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Old 10-01-2009, 11:50 AM   #1
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99 Jeep Wrangler 4.0L Rough Idle, Chugs On Acceleration

1999 Jeep Wrangler 4.0L 5 Speed, Runs Rough And Will Pop And Chug Under Acceleration, Under 3000 RPM. Sounds Like Vacuum Leak With Surging RPM At Idle, But There Are No Vacuum Leaks And No DTC's, I Have Also Tested The TPS, And This Problem Is Not Constant, Comes And Goes. I Have Changed The Plugs, Wires, Cap, Rotor, And Crank Sensor, Problem Is Still There. Any Ideas?? Please Help!!

Thank You,
Adam

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Old 10-01-2009, 12:01 PM   #2
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Bad fuel injectors?

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Old 10-01-2009, 12:37 PM   #3
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No guarantees but I would start by a throrough cleaning of the throttle body with an aerosol throttle body cleaner. This is done with the engine running after taking the air intake tube off from the top of the throttle body.

Look at the below drawing and especially note the "Idle Air Control Passage Inlet" slot inside the TB opening. That is where the engine gets ALL of its air at idle so especially try to get some of the TB cleaner spray into that slot which might help. Better yet is to physically remove the Idle Air Control Module (it's only held on by two small torx screws that I think are T-25) and GENTLY clean all the black gunk off its solenoid actuated plunger you'll see once the IAC has been removed. Don't push/pull/rotate that plunger. It's not delicate but don't push/pull/rotate it. Use a small brush like a baby bottle nipple brush together with TB cleaner to clean out the hole in the throttle body that plunger fits into, which is the backside of the Idle Air Control Passage Inlet slot. When this area gets dirty, and it always eventually does, it causes idle problems.

The off-idle problem could be caused by a dirty TPS (throttle position sensor) which is what reports to the engine computer where the throttle position via the accelerator pedal is positioned. A bad TPS can cause all kinds of erratic off-idle problems like rough acceleration. A bad TPS can only be replaced, it is not cleanable like the IAC is.

One last thing that can cause similar problems if they're not caused by a dirty throttle body interior, dirty IAC, or bad (dirty internally) TPS is a bad upstream 02 sensor. Try unplugging it from its wiring harness and see what happens. If the problems go away or improve, the 02 sensor is bad. This is the 02 sensor on or just below the exhaust header. Don't worry about any other 02 sensor(s) mounted on the catalytic converter(s) which are known as downstream 02 sensors.

Good luck and let us know how you do on this problem.
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Old 10-01-2009, 01:00 PM   #4
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Jerry,
Would you reccomend just taking off the throttle body, all the sensors and thoroughly cleaning it completely ?
Mine does the same thing.
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Old 10-01-2009, 01:35 PM   #5
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Quote:
Originally Posted by SPDNJOE View Post
Would you reccomend just taking off the throttle body, all the sensors and thoroughly cleaning it completely ?
No, I wouldn't bother with all that. In fact, cleaning the throttle body with TB cleaner with the engine running also cleans the interior of the intake manifold and maybe even does a little good with the intake valve area too.
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Old 10-01-2009, 03:41 PM   #6
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thanks Jerry
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Old 10-01-2009, 05:24 PM   #7
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Jerry Thank You For All Of Your Help, I Am an Apprentice Mechanic And This Has Stumped Me Good, I Have Cleaned The Throttle Body While Running But I Have Not Checked/Cleaned Any Sensors Yet, But I Will Do That Tonight And Get Back To You, Thanks Again.
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Old 10-01-2009, 05:52 PM   #8
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Throttle body cleaning

Jerry is probably right. A good throttle body cleaning is in order. Chemtool makes a kit that has a can of spray throttle body cleaner, a can of cleaner to hook up to a vaccum line (I always use the line out of the brake master cylinder), and all the hoses needed. Takes about 30 minutes, costs about $30.00 at O'Reilly's, is very simple to use and does an outstanding job.
Good Luck
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Old 10-01-2009, 06:29 PM   #9
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Jerry You Are A Life Saver, I Didn't Even Have To Go As Far As Cleaning Sensor's, I Un-Plugged The Upstream O2 And It Cleared Right Up, Went For A Drive And All Was Good, So Im Pretty Sure That Was It.

Thank's Again,
Adam
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Old 10-01-2009, 06:54 PM   #10
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Jerry, thanks for posting that excellent, succinct, and complete description and possible solution. That is invaluable to us not-even-weekend warriors.
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Old 10-01-2009, 07:10 PM   #11
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You're welcome guys, we all learn cool stuff every day here and I've certainly gotten more than my share of good solid answers out of this forum. as well
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Old 11-13-2009, 08:08 AM   #12
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I have been searching through posts because I have been having similar problems. I have cleaned the TB wile the engine was off but how would you do it with the engine running? I'm guessing pull of the intake, start the jeep and spray away? can anyone who has cleaned it with the engine running give me some details?

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