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[SIZE=medium]I need some help; I have a 1996 GC 4.0. 189K mi. The problems started on a road trip pulling a trailer on a mild grade, suddenly and momentarily the engine quit and the transmission dropped out of OD and lock up and continued to do so intermittently for the rest of the trip, it may also go for several hundred miles without doing it. And then it just starts to miss and jump in and out of gear, it never completely stops running and it always starts, since then I have been replacing parts, as a mechanic with over twenty years of pulling on wrenches it’s something that I despise. [/SIZE]
[SIZE=medium]Here is a work and parts that I have replaced, header, it was cracked, full transition service, including fluid flush and adjustment (all appeared normal and in good condition) fuel pump, sock and filter, checked fuel pressure, (drove it with a pressure gauge hooked up to it, no change during episode) checked battery and charging voltage, wiggled ignition key during episode, replaced wires, plugs, cap and rotor, ignition coil Crank PS, rebuilt distributer with CPS, checked all connections to crank an cam position sensors and wiggled them with the engine running, including the CPU, could not make it miss, swapped around the ASD relay (thought I found it, it went 180 miles without missing), removed under hood power distribution panel and inspected for signs of bad connections, removed and cleaned all ground connections, removed starter and inspected hall effect trigger points for signs of foreign object damage and finally installed a rebuilt CPU. Most of these parts were due with 180K on the vehicle. The CPU was expensive and unwarranted. [/SIZE]
[SIZE=medium]One thing that’s driving me nuts is how intermittent this problem is. If it just died I could diagnose it. The vehicle is a dry country vehicle no corrosion. [/SIZE]
[SIZE=medium]Here are some things I have noticed; the tach stays stable during an episode, it never does it when cold, it takes it about 15 to 20 miles or it may take 180 mi. plus to do it again. It always starts and has never completely quit running, (most) episodes occur under light acceleration or on light grades. Episodes may last for several seconds or minutes, then all returns to normal. [/SIZE]
[SIZE=medium]Trouble codes are for multiple misfires, all cylinders and Cam PS, I have also had codes for Cat efficiency, I believe due to the missing problem. What could I be missing?[/SIZE]
[SIZE=medium]Please reply to [email protected] Thanks for your time.[/SIZE]
 

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I can't tell what is wrong, but I can tell you that you should never, never, ever tow in overdrive.
That's why there is the overdrive off button.
I'd suspect that the under load issue is related and in the engine management system. Maybe something in the cam/timing chain/CPS?
ImperialCrown will likely have a better answer.
 

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+1. Never tow with overdrive on. If the transmission overheats, it automatically shifts down to 3rd until it cools off some. For my truck the threshold is 260F and it shuts the overdrive off, and doesn't enable it again until it cools down to 230F.
 

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Welcome to Allpar. I would pay attention to the cat fault code. It is most likely real. The reason that it takes a long time to set is that the PCM wants to be extra, extra certain that the problem is really the cat.
If you 'thump' the cat, does it rattle inside?
If the internal ceramic element has broken up, it can intermittently block the exhaust exit and cause exactly what you describe.
CARB no longer allows 'direct fit' cats from what I understand.
 

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ImperialCrown said:
Welcome to Allpar. I would pay attention to the cat fault code. It is most likely real. The reason that it takes a long time to set is that the PCM wants to be extra, extra certain that the problem is really the cat.
If you 'thump' the cat, does it rattle inside?
If the internal ceramic element has broken up, it can intermittently block the exhaust exit and cause exactly what you describe.
CARB no longer allows 'direct fit' cats from what I understand.
That is correct, the CAT must have the correct part number or an CARB. Exempt, Executive Order number on it.

I was remiss, welcome to Allpar, I see your location is close to my old stomping grounds, not much in the way of shops up there, but a trip to Bakersfield might save you some money in the long run, if you're going to need a CAT.
 

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[SIZE=medium]It’s true that the first time it happened I was pulling a trailer in OD. That was the last time I pulled a trailer, I don’t think they are related and that was several thousand miles ago and I had put 30,000Mi. on it scenes I bought it without a problem, scenes then I have been driving it to work and back, 20MI. each way. I had the Trans fluid changed and the Trans serviced no sign of an issue, the fluid was red and smelled ok. As for the cat, it only threw the cat code once and my smog guy says the Cat is ok, the vehicle does not lose power, (like a restricted exhaust) it starts to miss like someone is turning on and off the ignition switch, and drops out of OD and lock up at the same time, then in ten seconds or a minute it returns to normal, I can then stand on it and it hauls [I should have my mouth washed out with soap for using such terms]. Never back fires, never quits running and runs great the rest of the time, 18 to 19.6 MPG to work and back on hills and curves @ 50-60 MPH.[/SIZE]
 

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Maybe a bad ignition switch.
 

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I could be way off here, but I'll throw it out there since I don't see it mentioned anywhere. Have you checked/replaced the throttle position sensor?

When I first got my ZJ it exhibited some of this behavior. It wasn't shifting right, the engine would miss and it wouldn't even get in to O/D - all periodically. I don't even think it gave a TPS error code, just a cylinder misfire. I replaced the TPS, I think $30-40 at Autozone and 5 minutes under the hood, and it ran like a champ again. It seems like the TPSs fail at very different intervals too...mine went on my 4.0L at 101K miles and my roommates failed on his 5.2L at ~170K miles.

Good luck with a solution!
 

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pjfsu331 said:
I could be way off here, but I'll throw it out there since I don't see it mentioned anywhere. Have you checked/replaced the throttle position sensor?

When I first got my ZJ it exhibited some of this behavior. It wasn't shifting right, the engine would miss and it wouldn't even get in to O/D - all periodically. I don't even think it gave a TPS error code, just a cylinder misfire. I replaced the TPS, I think $30-40 at Autozone and 5 minutes under the hood, and it ran like a champ again. It seems like the TPSs fail at very different intervals too...mine went on my 4.0L at 101K miles and my roommates failed on his 5.2L at ~170K miles.

Good luck with a solution!
[SIZE=medium]Thanks for the input; A TPS will probably be next on the list. I did not know the TPS could make the engine miss; The only codes that I have been getting, have been for multiple misfire and Cat efficiency, I worked a mechanic for over twenty years, In my life time, I can only remember one other automotive problem put me on the ropes like this, but that’s another story. [/SIZE]
Bob Lincoln said:
Maybe a bad ignition switch.
Been there, done that. Thanks
 

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Dierjm said:
[SIZE=medium]Thanks for the input; A TPS will probably be next on the list. I did not know the TPS could make the engine miss; The only codes that I have been getting, have been for multiple misfire and Cat efficiency, I worked a mechanic for over twenty years, In my life time, I can only remember one other automotive problem put me on the ropes like this, but that’s another story.[/SIZE]
Maybe the misfire was just a coincidence. I'm definitely no mechanic, just a humble DIY learner.

I do remember it only misfired once and it was literally when driving it home from the purchase. Other than the TPS and having the radiator replaced, this truck has thankfully been pretty problem free. Big fan of the ZJs.
 

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Discussion Starter #11
Changed the TPS last night, it was only about $32.00 bucks, yet another shot in the dark, I have found other accounts on other sights of ZJs missing, or jumping out of gear due to a faulty TPS. Its worth a try.
 

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Dierjm said:
Changed the TPS last night, it was only about $32.00 bucks, yet another shot in the dark, I have found other accounts on other sights of ZJs missing, or jumping out of gear due to a faulty TPS. Its worth a try.
Here's hopin'!
 

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A faulty TPS on my truck several years ago did cause stalling immediately after hot restart, and did cause the transmission to kick down to 3rd on the highway with the slightest throttle pressure on light grades. But it didn't cause any misfires.
 

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[SIZE=medium]The pisser is how intermittent it can be, sometimes 15 miles and other times 180 mi. lately it’s been pretty reliable to say @ about the 15 mile point on the way home in the evenings it will start to jerk, miss and drops a gear for only half a second or so, sometimes it does it for such a short period it can be hard to detect, then it escalates to bucking and missing like someone has a telegraph key on the ignition and OD and lock up, I fear major transmission damage could result from it hammering between third and lock up and overdrive, of course, I back out of it and lock it into second, the missing will continue for anywhere from 10 seconds to several minutes, then everything back to normal like nothing ever happened and runs like a scalded cat, much like a child throwing a tantrum. This problem has gone from total frustration to a source of amusement and a personal challenge, I will find the problem, understand why it did it and I will fix it! [/SIZE]
 

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I don't know why I didn't think of it before, but it's probably an upstream O2 sensor. What was the numerical fault code?
 

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I don't have them on hand, but they were for multi cylinder misfire, some times for Cat Eff.
PS the TPS di not help, but it was wort a try. up stream O2 next?
 

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I would stop guessing and replacing parts and try to diagnose this?
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I don't know the capabilities of your scan tool, but you might want one that can record an event for later viewing to see what dropped out. There is a Co-pilot data recorder tool that could record the 90 seconds leading up until you push the record button for 3 different occasions. This tool is great for finding intermittents that are otherwise impossible to get a snapshot of and don't leave behind a fault code.
 

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[SIZE=medium]Can you suggest the correct tool for the job and an estimate of what the tool will cost? I have talked to local shops and to the dealer in Bakersfield, no one will give a guarantee or limit price on diagnosis for the problem, in other words I could easily spend more than the vehicle is worth and still have my problem. I worked on and repaired Bosh FI systems for years,(begining in the sixtys) never had issues like this, but then technical manuals and descriptions were readily available. [/SIZE]
 

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At the dealer, we would start with a repair order and configure the Co-pilot to watch the things that we thought might be related to or helpful in determining the failure. We would then install the tool in the vehicle (plugs into DLC under dash) and route the cables away from interference (like feet and pedals). Under the seat and floormat might be good. We would place the trigger button within easy reach of the driver and then would instruct the driver to push the button when it acted up.
The tool will record the 90 second period before the button push and a total of 3 separate recordings can be made. Back at the shop, the recordings are downloaded into the MDS and the graphs covering various sensor voltages, engine and vehicle speed, throttle opening, injector pulsewidth, manifold vacuum, spark advance and many other measurements could be viewed together on the display.
It is usually quite obvious "when" on the graph timeline that things began to fall apart. The diagnostic idea was to then determine what happened, what it affected, what the root cause was and then the best way to repair the problem.
Shop policy should begin with the minimum (diagnostic) fee and then they have to contact you before continuing. They will probably explain what has to happen next and if that sounds reasonable and believable to you, you can decide whether to continue or not. If you have an idea of a maximum that you do not want to approach, discuss that upfront at the service desk with them.
If that only buys a partial answer or a better idea of a starting point of what is happening with the vehicle, then maybe you can figure out the rest of the story yourself.
All information is useful to finding an answer. The Co-pilot is a tool that can help where intermittents are concerned and any other 'real time' or 'freeze frame' data tool just won't help.
 
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