jdjonesdr

I'll throw this out to see where it goes.

My 90 Cherokee has been running fairly well lately. But lately, it's developed an occasional miss. Not on just one, but three cylinders.

The beast was parked for two weeks while I was in the states. During that time, there were a couple of really good rain storms. (just giving you the facts)

Now, before I left, everything was hunky dory. Come back, jump in, start the engine, and it runs rough as hell. I look things over, see a couple of moist injectors, so I bought and installed new injectors. End of problem, so I thought.

A couple of days later, start the engine at 6 in the morning, and it's missing again. Barely make it to work.

We pop the hood at lunchtime, start the engine, and it runs like a champ.

After it sits in the sun for 3-4 hours, the problem goes away.

It's obviously a "cool/moisture" problem somewhere, I would guess. When it misses, it's cylinders 2, 4, and 6. Surely not a coincidence.

Do any of you more experienced guys know what's causing this, and the fix?

cruiser54

How old are the plug wires, dizzy cap, and rotor?

jdjonesdr

Everything is new within the last six months or so.

It acts like it's moisture, since it goes away after a couple of hours of hot sun, but the missing on 2,4,and 6 each time it happens has me a little perplexed.

cruiser54

Here's 2 things to try. Free except for some time spent.

I suggest unplugging EVERY electrical connection in the engine bay you can find, whether engine related or not, and spraying it out with a good electronics cleaner, visually inspecting the terminals making sure they haven’t retracted into the plastic holder, and then plugging it back together. There’s a critical 10-pin connector for the front lighting system located in front of the air cleaner and behind the left headlight assembly. Don’t miss that one. Also be sure that the connectors to the ballast resistor mounted near the air cleaner housing are clean and tight.
ALL of the relays should be removed, the terminals wire-brushed until shiny, and the receptacles sprayed out with contact cleaner. Then plug them back in. I do this on every Renix Jeep I purchase or work on for someone else.
Revised 07/23/2012

Renix Jeep ICU/Coil contact refreshing
　
　
The contacts between the coil and the ICU on your Renix Jeep can become corroded and loose causing a complete or intermittent no-start condition. I recommend the following procedure as a maintenance precaution to insure this is eliminated as a possible cause now and in the future.
The coil is attached to the ICU by two T20 Torx bolts. Remove these two bolts and lift the coil up off the ICU. You will see 2 pins and 2 sets of contacts. Clean both the pins and springy contact pieces with a good electronics cleaner and some fine sandpaper.
Squeeze the springy contacts closer together with some needlenose pliers. Bolt the coil back on to the ICU.
While you’re right there, unplug the connectors from the ICU and inspect the pins in the harness connector. Make sure the pins are not retracted into the connector. Spray out the connector and the receptacle of the ICU with the same good electronics cleaner you used earlier.
I feel this procedure should be performed at least once in the lifetime of a Renix Jeep.
　
Revised 04-04-2013

cruiser54

OOps. Ever done this? Super important!!

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, failed emission tests, and wasted money replacing components unnecessarily.
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.
For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.
If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.
　
Revised 03-04-2013

jdjonesdr

Hey Cruiser,

Great to hear from you again.

Actually, as part of the whole restoration process when I first bought the beast, you suggested all of this, and I went through the whole wiring connections cleaning process.

At least I thought I did. Until I read that part about the contacts between the ICU and the coil.

I was originally going to purchase a new coil as part of the restoration, but ended up not doing so because of the cost.

I wouldn't be surprised if that's where my problem originating now. I'm going to pull it apart today and check out those contacts.

After all, this truck should be able to drive through all kinds of inclement weather without missing a beat. The fact that it has an occasional miss like this when it rains without even driving it is really kind of embarrassing.

As an aside, is there anything that would make it miss on those particular cylinders? Why those three as opposed to say the first three cylinders, or any combination of cylinders besides every other one?

cruiser54

Hmmm. 6,2,4 are together in the firing order.....

Cracked cap? Carbon tracking inside the cap?

Parsnip

Not doubting you op, but how did you verify 2,4,6?

cruiser54

Good question.

MrSimon

I'll second the idea of doing basic tune-up stuff first, then checking all your grounds, then checking your electrical connectors/wires.

Even if it doesn't frix your problem .... its a darn good thing to do to an old Jeep.

jdjonesdr

Heh heh.. I see you guys haven't seen my detailed reconstruction of the beast! LOL
https://www.cherokeeforum.com/f64/he...public-151585/

I confirmed the miss by pulling off the injector connectors one by one, and then pulling the spark plug wires one by one. mech101.

I thought everything on the engine (sensor wise and ignition wise) was new, but I had missed the module and the coil. Those are on the way now.

And troubleshooting101, I pulled the cap to make sure moisture hadn't gotten in there somehow, and that all the cables were properly connected.

The only thing not stock is the big relay next to the battery/ignition. I installed bosch relays, and divided up the load between them; the idea being it would be easier to troubleshoot. (I also put relays on the lights and horns)

All of that is working fine.

I'm convinced it's moisture somewhere.

BTW, thanks for your contributions guys..

Fivefifty%

Did anything ever come from this? I'm having the same problem now and only on cold days. I have new plugs wires dist cap and rotor. Also I have done the Ford injector swap. Still missing on cold days on cylinders 2,4,6. I'm thinking about replacing and cleaning the coil and contacts but was wondering if that solved the problem or not. I have a 1995 XJ Spot 4.0