be taking the battery back to where I bought it. Optima's website says 3 years free replacement, then 3 years pro rated. I guess we'll see how good their warranty really is. Thanks for all of the help, guys.

Sounds like you may need a new serp belt as well, or at least it needs to be snugged up. It should not squeal at all, that means it is wearing out, worn out, or worn enough to be loose (which means it may be worn out enough to worth replacing).

Be sure to use a volt meter and test the running voltage after the battery swap, to make sure the voltage regulator and alternator are working properly!!!!!

Ecomike said:

Sounds like you may need a new serp belt as well, or at least it needs to be snugged up. It should not squeal at all, that means it is wearing out, worn out, or worn enough to be loose (which means it may be worn out enough to worth replacing).

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It's been on there around 10,000 miles or so. I replaced it when I hit the deer two years ago, and I drive it 100 miles a week.

Ecomike said:

Be sure to use a volt meter and test the running voltage after the battery swap, to make sure the voltage regulator and alternator are working properly!!!!!

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The factory in-dash volt meter has always shown good, even with this "bad" battery. I have a digital volt meter. What would be the best method of testing?

M4Madness said:

It's been on there around 10,000 miles or so. I replaced it when I hit the deer two years ago, and I drive it 100 miles a week.



The factory in-dash volt meter has always shown good, even with this "bad" battery. I have a digital volt meter. What would be the best method of testing?

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Time to snug that belt a little at 10,000 miles, and check for any oils or antifreeze getting on the belt from a leak. New belts need a little snug up usually after about 1 year or 10,000 miles in my experience, unless they were installed too tight to begin with, LOL, then it is usually bearings that need the work instead, LOL.

New battery, look for 12.76 Volts (apx) before starting and about 14.7 V running with no loads. Then with all the loads on, head lights, AC and blower on Max, E-cooling fan, brake lights, etc. all on, needs to hold about 13.5 to 13.7 or more volts minimum.

Ecomike said:

New battery, look for 12.76 Volts (apx) before starting and about 14.7 V running with no loads. Then with all the loads on, head lights, AC and blower on Max, E-cooling fan, brake lights, etc. all on, needs to hold about 13.5 to 13.7 or more volts minimum.

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Measure right on the battery posts?

YES.

Well, the good news is that the XJ is back on the road. The bad news is that I'm not sure what fixed it. LOL!

Tonight, I drove all the way to the next town where I bought the Optima battery four years ago. They hooked it up to some sort of device and told me that it is good, even after the guy at Autozone told me it was bad. I wasted all that gas and time driving up there for nothing. The two years I had left on the warranty wasn't that good anyway. They told me that it is prorated at $2.50/mo, and since they owed me 25 months, that would amount to a $60 savings. But, the battery increased in price $40 since I bought it, so I was still going to have to pay about $127 + tax. Then to add insult to injury, the only warrenty you get on a replacement battery is what warranty you have left on the old one. What a crock! I'm glad I didn't need it after all.

Anyway, I drove all the way home, and put the battery back in the XJ. It fired right up and idled as good as it ever did. I then drove it to AutoZone and had then hook up some sort of tester, which showed that both my alternator and battery were good. So, the problem was either a bad battery connection or the IAC. Either way, I'm not complaining now. LOL!