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TJ Wrangler 4.0L Intermittent Fast Idle

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Hey, Moses...I had another high-idle episode a week ago, so there may be an intermittent vacuum leak somewhere. I have removed & reinstalled the intake and exhaust manifolds about a dozen times over the years and am very familiar with that large gasket surface area. For years I have used a Remflex carbon gasket https://www.amazon.com/gp/product/B00IQHL6KC because I read (and observed) that it compresses much more than the usual blue Fel-pro gaskets, allowing it to better handle slight variations in the intake and exhaust surfaces. Do you have any experience with that gasket? Maybe it is unreliable and is the source of my intermittent leaks?

I took my front window-uppers off, and I definitely hear what sounds to be an exhaust leak. I had carefully inspected the exhaust system about 2 years ago for leaks (using the reverse-shop-vac & soapy water spray bottle method). Maybe I missed something. Your mention using a smoke generator, which has me thinking that would be the most reliable method to find leaks. I think I'll look into that.


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SomeBuckaroo…If the high idle is intermittent and not continuous, the intake gasket is an unlikely culprit.  Depending on the leak’s position, however, this could be a source of extra oxygen in the O2 area.  That would throw off the fuel trim.  Follow up on the exhaust leak, regardless of whether it cures the idle speed issue.  CO is not a gas to have in the cabin.

I looked at the Remflex gasket, it’s similar to the gasket Borla furnished with the header on my 4.0L.  The thick flanged Borla header has pluses and minuses.  It’s very heavy and stiff.  I’ve retorqued the manifold nuts and bolts at least a half dozen times since installing the header seventeen years ago.

SomeBuckaroo...As for actual causes of the idle flare-up, a vacuum leak is a possibility.  I had a vacuum leak on the XJ Cherokee that may be worthy of your attention.  The vacuum hose to the bulb reservoir ran past the battery.  Over time, the hose corroded and began leaking.  The engine management system compensated under most driving conditions.  My only clue was that the cruise control would kick out on a grade.  The manifold vacuum signal was inadequate.  I replaced the hose, end of problem.

Yes, a smoke machine would be helpful for both the vacuum leak(s) and exhaust system check.  Engine turned off and coil wire removed at the distributor cap, you could rotate the crankshaft slowly while the smoke machine feeds the intake and cylinders.  When the exhaust valve opens at the leaking header tube(s), the header or exhaust leak would show up.

In your case, there may be something else going on.  If the idle kick-up speed is precise, especially after all of the parts you’ve replaced, I would check the air conditioner clutch wiring circuit.  If the speed kicks up to 1000-plus rpm without the A/C compressor operating, this could be a false signal from the A/C circuit.  The HVAC controls could cause the idle to kick up whether or not the A/C clutch engages.  Worth checking, anyway.  If the idle flares up to random speeds, the issue would likely be a vacuum leak or something else.

On the 4-cylinder models, Mopar uses a power steering pump pressure sensor to kick up the idle when steering pressure goes up.  Primitive and mechanical, the system senses the steering effort when the vehicle is stopped and the steering wheel gets turned.  This boosts the pressure requirement and signals the PCM to raise the idle speed and compensate for the added load on the engine.  Your issue could be similar if the A/C wiring circuit is bumping the idle speed to a somewhat consistent rpm.  The A/C circuit may be activating without a compressor load, which would raise the idle rpm.  It’s worth considering an HVAC wiring or controls issue.


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