1999 TJ cutting out & other gremlins

Buddy Lee

Well-Known Member
Joined
Jan 16, 2007
Location
Knightdale, NC
Hi All, I hope someone can help me with some issues I'm having with my '99 TJ Sahara. It runs great 99% of the time. But then it randomly has odd issues. This started about a month or so ago prior to that it had no issues and had been running great.

First issue is that occasionally while I'm driving the engine will cut off for 1-3 seconds and then it goes back to normal. It has done this on the highway doing 75mph and on local roads going 20mph. I have not been able to find a common cause. When it cuts out the CEL will always come on and sometimes other lights as well. Mostly the lights go back off but, one time the anti-theft light stayed on and another time SRS light stayed on until I restarted it.

The second issue which may or may not be related but started around the same time is that when I try to back up the engine will try to stall out on me and even on flat ground I'll barely be able to back up. When this happens I end up having to rev the engine pretty high and slowly feather the clutch to get it to back up. Sometimes it will sputter and pop during this. If it stalls during this time I typically have a hard time getting it started again.

When this first started I did some searching and people talked about bad grounds or battery connections. My battery was pretty old anyway so I replaced the battery, cleaned all the terminal connections and also checked and cleaned all the engine and body grounds. Due to the anti-theft light coming on I thought it might be my transponder key so I switched keys but still got the same results.

Does anybody have any thoughts on what the issue could be?
 
Dumb question.
when it cuts off, is the engine shutting off or are all the gauges just dropping to zero ?
 
If the CEL has been on, there should be codes.
 
Ron said:
Dumb question.
when it cuts off, is the engine shutting off or are all the gauges just dropping to zero ?
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The guages stay on. it's like someone just turned the engine off and right back on. I'm thinking it might be the PCM. But was hoping it could be something simpler.
 
Not sure on the TJ but in my ‘97 cherokee it did a very similar thing and it was the CPS. Swapped it with a mopar unit after having bad luck with like 4 different parts house ones and ran perfect.
 
skyhighZJ said:
Not sure on the TJ but in my ‘97 cherokee it did a very similar thing and it was the CPS. Swapped it with a mopar unit after having bad luck with like 4 different parts house ones and ran perfect.
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The CPS was replaced about 4k miles ago. I suppose it could be going bad again but not sure how likely that is.
 
Inspect the wiring at the rear (downstream) O2 sensor. Many times they come dislodged from their plastic clip mount and the wires will make sweet love to the driveshaft or converter heat shield. If the 5v reference wire shorts to ground, what you're describing happens.
 
getstucksome said:
Same here on a 97 tj. intermittent on/off on the highway and no codes in the computer. Replaced the CPS and all issues went away.
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It's been replaced in the last year, but I still have the old one since that didn't end up being my issue. So maybe I'll put the old one back in this weekend and see if it makes a difference.
 
Jody Treadway said:
Inspect the wiring at the rear (downstream) O2 sensor. Many times they come dislodged from their plastic clip mount and the wires will make sweet love to the driveshaft or converter heat shield. If the 5v reference wire shorts to ground, what you're describing happens.
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I was feeling optimistic about this one. But everything looks nice and tidy under there.
 
Did you replace the CPS with a Mopar unit? Parts store and knock off ones will act up bad.

Bought an 03’ TJ and it didn’t run. Wound up being the fuel pump, but the PO has replaced the CPS with an eBay unit just throwing parts at it cause he didn’t know what was wrong. When I got it back running, it ran but very bad. I thought it was bad gas from sitting, but decided to put the original CPS back in for good measure (he’d warned me it was a cheap unit) and soon as I did that it ran amazing.
 
77GreenMachine said:
Did you replace the CPS with a Mopar unit? Parts store and knock off ones will act up bad.

Bought an 03’ TJ and it didn’t run. Wound up being the fuel pump, but the PO has replaced the CPS with an eBay unit just throwing parts at it cause he didn’t know what was wrong. When I got it back running, it ran but very bad. I thought it was bad gas from sitting, but decided to put the original CPS back in for good measure (he’d warned me it was a cheap unit) and soon as I did that it ran amazing.
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It was whatever Advance had in stock. So probably not Mopar. I'll put the original one back in this weekend and see.

I'll be replacing the fuel pump soon also. Not because it's bad. Just because I'm replacing the rusty skid and figured if I'm dropping the tank...
 
Buddy Lee said:
It was whatever Advance had in stock. So probably not Mopar. I'll put the original one back in this weekend and see.

I'll be replacing the fuel pump soon also. Not because it's bad. Just because I'm replacing the rusty skid and figured if I'm dropping the tank...
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Yeah that very well may be your problem. TPS and CPS, among many other sensors, should really only be bought from the dealer or some other means of OEM quality. I have seen too many parts store sensors like that fail.

As for the fuel pump, not a bad idea. Mine actually still worked, which is why the PO had such a hard time figuring out why the jeep didn't run. The flexible tube from the pump, thru the assembly had actually broken from being brittle. It would come on with the key and cycle like it should but wouldn't build pressure. the PO had actually cut an access hole to reach the wires to the pump so I didn't have to deal with dropping the tank. I made a put a panel over the area.

Anyways, I rambled on to say replacing the pump while you're there isn't a bad idea. Consider putting a Bosch in it. It's costly, but reliable and quiet compared to the $60 Amazon unit I put in. Man that thing is loud.
 
Update, swapping out the CPS did not change anything. Right now it seems to be most noticeable when I'm in Reverse. I'm not sure is so different about that. But if I'm nose down in a parking space it's almost impossible for me to back out of it. The Jeep will start right up and run smooth but as soon as I start to back up under load the engine will cut out and stall. Sometimes, depending on how steep the incline is I can hold it at about 2-2.5k rpm and feather the clutch and I can manage to back up but, the engine will sputter and pop and try to stall out the whole time. Going forward it does not do that. On flat ground it is ok most of the time.

So, to test this today I backed into the drainage ditch at my house and tried to back out the other side but it would cut off on me each time I tried. When I put it in 1st gear it would drive right back out however.

I'm thinking it has to be a short or bad connection somewhere or possibly the PCM which I'd rather not have to replace and have to deal with reprogramming etc.
 
Well, I at least figured out where my issue is coming from now I just need to figure out how to fix it. It appears that it is related to the PCM which was the one thing I was hoping it would not be because I can't seem to find a good affordable source for those.
I don't think we can upload videos so I put it on youtube. You'll need audio but in a nutshell. If I rotate the PCM towards the passenger side the engine will stall. I guess that is why it would do it in Reverse but not in forward because the body rolls in different directions.


Anybody have a good source for a replacement PCM? Mine has the chipped (SKIM) key so I don't think I can just use any old junk yard one w/o having it reprogrammed.
 
Makes me wonder if a wire in the loom is cut and shorting when it rotates..the pc board itself doesn't change when it turns
 
Ron said:
Makes me wonder if a wire in the loom is cut and shorting when it rotates..the pc board itself doesn't change when it turns
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I thought about that too. Before unscrewing the the PCM I shook, tugged and jiggled every harness and nothing happened. I know PC boards does not change but they can have bad solder joints or broken corroded internal connections. Lot's of stuff happens to electronics after 20 years of heat cycling and vibrations.
 
I have a computer out of a 99 with a 4.0 and an auto. Your more than welcome to it if you can use it. The Jeep it came out of had the sentry key, not sure if that makes a difference. @Jody Treadway might know?
 
Pless said:
I have a computer out of a 99 with a 4.0 and an auto. Your more than welcome to it if you can use it. The Jeep it came out of had the sentry key, not sure if that makes a difference. @Jody Treadway might know?
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Thanks, from what I've gathered off the interwebs and experience with other cars. The sentry keys need to be programmed into the PCM/ECU so if you switch computers then you need to have the keys reprogrammed. Which unless someone has a computer that can program the module can only be done by the dealer. I'm not sure if the automatic would make a difference. I know on newer electronically controlled cars it does. But I have no idea about the TJ's.
 
Buddy Lee said:
Thanks, from what I've gathered off the interwebs and experience with other cars. The sentry keys need to be programmed into the PCM/ECU so if you switch computers then you need to have the keys reprogrammed. Which unless someone has a computer that can program the module can only be done by the dealer. I'm not sure if the automatic would make a difference. I know on newer electronically controlled cars it does. But I have no idea about the TJ's.
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I'm pretty sure you need a manual computer if you have a manual. Otherwise it will throw a CEL trying to lock up the convertor.
 
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