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My Jeep died on me. Runs fine cold but bucks, misfires, and dies when hot. Pls help!

2K views 19 replies 5 participants last post by  jtinglof 
#1 ·
This is a 1990 Jeep YJ 2.5L TBI. It has a relatively new rebuilt engine.

Over the past few weeks I started experiencing misfires and bucking while driving, usually in 2nd or 3rd gear, and only after I had driven it enough to reach operating temperature. It started getting worse so I replaced the spark plugs, distributor cap and rotor, and checked the spark plug wires, but nothing helped.

Yesterday I drove 10 minutes to the store completely fine, and 30 seconds after leaving the parking lot it bucked a few times and completely died. I pulled off into another parking lot and tried to start it back up and the RPMs went crazy for a few seconds as it tried to stay running but quickly died again. I waited 5 minutes for it to cool down a little and it started fine and ran long enough for me to drive around the parking lot to see what was up. It bucked, misfired, and died again. I limped home by waiting 10-15 minutes to let it cool, driving a couple minutes, and pulling over is it died.

I was really worried so I searched around on the forums and found that a bad coolant temperature sensor can cause these problems. I just pulled that and tested it by putting it in water on the stove and bring the water to a boil. The resistance steadily decreased from about 1.9 k ohms to 0.18 k ohms which seems normal from everything I've found online.

Now I'm worried again, and have no clue what the issue could be. Anybody have similar problems that they fixed or any advice at all?
 
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#2 ·
I took out the crankshaft position sensor after doing some more research and thinking it could be thermally failing. It was pretty nasty and covered in grease so I cleaned it and put it back in because I saw that worked for a few people. No luck, it just died the same way as I was testing it out. Drove fine for 6 minutes. Turned it off for a few minutes and then drove for another 2 minutes before it cut out. I guess I will order a new CPS in case it's still busted.

Anyone have any other ideas?
 
#4 ·
I just replaced the CPS and it didn't help. It seems to die when I'm engine braking. I just test drove it and it accelerated ok I guess, it was a little slow, and then I came up to a stop sign and let off the gas to slow down and the RPMs just dropped and it stalled out. I don't know if every time it's died has been in that way though.

This is my daily driver and I just graduated college so I'm almost broke. I know a bit about cars but not enough to figure out what could be causing this. I would bring it in to my mechanic but I don't think it would make it and again, money is tight. Please, any thoughts on where I should check next would be much appreciated.
 
#6 ·
I put in a new fuel filter and the old one was in bad shape. I thought that fixed the issue. I took it on a few short test drives and it seemed better but after a slightly longer drive it still jerked a few times and the RPMs dropped like it wanted to die, but it never completely died. So the filter seems to have mostly fixed it but there's still something wrong. Where in the fuel system should I check next? I have never used a fuel pressure gauge, but I guess I will look into that. The IAC is fairly new.
 
#7 ·
I am thinking either the fuel pump or ignition coil is next. I read that the fuel pump can fail from trying to push too hard through an old fuel filter. Since replacing the filter seems to have helped a lot, I assume it was a bad filter. So it's possible the pump is failing, but is it possible to sporadically go out, only when hot? The only thing is I don't want to drop the gas tank if I don't have to, and I'm tired of throwing money at parts that don't fix the problem. I also read that the coil can get too hot and screw up the sparking process. I guess I should check out the fuel pump. Is there any way to visibly check if the pump is bad or something without just buying a new one?
 
#8 ·
You start by checking the fuel pressure, the pressure gauge will either screw into a fitting on the fuel rail line or go inline with the pressure line, sounds to me like a fuel pump issue
 
#10 ·
Good to know about the distributor, thanks. I'm sort of stuck at the moment. I looked all around town for the special fuel pressure test fitting that I need to connect to the pressure gauge and nowhere has it, even the Chrysler dealer which the Haynes manual says is supposed to have it. I found a link for one at O'reillys but there isn't one near me so I would have to order it.
 
#12 ·
I have been having the same problem with my 98 2.5. I just pulled and cleaned out my crank position sensor and it seems to me okay after a test drive. I had the distributor problem with GM and Ford distributors before with condensers in them. So I think these Jeep one are a different set up.
 
#14 ·
I just took it out on another longer drive to really test what makes it happen. It seemed to drive at a constant speed and accelerate mostly fine, but it was a little hesitant. What really makes it happen is when I'm in 3rd gear above 2000 RPMs and let off the gas to engine brake. It immediately started to slow down faster than usual and it started to misfire and lurch. I took it out of gear and coasted as I pulled over along the curb in case it died and it acted like it wanted to die but caught itself and then idled normally. Then I could put it into first and drive off normally. Before the new fuel filter it would do this same thing but much more violently and then it would quickly stall out. It hasn't stalled yet since the new filter.

Also recently the exhaust, like near the tailpipe, sounds more gurgly, it sounds like a boat in the water. Not sure if this has anything to do with my problem or if it's just an exhaust leak.

Any ideas on which is most likely the culprit? Right now it sounds like it's either the fuel pump, ignition coil, or distributor. I can fix the first two, but I wouldn't mess with the distributor, I would have to take it to the shop.
 
#15 ·
I also just wondered if it could be the new CPS that's causing this. I replaced my old one and then shortly after, replaced the fuel filter. Maybe the fuel filter fixed it and now the new CPS is causing a different problem. I ordered a cheap CPS off eBay, and the seller didn't even list which brand it was. It was only $10 but it looked just like the original so I thought it would be fine. I know sometimes these cheap parts cause issues in old Jeeps, even brand new.
 
#16 ·
Ok so I swapped back in the original CPS and it seemed to run fine on a short drive. I took it out again today on a similar drive. Drove for 20 minutes, turned it off for 5 minutes, and drove home. It died while idling once and then sputtered a few times on the way home before dying as a pulled up to my house. I was able to get a video of it trying to run once I got home.



Once I let it cool it idled normally again. So it does seem like the fuel filter made it at least more intermittent. This looks like it is most likely just starving for fuel right? But can the fuel pump only go out like this when its warmed up? If not fuel then is it spark? Again I guess it could still be the ignition coil or distributor? Anybody experienced something like this? I'm just in tight spot because I start work Monday and this is my DD but I don't have much money until I start the job so I don't want to keep trying parts that don't work.

Thanks for everyone's assistance so far!
 
#17 ·
Welp. I have up and took it to my mechanic. I didn't want to mess with it anymore since I'll need it to get to work. The crazy thing is made the 35 minute trip to the shop tonight without any problems. So that means it only occurs when I turned it off and started it back up while it was still hot. That sounds like to me it has to do with the ignition coil. Thanks for the advice guys even though I wasn't able to figure it out. I'll update with what the machines finds out.
 
#18 ·
Something that caught my attention is you mentioned the exhaust has a gurgley sound. Sometimes baffles break free in the muffler and partially block exhaust flow. It can be intermittent. That condition fits the description of your symptoms too. Hope the shop comes back with a good repair for you!
 
#19 ·
My mechanic just let me know it is the ECU under the dash above the heater. I think he's also replacing the EGR valve and doing a couple other small things but all in all it will be about $900...I did not expect it to cost that much but oh well :(
 
#20 ·
Are you still having the issue? I have a zj with a 5.2 I know it's a different motor and model, put I just fixed that same problem with my jeep today. If you put a new temp sensor in and had no luck change the temp switch. my jeep was doing the same thing. The temp switch was telling the computer the engine was cold. When a motor is cold The computer feeds more fuel to the engine. So if the switch is bad it could be giving the computer a false reading. So it is being feed to much gas causing the engine to stall. Try to change that temp switch, it fixed mine. I changed the sensor and it didn't fix it but the switch did.

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