Mystery Rough Idle -
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post #1 of 3 Old 07-09-2012, 02:41 AM Thread Starter
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1998 TJ Wrangler 
Join Date: Jun 2012
Location: Pasadena
Posts: 284
Mystery Rough Idle

So the XJ seems to be idling a little low these days. Spark plugs are new, wires are new, cap and rotor are new. Seafoamed the whole thing recently and cleaned the throttle plate. Didn't blow much smoke at all. Still seems to idle rough.

The only other things that I can think might cause it are A) My oil could probably use a change. B) The coolant is a little low and for some reason this has seemed to affect the idle before C) The fuel filter is clogged or D) The tiny vacuum leak in my brake booster is causing it.

Thoughts? Thanks guys!

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post #2 of 3 Old 07-09-2012, 04:14 AM
It's the crank sensor!
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1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota
Posts: 8,847
*Start with a fuel filter. A few bucks and 15 minutes of your time. It's located on the drivers side about halfway back on the frame rail. Be sure to depressurize the fuel system before replacing.

*Oil and coolant should not be related to your symptoms but always keep up with fluid changes as they are the lifeblood of your vehicle.

*Any vacuum leak is not good but usually doesn't result in a "low" idle. But replace any parts that have a vacuum leak as a matter of course.

*Testing of engine management sensors is never a bad idea. You'll need a meter and the ability to use it. Good luck!

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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post #3 of 3 Old 07-09-2012, 07:21 AM
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1990 MJ Comanche 
Join Date: Oct 2009
Location: Prescott
Posts: 11,511
Aside from what Mr Walker suggested, I would check to see that the intake manifold bolts are snug. They have a habit of loosening up.

Then, there's the ground refreshing which is needed at least once in a REnix Jeep's lifetime. If you haven't done it, now's the time.

Renix Ground Refreshing

The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, and wasted money replacing unnecessary components.

The components grounding at the dipstick tube stud are:

Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.

The factory was aware of the issues with this ground point and addressed it by suggesting the following:

Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.

Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.

While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.

Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.

First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4” socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.

A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18” long with a 3/8” lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.

A further improvement to the grounding system can be made using a #4 cable, about 10” long with 3/8” terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.

If you want to upgrade your grounds and battery cables in general, contact Jon at He makes an incredible cable upgrade for a very reasonable price.

Revised 11-28-2011
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idle , low , rough , vacuum leak

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