Jeep WJ 4.0 Problem

[RockMonkey]

Chicky drives a 2001 4.0 WJ. Yesterday on the freeway about 10 minutes into her commute the engine "stumbled" briefly, and then ran fine the rest of the way home. Today, about 10 minutes into her commute on the freeway the engine shuts off and will not start again. While waiting for rescue she keeps trying to start it. After 15 minutes or so it would start, run for a few seconds, then die. It took me about 2 hours to retrieve my trailer and get back to it to tow it home. When I got there I was able to start it and drive it up onto the trailer (that saved a lot of work...). When I got it home I was able to start it and back it off the trailer just fine. I even revved it up for a few seconds and it ran fine. So, it appears that it runs fine until it gets hot, then shuts off. No check engine or any other idiot lights. What would cause that? My first thoughts are the CPS, or the coil, followed by the fuel pump. I've never had a CPS go out this way, usually when they go, that's it. I had a coil failure once before on a 4.0, and I think it ran/stumbled a little bit a couple times after it was gone, so maybe it could be that. I can't really figure out how a fuel pump could do that, and I've never heard of vapor lock in one of these... What do you think?

 

[rholbrook]

Fuel Pump, problem is, when it goes, it will be when you put a tank full of cold fuel on a hot pump and you'll end up dropping a full gas tank to change it. Happens every time they go out.

 

[RockMonkey]

Fuel Pump, problem is, when it goes, it will be when you put a tank full of cold fuel on a hot pump and you'll end up dropping a full gas tank to change it. Happens every time they go out.

So, you think the fuel pump itself gets hot after a while and stops working? I'm hoping for not the fuel pump. I don't really want to drop the tank... UDOT incident management gave her a gallon of fuel despite being assured that she was not out of gas.

So, how could I test the fuel pump? Let it idle until it dies (hopefully it will die), and then check the fuel pressure?

 

[mbryson]

I had a CPS act like that on my 4.0L XJ. In fact it was NEW one from Autozone.

 

[Corban_White]

Probably not related, but my buddy had an 88 MJ with the 4.0 that had that EXACT problem. We replaced the fuel pump, plugs, wires, cap, rotor, distributor, and everything else I could think of and it finally turned out to be the computer. Good luck, intermittent problems suck.

 

[lewis]

I had a similar problem with my zj. It turned out to be the cap and rotor. It would get hot and take a dump. I would replace it and a new one would only last 3 months until it started doing it again.

 

[RockMonkey]

I had a similar problem with my zj. It turned out to be the cap and rotor. It would get hot and take a dump. I would replace it and a new one would only last 3 months until it started doing it again.

So, did there turn out to be another problem?

 

[lewis]

No, for some reason they would just get too hot and burn them up. I never bought a dealer one to see if it would last. The aftermarket ones were all were the same as far as how long they would last and I finally got tired of it and sold it.

 

[Devel]

my 02 TJ did that when my CPS went out

 

[Marshall]

I had a CPS do exactly as you are describing on a 94 XJ with a 4.0, and I have also had them quit all together and not start anymore. On my 94 it would start and run, and then just die while driving...no matter where the RPMs were. I THINK you can check it by measuring the voltage to your fuel pump. It should come on momentarily when you turn the key on, and then constant once started. If you lose voltage a few seconds in to running then your CPS probably isn't reading correctly. This will also tell you IF you are getting the voltage to your fuel pump when you should be, and if you are and the fuel pump isn't running then there is obviously something wrong with the pump. I am not a professional mechanic so I am sure someone else on here can make more sense of what little information I have given. I would start with the CPS, relatively cheap and easy to swap.

 

[rockreligious]

I had both my ZJ Grand, and My WJ Grand do that very thing, and both times it was the fuel pump, that said while researching for possible problems i found that its common to have CPS do that, I bought a new CPS figuring that I would change it, and if its not it I have a spare CPS since its common to go out, it wasnt it was the pump. (both times)

 

[Bart]

I'm voting CPS, and it may be the wires themselves. I had my 90 XJ do this. I replaced the CPS and it still happened. I fought it for months and finally hard wired the connectors on the CPS. That fixed the probelm.

 

[Gravy]

cps or fuel pump. exact thing happened to my 99.

2001 is distributor-less so it wont be that.

 

[RockMonkey]

cps or fuel pump. exact thing happened to my 99.

2001 is distributor-less so it wont be that.

Good to know. Do you happen to know if the CPS on this 2001 is the same as the one on my 1992 XJ? I happen to have a spare one for the XJ, so if it's the same it would be free to try.

 

[Devel]

Good to know. Do you happen to know if the CPS on this 2001 is the same as the one on my 1992 XJ? I happen to have a spare one for the XJ, so if it's the same it would be free to try.

They should be different, if u do buy one make sure u buy it from the dealer, after i installed a borg warner one it misfired on cylinder 6. then the dealer one and its ran great ever since

 

[RockMonkey]

It's frustrating trying to trouble shoot a vehicle that runs perfectly... I let it idle for 20 minutes. Ran like a champ. We drove it around the neighborhood for a while, made a right turn and it shut off. I flipped it around and starting coasting towards home, and decided to try to start it, and it started right up. Drove around the neighborhood a bunch more and not even a stumble. Then I took it back home and let it idle in the garage, and it shut off twice, but started right back up afterwards. I took it back out on the road, drove it up and down the street, high speeds, low speeds, sharp corners, etc. Nothing. No troubles at all. Got it home and let it idle and it had no troubles at all...

SO. I finally found the plug for the CPS (it's on the other side of the engine, and way lower than EVERY OTHER DAMN JEEP). I tugged on it and it came apart, a little too easily. Of course the Jeep wouldn't start after it was unplugged. I tried to plug it back in and I can't get it to fully engage. It doesn't "click" into place. When I look at it with a flashlight the plug is dirty like it hasn't been fully plugged in for a long time, and I cant see the little plastic nub that should lock it together with the other half. I think there's a good change it's just a loose CPS plug. Once the engine cools down I'll try a bunch harder to get it plugged in fully. If that doesn't work I'll get a new one from the dealership. It looks like that's probably the problem.

 

[mbryson]

It's frustrating trying to trouble shoot a vehicle that runs perfectly... I let it idle for 20 minutes. Ran like a champ. We drove it around the neighborhood for a while, made a right turn and it shut off. I flipped it around and starting coasting towards home, and decided to try to start it, and it started right up. Drove around the neighborhood a bunch more and not even a stumble. Then I took it back home and let it idle in the garage, and it shut off twice, but started right back up afterwards. I took it back out on the road, drove it up and down the street, high speeds, low speeds, sharp corners, etc. Nothing. No troubles at all. Got it home and let it idle and it had no troubles at all...

SO. I finally found the plug for the CPS (it's on the other side of the engine, and way lower than EVERY OTHER DAMN JEEP). I tugged on it and it came apart, a little too easily. Of course the Jeep wouldn't start after it was unplugged. I tried to plug it back in and I can't get it to fully engage. It doesn't "click" into place. When I look at it with a flashlight the plug is dirty like it hasn't been fully plugged in for a long time, and I cant see the little plastic nub that should lock it together with the other half. I think there's a good change it's just a loose CPS plug. Once the engine cools down I'll try a bunch harder to get it plugged in fully. If that doesn't work I'll get a new one from the dealership. It looks like that's probably the problem.

Yea! I bet that's the issue more than anything.

 

[rockreligious]

Yea! I bet that's the issue more than anything.

sounds likely the problem, that wire is the messenger to the computer of what position your motor is, without it the computer doesnt know when to fire, inject, advance, or even run.

 

[Grim]

wow , my wife keeps saying hers is doing the same thing , and every time i drive it, it works perfect. so where is this thing you call a CPS ?

 

[RockMonkey]

wow , my wife keeps saying hers is doing the same thing , and every time i drive it, it works perfect. so where is this thing you call a CPS ?

Crank Position Sensor, and it's mounted to the top of the transmission bellhousing. The wire from it plugs into the wiring harness near the back of the motor. On most Jeeps I've worked on it's on the driver side near the back of the valve cover. On this WJ the plug is on the passenger side behind the oil filter, where it's really hard to get your hands on it to make sure it's plugged in fully.