96 ZJ dying again! - JeepForum.com
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post #1 of 53 Old 01-10-2019, 10:47 AM Thread Starter
xtrattitude4u
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96 ZJ dying again!

I've got the dreaded mysterious Jeep dying issue again.

My Wife's 96 Grand Cherokee 4.0 Laredo (175k) started "bucking" on the freeway, then would run fine.

Soon after, she was on the way home, and she said it lost a lot of power, and increasing throttle was not making it go any faster. Then it backfired and started running normal again.

A few minutes later, it died completely and had crank no start. No check engine light. It started intermittently, enough for the tow truck to get it on the truck.

When I got it home, I scanned for codes with Elm327 Bluetooth scanner and Torque app, no codes stored.

Now that it's home, it will idle for a few minutes and just die with no codes. Then it starts back up as though nothing happened.

When this issue happened last time, it was the crank sensor, so I replaced it with a genuine Mopar crank sensor, maybe a year ago.

I tested the ignition coil and it was out of resistance spec, so I replaced it. still dying.

I checked the distributor and the cap and rotor both were due. I also heard the camshaft sensor (coil pickup) can cause these issues, and the dizzy had some play and the bearing wasn't in great shape. So I replaced the whole distributor yesterday, after getting 2 in a row that were built wrong, btw 😞

And it's still dying, but starts right up after. I drove it around the block, but in true Jeep form, it doesn't want to act up while the person trying to fix it is paying attention.

The O2 sensors we're both replaced within the last year, and I'd also expect a code to show up if they were causing it to die.

I've never wished more in my life that a vehicle would just break for good, so I can figure out what's wrong!!!

Looking for any other suggestions.

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post #2 of 53 Old 01-10-2019, 11:03 AM
jtec
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you post "And it's still dying" -

intermittent has us checking and double checking the CKP.

with a gauge what is fuel pressure?

do a vacuum leak check, the old school carb spray trick makes a good easy quick test.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 53 Old 01-10-2019, 12:07 PM Thread Starter
xtrattitude4u
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Quote:
Originally Posted by jtec View Post
you post "And it's still dying" -

intermittent has us checking and double checking the CKP.

with a gauge what is fuel pressure?

do a vacuum leak check, the old school carb spray trick makes a good easy quick test.
Sorry don't understand the first part, not sure what CKP is.

Ill get a pressure gauge and test today!
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post #4 of 53 Old 01-10-2019, 12:43 PM
jtec
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you posted "still dying" I wanted an up date - you also mentioned it does not act up now ?

CKP is a trade acronym for crank position sensor AKA CPS

with your scanners did you look at 'pending' codes?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #5 of 53 Old 01-10-2019, 01:13 PM Thread Starter
xtrattitude4u
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Quote:
Originally Posted by jtec View Post
you posted "still dying" I wanted an up date - you also mentioned it does not act up now ?

CKP is a trade acronym for crank position sensor AKA CPS

with your scanners did you look at 'pending' codes?
Ok got it. What I mean is it is still dying after a few minutes of idling, but then starts right back up so there's no way to really narrow it down.

But because it's doing that I don't really want my wife to drive it to work right now, had to tow it once. I'm hoping I can get it to at least crank no start while it's home so I can see if there's spark or not while the problem's happening.

I think the app says "no stored or pending codes", but I'll double check again.
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post #6 of 53 Old 01-10-2019, 03:23 PM
montync
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My 98 4.0 would Idle and shut down. I cleaned the intake, idle air control, throttle body, replaced a leaking exhaust manifold (cracks at weld joints) and threw new plugs in. It solved the dying while idling. It has a rough cold start but that I believe is due to the fuel pressure regulator and aged o2 sensors and is fine once warm.

You say it was bucking at highway speeds? That would lead me to check the coil.

every time I have had a bad coil (on other vehicles) it would buck (misfire), and idle like crap (or die). Once shut off for a little while (warm weather) it would start up fine but idle and driving would degrade as the coil heated up. colder weather it would die, but crank up and last a little longer. I kept a frozen bottle next to a coil when hauling trash a few miles down the road one time because I was too cheep to but a new coil.

Google how to test the coil with a multi meter. Test it cold, then test it after running and dying. If it is the original coil or the coil has some age may be time to change it.
as always check grounds and battery cables for corrosion, clean and fix where necessary.
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post #7 of 53 Old 01-10-2019, 04:31 PM Thread Starter
xtrattitude4u
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Quote:
Originally Posted by montync View Post

You say it was bucking at highway speeds? That would lead me to check the coil.

Google how to test the coil with a multi meter.
I did, and it was out of spec on the secondary winding. I replaced it.
(And the first new one I bought was out of spec, so got another one.) Stated that in the first post. Also replaced the distributor.

The only thing left in the ignition system I haven't replaced that would cause it to die completely is the coil wire and the ignition relay. I'm considering replacing those next if I can't figure it out, but I'd much rather test/verify than throw parts at the problem. I've only replaced things that tested bad or were obviously needed so far, so I feel good about that.
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post #8 of 53 Old 01-10-2019, 04:56 PM Thread Starter
xtrattitude4u
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Ran fuel pressure test.

First of all, there was not even a dribble of fuel when checking the port, 0 psi. I hooked up the gauge and turned key to position 1, pressure climbed to 10 psi and stopped. Turned key off/on, pressure climbed to 30psi and stopped.
Turned key off/on once more and pressure Rose to 40psi and stopped.

Started her up and it is pegged at 49psi on the nose, where it should be.

While tinkering around under the hood, I gave the ECU a good tap, and the engine stumbled.
I got it to do this at least 3 times, but it didn't die completely.

Wiggled the looms to check for a loose wire and got no result there. Wiggled the connectors and the gray one (far left) made it stumble again.

I think this is my issue.

I forgot to mention, I think I had the problem with the 2 screws on the PCM shorting out beore, so I backed them out a couple turns and that seemed to fix it at the time. They are still backed out like that, maybe 2-3mm out.

So maybe there is a loose solder joint in the ECU, or the screws damaged it.
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post #9 of 53 Old 01-10-2019, 05:23 PM
kg6mov
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The screws can’t short the PCM, that’s an old wives tale.

You have a bad connector on your pcm, you messed with the screws and it got better because you messed with it, now it’s back.


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post #10 of 53 Old 01-10-2019, 05:27 PM Thread Starter
xtrattitude4u
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Quote:
Originally Posted by kg6mov View Post
The screws can’t short the PCM, that’s an old wives tale.

You have a bad connector on your pcm, you messed with the screws and it got better because you messed with it, now it’s back.
I moved the coolant tank out of the way, and grabbed my t15 torx bit.

I started turning the left PCM screw to the right, to tighten it, and boom the Jeep died hard.

I didn't even turn it 1/8th of a turn.

I don't think it's an old wives tale.

EDIT:

Nevermind, you're right.
I took the PCM apart and there doesn't seem to be any way of the screw affecting any circuits.

I think I have a loose solder joint on the PCM header, checking that out.
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post #11 of 53 Old 01-10-2019, 06:19 PM
RedRiverT
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Your post #1 sounds exactly like what mine was doing, until after I replaced the PCM. I did just like you, change something and it would run fine for 2 days or a week or 2 weeks. The bucking, backfiring, shutting off, not firing at all, then boom next time you turn the key it would start right up and run fine, sounds just like mine. LKQ Pull-a-Part here sells PCM's for $28 with a 30-day warranty. You can use one from a 96 or 97, but not a 98, I'm pretty sure. And it matters whether there is factory security or not.
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post #12 of 53 Old 01-10-2019, 06:23 PM
RedRiverT
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Oh yeah, if it is a loose connector as kg6mov suggested, you can try to hold them on tighter with zip-ties around the PCM. That has worked for a bunch of people.
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post #13 of 53 Old 01-10-2019, 08:05 PM Thread Starter
xtrattitude4u
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Quote:
Originally Posted by RedRiverT View Post
Your post #1 sounds exactly like what mine was doing, until after I replaced the PCM. I did just like you, change something and it would run fine for 2 days or a week or 2 weeks. The bucking, backfiring, shutting off, not firing at all, then boom next time you turn the key it would start right up and run fine, sounds just like mine. LKQ Pull-a-Part here sells PCM's for $28 with a 30-day warranty. You can use one from a 96 or 97, but not a 98, I'm pretty sure. And it matters whether there is factory security or not.
Yeah it does have factory security, which makes it a bit harder. I've had to replace it once before, when the rear 02 sensor wires came loose and wrapped around the driveshaft, shorting it out.

Funny thing, I was told the dealer would have to program the security for me, but it seemed to sort itself out after a couple starts, which I thought was nice.
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post #14 of 53 Old 01-10-2019, 08:55 PM
montync
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Originally Posted by xtrattitude4u View Post
Now that it's home, it will idle for a few minutes and just die with no codes. Then it starts back up as though nothing happened.
For some reason that is the last part I read this morning, I didn't see the rest about the coil. My mistake.

Hope it is something simple and cheap!!
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post #15 of 53 Old 01-10-2019, 09:41 PM
ZeeJay1997
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Another reason messing with the screws will cause things to change is corrosion in the internal front module connector.

None these PCMs are 100% sealed along the front module and the thermal cycling tends to make the PCM "breathe", thus drawing moisture through the seams. One of these seams is along the top of the internal connector so the pins and sockets get corroded. Been there, done that.


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