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Inconsistant Check Engine Light

B

bjoehandley

NAXJA Forum User
Location
WeGo, Chi-Town, Ill
My '98 Limited (4.0l/AW4/242 @ 140k miles) has been having the check engine light pop on every so often then going out at what might be random times, usually comes on when the fuel gets low and shuts off when the tank is full, but when I had the code pulled it the last time, it came up P038. So far as In know, the gas cap and exhaust are 100% factory installed including sensors, gaskets, and clamps.

Anybody have any ideas as to what's happening?
 
Download the exact trouble code with an OBD-II code reader. It will be a 4 digit code with a P in front. Generic OBD-I two digit codes are useless for troubleshooting.
 
Was that a P0138?
 
Ah, I remember it well! P0138-O2 Sensor. It's why I got rid of our '98 ZJ. It all started with a bad fuel pump. Plastic tank. Dealer insisted Sunoco gas was dissolving the tank. Sunoco said that it was bacteria that live in gasoline, clogging the screen-sock on the pick-up. Snotty attitude from dealer, wouldn't honor the GOVERNMENT MANDATED 8 year, 80k mile emission warranty, wouldn't even look to see if there was a problem. Told me I needed to give them $140 to diagnose the problem. All the ECU needed was a flash. I bought a code scanner, and kept it in the car, and even my wife would hook it up when the CEL would come on! I told this to the Subaru dealer when I traded it for our Outback. They flashed it, and I recently, met the current owner, who loves it! I'll never spend another cent with that Jeep dealer again! Bought the Subie, and haven't looked back...Although, I'll keep my XJs!
 
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Yea it was P0138 (knew that didn't look right!), Thanks Joe

Is it possible the dieing muffler could be the cause of this? It is the factory piece and in bad shape. I don't know much on the OBD2 stuff and kinda wish it worked a bit more like the old electronics in the Lebaron and Shadow I had (three key twists gave me a code and I got those off the internet!) Weird part is the on/off all on it's own bit, that's what is annoying the crap out of me. The truck usually runs like great at the same time, sometimes stronger than normal too which confuses me about it too I do usually keep the gas cap nice and tight as well.
 
I would check the harness at the downstream CAT o2 sensor, check for anything frayed.
 
I'd do that, hopefully the weather will cooperate Monday, seems like it's either raining or I have a bunch of other stuff to do on my days off that are more important than anything else I might have in mind for my days off:smsoap:
 
Do you know if the O2 sensor has ever been done? If not, at 140k, your car is CRYING! Ten minute job, great payback if you need it. My '92 was doing 15-16 MPG highway, now can do 23! Your results may vary.
 
heyhar said:
Do you know if the O2 sensor has ever been done? If not, at 140k, your car is CRYING! Ten minute job, great payback if you need it. My '92 was doing 15-16 MPG highway, now can do 23! Your results may vary.
Click to expand...

While replacing the o2 sensor on a regular schedule is best, P0138 indicates the downstream CAT sensor, and is indicating a short, so it is time to test before replace.
 
I was under the impression that P0138 referred to the 'Upstream', or the first sensor, the one that controls the mixture, not the one in or after the cat, which merely indicates the effectiveness of the cat. When I was chasing the same code in my '98, I got to where i would drive with the scanner plugged into the port. If it threw a code while driving, you could actually feel a difference in performance, and the MPGs would drop, according to the display in the OHC. Reset the code, and it would perk up, and the mileage would increase. It's been almost seven years since I've had to play this game, so I could be a little fuzzy on it...
 
I bought the truck with almost 54k on it on it back on '03 and so far as I know, literally the whole exhaust system should be what it came from the factory with other than a clamp holding the tail pipe to the rear hanger (weld on that broke years ago, so I just clamped the two together!) I figure it's got to be close to the time to swap at least one of them, I just wish they weren't so damn expensive. Just one of the sensors for my truck costs nearly as much as the sensors did for both of the K-Cars I mentioned above:(
 
The whole code thing makes it sound like it's the front sensor, but when I searched for that on here a week or so ago, it sounded like it was more of a wiring problem with the rear sensor than anything else!!!1
 
Ok, post up what you find.

Good luck.
 
One other thing that I had forgotten about, right before this happened we had gotten a ton of rain right over a night or two and ended up doing a couple "water crossings" trying to get to work and around town and a couple says later the light started coming on.


I'm hoping I'll get to look under it tomorrow for that and to measure how long the muffler is (I'm thinking about throwing a Thrush 25" welded muffler under it), hopefully it won't rain..................again:soapbox:
 
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I know it's been nearly a year, but I've finally and hopefully permanently fixed the check engine light by having a new cat (2.5" Magnaflow) and a new O2 sensor when I had the stock muffler replaced (2.5" Magnaflow there too!)
 
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