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I've had this a problem a lot with this vehicle. I have a 99 Jeep Wrangler TJ. I believe the car's computer has a battery that is a dead (like a watch battery). For some reason when it stalls or when it needs a jump it forgets how to keep itself running. I then have to manually control the amount of gas going to the car to keep it idling for sometimes 15 minutes. Then if I turn it off before I let it stall, I can start it back up and it'll work fine.

The process that best works is starting up with heavy gas, and slowly taking the gas idle. Then restarting the car and it holds out fine.

Is there a faster way to do this? What's happening? How do I fix it?

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  • Cars don't have a "watch battery" they only use main battery.
    – vini_i
    Commented Dec 21, 2015 at 1:48
  • Need a bit more clarity here. What do you mean by "manually run the car"? When the car struggles to start after stalling is the engine cold or hot? Or does it happen regardless of engine temperature?
    – Zaid
    Commented Dec 21, 2015 at 1:58
  • @Zaid updated., engine is cold but it happens regardless.
    – Evan Carroll
    Commented Dec 21, 2015 at 2:12

2 Answers 2

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The engine computer looses is idle speed learning whenever it drops below nine volts even for a short time. The problem here is the starting battery or its connections. Hook up a voltmeter across the battery terminals then crank the engine for 15 seconds. The voltage should remain above 9.6 volts. If the voltage drops below nine the computer will loose the idle speed learn. Your "run it until it idles" process is just fine. The problem should clear up when the battery problem gets repaired. It may need a new battery or battery terminal repair.

The computer does not have any internal battery for memory keeping.

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  • So it's normal to lose the "idle speed learning"? They can't store that in some place not volatile? What a horrible engineering job.
    – Evan Carroll
    Commented Dec 21, 2015 at 4:56
  • It is standard in automotive engineering to not have a backup battery. It is not an engineering decision. Every one unit of cost raises the retail price many times. Extra weight increases fuel consumption. It is a management decision.
    – Fred Wilson
    Commented Dec 21, 2015 at 5:15
  • We're talking a watch battery, or an nvram chip. The radio in the car has one.
    – Evan Carroll
    Commented Dec 21, 2015 at 5:34
  • Agreed, either would work.
    – Fred Wilson
    Commented Dec 21, 2015 at 6:56
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    Another consideration is, when the computer loses power and resets, this is a way to get the vehicle back to factory standard for the fuel trims. If there was a battery in there, the process would be much harder and would not be able to be done by "the average Joe". As stated, there are tradeoffs in all engineering feats. This is no exception.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 21, 2015 at 11:45
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Sounds like either a dirty throttle body, bad idle speed motor, or bad coolant temperature sensor.

I would start with cleaning the throttle body. If there is gunk in there, the AIS may not be able to let enough air by to raise the idle.

The AIS automatically set the idle speed based on several factors, mainly engine temperature. The cold idle should be around 1500-2000 RPM. If it is lower and tries to idle around 900 or lower, then the AIS is likely to blame.

The low idle could also be due to the coolant temperature sensor telling the ECU that the engine is warm when it is actually cold. When the engine is cold, the ECU raises the idle, richens the fuel mixture and retards the timing. You can test the CTS with an ohm meter. You should be able to find the values in a repair manual or online. Be aware you might have 2 coolant sensors, one for the ECU and one for the gauge.

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  • No jeep wrangle idles at 1500 rpm...
    – Evan Carroll
    Commented Dec 21, 2015 at 18:15
  • Your TJ shouldn't be idling at 1500 rpm. It should be idling around 1000 at start, then drop to around 650 rpm when it's warmed up, assuming you're talking about the 4.0L inline 6. I don't have any data on the 4 cylinder, but 1500 still seems too high.
    – Paul Dufresne
    Commented Apr 16, 2016 at 4:07

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