What would cause this?? - JeepForum.com
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post #1 of 4 Old 02-10-2013, 12:55 AM Thread Starter
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1998 TJ Wrangler 
Join Date: Jun 2012
Location: Pasadena
Posts: 286
What would cause this??

I believe one of my sensors is bad, but I'm not sure which.

Here are the symptoms:
1. Slightly rough idle.
2. Occasionally stumbles from dead start or when hitting a bump.
3. Has power, but won't crank. (RARELY)
4. Dies when hitting a bump or on a turn. (RARELY)
5. (Possibly unrelated) When cruising, the transmission tends to slip and the engine revs without engaging, but clunks into gear afterwards.

SOOOO... MAF? MAP? TPS? Crankshaft Position Sensor? Fuel Ballast?

So many sensors, so little money... Which one??

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post #2 of 4 Old 02-10-2013, 06:31 AM
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1996 ZJ 
Join Date: Nov 2009
Location: michigan
Posts: 969
the starting problem sounds like neutral safety switch. Try pushing forward on the shifter when starting and see if that make a difference.

I had symptoms like your hitting bump problems once, turned out to be a wire not routed/secured properly and it rubbed on something until the insulation was gone. When I hit a bump the thing would die.

96 ZJ Laredo, 4.0, 242, 230K
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98 XJ Sport, 4.0, 231, 140K
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post #3 of 4 Old 02-10-2013, 08:12 AM
It's the crank sensor!
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1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota
Posts: 8,860
1. Items 2-4 could be caused by bad engine grounding. Grounds are critical, especially for 87-90. More on that below. You MUST start with this!

2. Always review basics. Is tuneup hardware fresh? New Champion copper plugs gapped to .035, quality plug wires, distributor cap, rotor, fuel filter. Clean your throttle body and idle air control as they get carboned up over time and miles.

3. Most everything on a RENIX can be tested with a manual and a meter. And the basic ability to use both. Good link to how to test things on the RENIX below...


4. I agree that #3 could also be the start of a neutral safety switch issue; problems with this switch are very common on the XJ. If wiggling the shift lever while turning the key or starting in neutral helps when you have this symptom, suspect the NSS, which often can be "refurbished" according to below procedure.

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.

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 #4 of 4 Old 02-10-2013, 11:59 PM
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1991 XJ Cherokee 
Join Date: Feb 2011
Location: Bakersfield
Posts: 1,297
X2 on what TJ said. Actually bad grounding OR bad battery connections could cause 1-4.

5 could be low transmission fluid level or need for TPS adjustment.

It doesn't have a MAF, it's MAP, which is basically the same thing. That could cause rough idle.
Reptile is offline  

ckp , maf , MAP sensor , sensor , tps

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