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Unread 08-24-2013, 07:59 PM   #1
bergoff
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Join Date: Nov 2011
Location: freeport, il
Posts: 16
4.0L runs good for 30sec then rough

I have a 99 4.0L with 96k. All of the sudden it has been acting strange. It will start and idle good for 30sec. Then it doesn't want to rev and will backfire through the tb and sputter but won't die just idle rough and won't rev. It will do this for about 1min then it's fine for the rest of the trip. If it sits not running for longer than 30mins it will do it all over again. Sometimes the revs will come down real slow. And also there is no check engine light or codes.

Today I pulled the tb and cleaned the iac and installed a new tps. When I was in the driveway it was idling rough and I could hear the tb sucking air like crazy. And if I remember correctly my Ram with a bad plenum gasket did the same thing. So do I have a vacuum leak or is it something else? I don't know if its connected but my ac will switch on its own from vents to defrost and then back to vents. Kind of weird and one of my WJ's quirks.


Last edited by bergoff; 08-24-2013 at 08:28 PM..
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Unread 10-27-2013, 07:55 PM   #2
str8six
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1997 TJ Wrangler 
 
Join Date: Nov 2008
Location: Florence, KY
Posts: 12
I am having the same issue with my 2000 Gr Cherokee 4.0. It will start up and run perfectly, once it warms up a little and sometimes it happens once it warms up all the way, but it happens everytime I drive it. It bucks and backfires, and misses. It then stops all together after giving it some gas, and then its fine the rest of the time until it gets cold again and I start it and drive it again.

Im just starting to diagnose, was wondering if you came up with anything, or if anyone knows where to start. I'm thinking intake leak. Faulty fuel pump. Faulty fuel relay.
It has new plugs, new air filter, it has 177,000 miles on it. It's definatley weird, that it only does it once it's warm, and only once at idle , but then at idle for the rest of the time it's running, no issue.
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Unread 10-28-2013, 10:53 AM   #3
rockatansky666
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Join Date: Jun 2013
Posts: 154
Bad TPS can cause running issues…just a thought. Easy to test with a multi metre
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Unread 10-28-2013, 01:19 PM   #4
str8six
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1997 TJ Wrangler 
 
Join Date: Nov 2008
Location: Florence, KY
Posts: 12
I fixed the issue. It was the front O2 sensor. It had a faulty heater circuit. It wouldn't heat up as fast as the car or something like that, so as soon as the jeep got to operating temp, the front 02 sensor gave the computer the wrong reading, causing it to run very lean. But new O2 sensor and all is good.
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