96 Cherokee Rich smelling exhaust

[brsexton]

Hello all. I have a 1996 XJ 4.0L Auto 4x4 with 162000 miles. It runs very good, but has a very strong rich smelling exhaust. It smells like it is running really rich and is very strong at idle. However, the Jeep doesn't drive or idle like its over rich. It drives and runs very good. It did have a crack in the exhaust manifold which was fixed, but that didn't help the smell issue. Was thinking the fresh air was tricking the O2 sensor, but nope. There is NO check engine light or codes. All of the ignition looks like it is fairly new. The wires and plugs look to be in good shape. I have ran 2 bottles of fuel injector cleaner. I am at a loss as to what to do next. I just don't think its much because it runs so dang good. It just stinks really bad. I just had the computer flash and cat inspection recall performed and the cat was fine. Any help would be great.

[fredjacksonsan]

First, if it's running really rich then your MPG will go down the tubes. What mileage are you getting?

Second, I assume you're smelling the exhaust after it's warmed up....if you're checking it when it's cold, then it WILL be rich.

Third, if you have a bad O2 sensor, it won't necessarily throw a code, but when they fail, they fail in the rich position so your vehicle will continue to run. If you're getting poor mileage and the inside of the tailpipe is wet and black (but it's otherwise running fine) then I'd guess O2 sensor.

[brsexton]

Originally Posted by fredjacksonsan

First, if it's running really rich then your MPG will go down the tubes. What mileage are you getting?

Second, I assume you're smelling the exhaust after it's warmed up....if you're checking it when it's cold, then it WILL be rich.

Third, if you have a bad O2 sensor, it won't necessarily throw a code, but when they fail, they fail in the rich position so your vehicle will continue to run. If you're getting poor mileage and the inside of the tailpipe is wet and black (but it's otherwise running fine) then I'd guess O2 sensor.

Yeah, I am getting pretty bad MPG's I only get 200 miles to a tank of gas. Not sure what that is, but it seems pretty low to me. I haven't had this Jeep very long, so I can't say if that is normal or not.

Yes, it stinks very bad when warm. It seems to be really bad at idle after I have been driving, so yeah it does do it after warm. I would think an O2 sensor that far off would throw a code, but it does seem to point in that direction. Is there any way to test an 02 sensor?

[fredjacksonsan]

Sounds like you're getting only 10 or 11 mpg...even in town, that's below what you should expect (16 in town and 20/21 highway are more usual).

I've seen numerous posts where a bad O2 sensor didn't trigger any codes - so that's still my thought. There are ways to test the voltage to/from an O2 sensor, do a search in the forum and you'll find the posts. I don't know what the voltages should be.


There may be 2 O2 sensors on your model, one above and one below the catalytic converter.

[brsexton]

Originally Posted by fredjacksonsan

Sounds like you're getting only 10 or 11 mpg...even in town, that's below what you should expect (16 in town and 20/21 highway are more usual).

I've seen numerous posts where a bad O2 sensor didn't trigger any codes - so that's still my thought. There are ways to test the voltage to/from an O2 sensor, do a search in the forum and you'll find the posts. I don't know what the voltages should be.


There may be 2 O2 sensors on your model, one above and one below the catalytic converter.

Wow, that is very low. But I sort of knew it couldn't be good as rich as it smells. My Jeep does have 2 sensors, but I have read that the one in front of the cat is the one that makes the adjustments in the fuel. The one past the cat only checks the gases exiting the cat. So I assume if I replace the O2, that I will only replace the front one. Thanks for you help and I will search on how to test the sensor.

[fredjacksonsan]

Good luck with it, let us know what your mileage gets up to afterwards.

[sector95]

Replace both sensors. The cost is not that great for the sensors so why go through all the trouble just to replace one only to find out you needed to replace both. Normal lifespan for an O2 sensor is about 100K miles. If yours are original they are well past their prime....

MPG= mileage traveled divided by gallons used.

BTW, this strong smell doesn't happen to have a somewhat rotten egg oder to it does it?

Mike
00XJ