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Old 02-14-2013, 12:14 PM   #1
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Wrangler cuts out for a bit then fine?

I've got a 99 Sahara with 143k. When I purchased it the idle would go up and down (pretty dramaticly) until warm. I replaced the cap, rotor, wires, plugs, tps and idle speed thingy- also cleaned the throttle body.

Now it runs great, EXCEPT sometimes for maybe 15-30 seconds it will act as though it's running out of gas. I can put it in neutral and the motor will rev fine, but in gear it bucks. This most often happens if I turn the motor off, then restart- after about 30 seconds this will happen and once it stops (again, after 15-30 seconds) you can drive for 8hrs, no issues.

I am planning to replace the map sensor and the O2 sensors. I'm running a quart of ATF now as well as Seafoam.

What might cause this?? Thanks in advance!

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Old 02-14-2013, 12:25 PM   #2
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I would first drain that ATF out of the engine. ATF can only be run as a strong detergent in the engine oil for a short period of time & it needs to be drained right afterward.

The first thing I'd replace is the upstream 02 sensor, the one mounted on the exhaust header down pipe. That is the only 02 sensor on your '99 TJ that has anything to do with engine performance/air-fuel checking. The other 02 sensor, mounted on the catalytic converter, only verifies for the engine computer that the cat is working properly.

And did you clean your throttle body thoroughly enough to have also cleaned the IAC (idle air controller) well enough? The IAC has a lot of effect on idle RPMs since it is responsible for providing all (!) of the engine air at idle RPMs. When the IAC is dirty, it can hang and cause erratic engine RPMs.

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Old 02-14-2013, 01:10 PM   #3
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Thank you. Yes, ATF only for about 50 miles, then it will be drained.

I replaced the IAC, so it's brand new. The problem with the idle going up & down when the Jeep was cold was resolved.
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Old 02-18-2013, 04:13 PM   #4
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Anyone experience this? Replaced the front O2 sensor waiting for the new MAP sensor...
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Old 02-21-2013, 08:24 AM   #5
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Well, the MAP sensor, O2 sensors fixed this issue.
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