VERY solid 1999 cherokee sport, Auto, everything works. No codes. 242,xxx miles.

New fully charged die hard, EVERY ground, ecu mounts and grounds, to firewall, block, cam sesnor, coil, even negative ground on fender(came out with hand ratchet easy), EVERY ground is removed and CLEAN, and dielectric grease..

Cherokee randomly doesn't start(we don't drive her often enough, 10 X's the last year, and need to sell her).
Replace CKP (Dunno why so many cry about it. Took 15 minutes, from below). Probably original, looked like factory part numbers. Epoxy on end dried/melted through.
Fires right up. Drive around block. Several blocks. 5 minute drive.
Put it in reverse to park her, she dies.
Will not start back up. Coughs a little bit when cranking, but no go.
So some spark. I can also hear the fuel pump prime. I will probably try a shot of ether to see if it runs a second or two. Which would indicate fuel supply/pressure issue.
However, No communication (iso 4191 page, nothing goes further(?), with NAPA code reader. Sits there on that page and does nothing after several minutes.
Unplug new CPK sensor. Still no communication.

Disconnect battery, reconnect. Same thing. And no check engine light, or any indication of a problem on dash.
No communication.
Bucks slightly, but no idle. Like the fuel pump isn't kicking BACK on as the injectors fire during cranking.

Reader blasts right past that page on my wifes 74,xxx mile 2001 cavalier, and reads her ecu perfect(and still no codes, haha)

So if a frying CKP was screwing with the ECU, could the electronic circuits changing when shifting into reverse, with a nice strong current from a new CPK, finish it off?
No coming up on much of testing a Cherokee specific procedure for bad ECU before dropping $150.

Expert and guru advice, and bizarre jeep electronic stories(with cures and some diagnostics) welcome.

 

dielectric grease is non conductive...you may have done the exact opposite of what you were intending to do with the grounds.

 

Nah, the grease squishes out just fine. Non-conductive grease is used in all kinds of connections, contacts, bulbs, and switches with no issues.

This does sound like a possible failing PCM with bad capacitors to me. I would try to confirm if you have spark and whether the injectors are firing before dumping a bunch of money on a new PCM. Any junkyards around that might have a used one?

 

There is some stuff in the wiring section at Lowes etc. for preventing oxidation where the ground wire for your service panel joins the ground rod. That would be the correct stuff to use for your grounds. I think it's called NoOx.

The dielectric grease is used to seal the spark plug boot from moisture while withstanding high voltage so you don't get carbon tracking. It doesn't go on the electrode itself, just the boot.

FWIW I killed a Subaru by putting dielectric grease in the three big connectors for the engine harness. Took a lot of work and several cans of contact cleaner to get it running again. Fine ever since.

 

The engine wouldn't "Buck" when first cranking if it wasn't getting any fuel, or spark. It's not a simple "crank and that's all" like when the CKP fried out.
Like I mentioned, I can hear the fuel prime, tries to fire a few times, getting weaker as it cranks, but FP NOT kicking back on while cranking. Then prime is lost and won't idle.
I WOULD say cam sensor, but no ECM -reader communication.

Personally, in my experience, buying 20+ year old or older electrical parts from a junkyard that were most likely rarely washed in the salt belt is a way to make yourself headaches. Half the price of a refurbished one with added bonus of no guarantee it will work in the first place is a gamble to waste time and money, and asking for some different circuits to not work(as they are VIN/trim level programed).

Stuff from lowes sounds expensive and designed to withstand rain and sun. My big tube of Dielectric from the auto store was @ $7.
Dielectric grease only >>seals<< a contact, and won't melt out with heat, or easily catch fire, like axle grease CAN with long term spark. Like dude said above, CLEAN metal to metal easily squeezes it out. Will NOT change how much metal to metal contact you get when clean metal-metal, and not over or under tightened. You can do the same thing with spray paint on a >>CLEAN<< metal ground. paint merely seals and fills voids that salt and crud can get into. Dielectric is "safer" though.

If you killed your subi, it wasn't the grease. You changed, bumped, or MUCH more likely, tightened/cleaned something enough on the second go around, promise you. I have been using it on ALL >>clean metal<< connections and grounds for >>SEVERAL<< decades.

Works esp well slathered in semi truck 2 circuit 1157 clearance bulb sockets. If you DON'T use it, the 2 lead contacts on the bulb will go away incredibly fast from arcing and bouncing down the highway, or the brass socket will go away within a couple of years from salt and ground corrosion(like a battery ground is always the one that becomes gross). It is a MUST to keep DOT away.

You should see how almost LED level BRIGHT, my 1974 cj5 1157 brake and turn lights are. Driven 2 years daily even in Ohio winters, and in service now for almost ten years. Still as bright as new. Fitted a semi size "FleetRite" battery, and wired the entire Cj5. Not one bolt or nut I haven't removed at least once. EVERYTHING is custom and or self engineered. And completely reliable for 10 years. Can sit all winter, and fire the first time I try.

Not my first rodeo, yo, lol I build stuffs.

 

Oh, excuse me then, I thought you were asking for some help, but apparently not.

 

It's Ok. I AM looking for help from Master level guys who KNOW and UNDERSTAND the @ '99 XJ electronics.
But if you don't KNOW what you are saying you shouldn't speak. Not MY job to raise you.
Be mad at your parents, not me.

 

If cps is not Mopar part, swap it. Then for sure it isn't a marginal unit.

 

Some more spitballing from me:

I suggested swapping in a used PCM because the local pull-n-save junkyard charges under $20. A temporary swap to see if it changed the symptoms can be helpful. Usually PCMs outright die or an injector driver blows due to a shorted harness, but sometimes they can get heat sensitive and get flaky.

It sounds like you have something coming back for a crank signal if it's trying to start, but if it's an intermittent signal you might get just a few sputters and backfires. I assume you see the check engine light for a few seconds at initial key-on, meaning it's not totally dead. It sounds like you're getting the initial key-on fuel pump prime, but it doesn't run the pump when you try cranking. The PCM normally looks for a good crank signal above a certain rpm before kicking the pump on again. I believe the HO engines need both the distributor and crank sensor to be working to start, although it often will not set a code if the crank sensor is bad. Non-mopar crank sensors tend to be of low quality with short lifespans.

That it died when you hit reverse is interesting. I would look closely at the wiring harness, wiggling and shaking it, especially where it transitions between the head and the firewall. It's prone to fatigue and chaffing, particularly if you ever had bad motor mounts. I've seen random dying due to the O2 sensor wiring against the exhaust as well. Other less likely things to check are that the flexplate is good shape and that the bellhousing is still bolted up tight.

 

Plain silicon grease versus the more specialty stuff has been hotly debated for years, even arguments about not using it on multipin connectors as it can cause shorts. Noalox adds some anti-corrosion chemicals to help prevent aluminum conductors from oxidizing and building up contact resistance that can start a fire. Some brands add ground zinc particles claimed to help cut through the oxide layer on aluminum, which to me is a band-aid for poor conductor prep. The Noalox grease itself isn't actually conductive even with the zinc particles, and you can verify that with an ohmmeter. The main benefit of the grease as you correctly point out is to exclude moisture and oxygen to minimize corrosion. It's important to use it sparingly though, and not pack the connector full to the point of bending pins when you push it together.

 

A wire ready to send 50+AMPS at 240V in to the ground is a little more "sloppy" and "100% different use" than ECM or Bulb sockets.

I read several times about a genuine MOPAR part. But she is going up for sale or trade towards a minty '97-05 Wrangler
I still have almost a full exhaust and a valve cover gasket to put on it yet. We were going to keep it, and spared no expense like the die hard and brand new,
PROUDLY "only" 225/75 R 15's Hankooks. Because dumb kids rolling them making insurance WAY too expensive for what it is.
Insurance agent said it.
Too bad. We really wanted to keep her long term. She's a solid bones gem. Interior is MINT too.

I appreciate the spittballing lawson. At least you had the respect and courtesy to aim. Thank You Brotha. I already checked all those things.
Seems like the consensus was the Cam sensor almost never goes bad. During my searches, and contemplation, the fact if fired "sorta" 2-3 times before losing prime steered me away from CAM sensor.
For the $50 diagnostic if tests "OK", or $100 for a repair(if repairable), or credit towards a mint refurbished ECM, it's worth the time savings. A guy could spend DAYS scratching his head, probing, testing and fiddling.. Not this guy. I'd rather pay the 100 and completely eliminate the HUGE myriad of symptoms a "BAD" ecm is known to cause. Once ALL potential issues an ECM can cause is 100% eliminated, that would be the first thing I'd test if no start. Gut says it will.

Thanks again.

I will make a very detailed inspection for obvious things while waiting for my ECM to come back.

I'm just some idiot dumb enough to turn wrenches for almost 40 years. On F-15E Strike eagles(335thFS), service repair and operating :Peterbilts, Frieghtliners, Internationals, MACKs, Caterpillar 980B's, track hoes, Leibherr A922, overhauling many diesels, including 4FB1's, DT466E's, kabotas, swapping 15 spd truck trans, axles, full brakes, radiators in semis..., personal fleet of a wide variety.

Thanks for arguing about dielectric grease, kids. Huge "help", Now read my signature.

 

I would guess cap and rotor
fuel pressure,
ignition coil,
intake temp sensor,
throttle position sensor
crank position sensor

bad gas...

 

Another thought would be maybe a failing IAC. Not setting back correctly maybe causing a hard start, and not working to let a bit more air in when you drop into reverse?

 

Repeating a completely wrong old wive's tale is not helping.



Oh, please, not this again. NoOx is for ALUMINUM connectors at HOUSE VOLTAGES. It's not made for 12v systems. Dielectric grease IS. And by the way, NoOx is every bit as much an insulator as dielectric grease.


Read this . Follow the links, too.

 

Heh. Yah think? I wish people would stop with these old wive's tales! "Dah ah leck trick GREASE?? Run, Martha, it's wun ah them thar IN soo lators!"



Almost, but they do. I've had one. Can't completely rule it out.



Good plan.


Let us know. It will be educational for others, if nothing else.