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Old 12-23-2012, 01:15 PM   #1
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2500 RPM Idle? Need some help!

'97 TJ, 4.0. Got a sweet deal on a Borla Header. Got every thing on and welded. Now on start up the motor goes to 3000 RPM the down to to 2500 and stays there.

Initial thought was maybe I kinked, tore, or just left a vacuum line unhooked somewhere. Everything looks good. Don't hear any hissing eitheir.

I torqued everything to spec, but the bolts right under the manifold couldn't fit the tq wrench. "Feels" like spec.

There is also no interference with the throttle.

Any Ideas?

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Old 12-23-2012, 06:41 PM   #2
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I guess with so many views; no ideas. I'm gonna go through the hoses again.

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Old 12-23-2012, 06:46 PM   #3
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Your intake manifold is leaking where you bolted it back to the head, which is a common problem after changing headers. The usual problem is not having properly positioned the intake manifold over the small positioning studs on the head. When it leaks, it lets air into the engine the same as if you were stepping down on the gas pedal.

Caution that the Borla header is known for cracking.... unless they added the flexible joints that prevent the cracks like most other aftermarket headers now have.
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Old 12-24-2012, 12:08 PM   #4
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I guess I could have of that. Thanks. No flex joints in this one.
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Old 12-24-2012, 12:38 PM   #5
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Did you use a new gasket?
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Old 12-24-2012, 01:05 PM   #6
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Jerry is likely correct .
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Old 12-24-2012, 01:42 PM   #7
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I've also seen this happen when my battery terminals weren't tight, and when my TPS went out. Just some other things to check... Also clean the throttle body
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Old 12-24-2012, 08:09 PM   #8
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I reseated and tq'd everything. The throttle body is clean and new manifold gasket. Still runs at 2500. When I bought the TJ last Aug it had a CEL and I put a new TPS on it which solved that. A Chevy friend of mine is saying it's the idle air control.
My DD is a same year XJ. I'll swap the TPS and IAC before I spend the $.
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Old 12-24-2012, 08:14 PM   #9
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A dirty IAC can also cause that if it sticks open which will let in too much air. A sticky IAC can usually be restored to proper operation by removing it (two T25 torx screws hold it to the throttle body) and cleaning its solenoid actuated plunger with aerosol throttle body cleaner and a toothbrush. Use more of the cleaner with a baby bottle nipple brush to clean the oriface in the throttle body the IAC's plunger fits into. Rarely does an IAC actually need to be replaced, a good cleaning is usually all it needs.

However, that is very (!) surprising that reseating the intake manifold didn't fix this problem... that is by far the most common cause of a high rpm problem. You're 100% absolutely certain it was reseated & positioned properly? Sometimes it takes 1-2 tries to get it right.

Click on the IAC photo to make it bigger.
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Old 12-24-2012, 11:02 PM   #10
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An IAC can cause this, but the idle was fine before the manifold swap.
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Old 12-24-2012, 11:44 PM   #11
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Quote:
Originally Posted by WaterDR View Post
An IAC can cause this, but the idle was fine before the manifold swap.
My thought exactly... which is why I pushed back on the intake manifold positioning again.
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Old 12-26-2012, 05:29 PM   #12
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Quote:
Originally Posted by WaterDR View Post
An IAC can cause this, but the idle was fine before the manifold swap.
That did cross my mind, but not to sound cocky or anything, there is no way is not seated right now. If the swap dosen't work then I'll reseat it again. I'll swap it tomorrow. I'm hung over from X-mas duty.

I've had the intake/exhaust taken apart on my XJ four (maybe more) different occasions over the past years and never had this problem.
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Old 12-26-2012, 07:19 PM   #13
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One thing to note also. The previous owner rode this rig pretty hard in the mud and clay around here. He put an airaid intake on it and it was filthy also dirt was caked up at the top of the coupling for the intake tube and TB. I degresed the motor, tune up, fluids, checked rust, and got all the dirt out of the frame. when I got it home.
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Old 12-27-2012, 11:18 AM   #14
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I swapped the IAC from the XJ. Now it runs even higher. The XJ runs fine with TJ IAC. I guess I'll be taking everything apart again. Just some other thoughts; Is it possible I damaged the O2 sensor or maybe damaged a wire to the TPS or IAC and its grounded or shorted somewhere?
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Old 12-27-2012, 11:33 AM   #15
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A bad 02 sensor won't cause that high rpm. That high rpm still sounds like a leak between the head and intake manifold.
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Old 12-27-2012, 11:55 AM   #16
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Timing off can make it idle higher or lower so if all else fails check that
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Old 12-27-2012, 11:57 AM   #17
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The TJ's timing is under control of the PCM and is not user-adjustable.
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Old 12-27-2012, 01:16 PM   #18
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Oh. Well sorry man
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Old 12-27-2012, 01:19 PM   #19
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Quote:
Originally Posted by Jerry Bransford
The TJ's timing is under control of the PCM and is not user-adjustable.
That's good to know
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Old 12-27-2012, 01:21 PM   #20
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Crank and cam correlation?
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Old 12-27-2012, 02:02 PM   #21
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The gasket from Borla did not have the "hardend" seats (i guess that's what you call'em). So, maybe I kinked the gasket. Gonna listen to Jerry and try it again.
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Old 12-29-2012, 02:23 PM   #22
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Put a Fel-Pro gasket on it. It works now. Thanks everybody.

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