Need help re. Jeep catalytic converter...

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buffoni88

buffoni88

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I know there are Jeep forums, but I don't want to go creating another account somewhere else, plus I know all of you have tons of knowledge...so here I am. Here's my issue: My 1996 Jeep Cherokee (4.0, in l6) has been getting absolutely HORRIBLE mileage lately. As soon as I hit 160 miles, the light comes on (when I refill, I put about 15 gallons in). Roughly 10 mpg on average. I have replaced spark plugs, air filter, fuel filter, and just upgraded my fuel injectors today with the Neon ones (4 ports instead of the stock 1 port). I just filled up, so I'll see if it adds to my mileage, but I want to be prepared. So here's where I need help.

a. What else can you think of that might be causing such horrible mileage? I used to get around 300 miles to the tank, on average.

b. I am stationed in Delaware, but my Jeep is a California original vehicle, with a CA emissions catalytic converter. I haven't gotten it pressure tested yet, but if it is bad, and I were to put a non-legal CA cat on it, with the 02 sensor port, would it throw a code? Would it negatively affect performance? And what if I were to get stationed in CA again, would they make me change it, even though I'm military?

Thanks ahead of time...sorry for the novel. It's driving me crazy putting so much gas in this thing!
 
Wish I could help, I have a 96 4.0L. Get 20 mpg most every tank till it get cold outside then around 18. My converter is starting to rattle quite a bit but, I'm living with it.
My daughter also had a 96 about 7 years ago and there was a recall on the converter.
I would surely think that if it's getting plugged, mileage would go way low.
 
Consult a local exhaust guy, As a Ca.guy, don't see a problem with replacing a Ca. catalytic, with a 49 state. ( As long as you are not living in California, I wouldn't sweat it.) I don't think , the catalytic converter is the problem, here. Have you done a OBD 2 scan, for codes yet ? Is the check engine light on, or has been on?. O2 sensors, been changed?
 
Abodybomber said:
I don't think , the catalytic converter is the problem, here. Have you done a OBD 2 scan, for codes yet ? Is the check engine light on, or has been on?. O2 sensors, been changed?
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With the mileage down about half from before, i guess it could be a clogged exhaust, as I had a Ford work truck with that issue before.....although when it heated up all the way it wouldn't get out of it's own way and required almost full throttle to keep up with traffic :D.

Like "bomber" alluded to, your problem sure sounds like the ECM is pulling timing out for some reason to me.
 
I'm in Kalifornia and replaced a CAT with an aftermarket one. Never had a problem during any smog test after that. Truthfully the aftermarket one passed the test better than the original one.
 
If it's running rough, try checking the bulkhead connectors in the engine compartment firewall. My son's Cherokee was running like crap and we did the tune-up thing and still crap. Took it to a more specialized mechanic and he found that the bulkhead connector was not getting a good connection.
 
OldmanRick said:
With the mileage down about half from before, i guess it could be a clogged exhaust, as I had a Ford work truck with that issue before.....although when it heated up all the way it wouldn't get out of it's own way and required almost full throttle to keep up with traffic :D.

Like "bomber" alluded to, your problem sure sounds like the ECM is pulling timing out for some reason to me.
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Ricky, pretty how much how it looks. I won.t say anything, until I see a scan. he is looking for a Jeep tech, on the short notice.
 
Abodybomber said:
Consult a local exhaust guy, As a Ca.guy, don't see a problem with replacing a Ca. catalytic, with a 49 state. ( As long as you are not living in California, I wouldn't sweat it.) I don't think , the catalytic converter is the problem, here. Have you done a OBD 2 scan, for codes yet ? Is the check engine light on, or has been on?. O2 sensors, been changed?
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I forgot to mention that...I have already changed both 02 sensors. I drove it from CA to Delaware last year. Check engine light did come on, it was for a bad 02 sensor. Light went out, a year passed and I finally changed it when my mileage dropped.
 
I realize this is probably a stupid thing to ask, but you have checked the air filter right?
 
If the temp sensor, not the sender, is telling the computor it is not warmed up it could use more fuel.
 
TrailBeast said:
I realize this is probably a stupid thing to ask, but you have checked the air filter right?
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Forgot to mention that also. I did change the air filter as well. I'm going to go to auto zone to see if any codes show up. I just wanted some out of the box ideas for something else that could be the problem.
 
4.0s are not known for great mileage. My '01 Cherokee averaged about 16 mpg on a good day.

The cat is likely not the problem. You would have a MIL on and if it was in really bad shape, it would be flashing. If there currently is no MIL on, then there is no pressing issue with any emissions-related equipment. You might want to have someone check you fuel usage map with a scan tool though.

O2 sensors do not produce codes by themselves unless it's a heater problem. Codes usually appear when there is extra air in the system from an exhaust leak or intake gasket creating a lean condition. The computer compensates for the lean condition by - you guessed it - dumping more fuel.

Low mileage could be do to any number of things including tires, driveline/trans issues, worn out engine etc. Check for an exhaust or intake leak. Clean the throttle body and run some fuel system cleaner through it, see if that helps any.
 
No need to go to a parts house for a code scan on a Mopar. Cycle the ignition key on-off-on-off-on (three times without cranking engine) and the codes should display on your odometer.

A 4.0 in an XJ with reasonable sized tires and gearing should be getting 16-20 MPG. You definitely have a problem. If you have over 100K miles, I would suspect a clogged cat and/or muffler especially if your engine uses any oil at all. Your model year does not have an O2 sensor downstream of the cat so you would not get a code if it was clogged, missing or if the guts had magically fallen out. You would notice a power loss above 3000 RPM if it were clogged (or a slight gain if the guts fell out, not that I would suggest such a thing). Your coolant temp sensor or MAP sensor could also be a cause. This sounds like O2 sensor or cat to me though. (DON'T use Bosch O2 sensors BTW)
 
Just did the on-off thing, the only thing that came up was 12, which is the battery disconnected in the last 50 cycles. With the new injectors installed yesterday, I've driven about 50 miles so far, and the gauge hasn't moved yet...so it's looking somewhat promising. My next move will probably be the cat. I drove with a bad 02 sensor for a year, so I'm sure something got messed up with it during that time.
 
buffoni88 said:
Just did the on-off thing, the only thing that came up was 12, which is the battery disconnected in the last 50 cycles. With the new injectors installed yesterday, I've driven about 50 miles so far, and the gauge hasn't moved yet...so it's looking somewhat promising. My next move will probably be the cat. I drove with a bad 02 sensor for a year, so I'm sure something got messed up with it during that time.
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If you ran too rich, you may have some unburned gas in the exhaust which can cause the O2 sensor to fail. Try cleaning it off, or a new one if cleaning doesn't help. Gas can puddle in the cat and make it fail early also .
 
-Do you recall ever experiencing spark knock(pinging)? If, for what ever reason, the computer has pulled out timing,and hasnt put it back yet, that would definitely negatively affect fuel mileage.
-So will a too-cool coolant operating temperature, ie, a failed t-stat.Or a slow MAP sensor.
-Or short trips, or soft tires,bad alignment,extra weight,elevated ride height,driving style, driving below TC lock-up rpm,dragging brake calipers, extended idle times.....
 
You could pull the valve cover pretty easy on those and watch all the rockers to see if they are all moving full travel. A flat cam lobe on the exhaust can kill mileage.
 
I once had a 1992 XJ with a 4.0 and 5 speed. At best I would get 22-24 mpg on the freeway.

I sold it to friend with 250k miles on the odometer, it was still getting about 19 on the freeway at that point.

It did have an injector problem once years ago the mileage dropped to about 11-12 average before I replaced it. Then the O2 sensor went, it ran like crap and stunk really bad, never did calculated the mileage as I quickly changed it out. So I can't tell you if it would affect mileage or not. (which I'm sure would by the way the exhaust stunk)
 
10mpg.....uhhh ya....thats terrible.

Catalytic Convertor....and be done with it.
That is WHY they are there......they make Vehicle Buyers out of Ordinary People...since 1974.
Just like FORD's....making Mechanics out of Ordinary People since 1909.

ASE Professional Technician.....20 years experience.

Prove me wrong......:violent2:
 
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