O. K. don't laugh, I have a 1991 cherokee with 253,000 miles showing, it runs great has no leaks doesn't use any oil, and it drives great, however!!!!!!!!!!!!! it does have a rich exhaust smell and the tail pipe is smutty black. occasionally is does develope a slight miss when idling, but clears when you rev the engine slightly. I'm thinking with that many miles it needs injectors for the miss but i don't think that's my rich exhaust problem.
check engine light has come on but goes off. Maybe Ox sensor??????? It still gets about 17 MPG but of coarse I'd like to get better. It is a 4x4

 

Describe the smell more. Does it smell like eggs, or is it just a typical exhaust odor but stronger?

If it smells like a strong exhaust type odor, then you may just need a new muffler or it maybe a clogged cat.

If your check engine light is coming on occassionaly, I would lean more towards a clogged cat. If this vehicle is fed emissions, there is a O2 sensor right past the cat.

17mpg is not bad at all for a Jeep 4x4. I had a 01 Cherokee with 4.0l and was lucky to get 17 on highway,

 

It sort of smells like the old carbs when they were adjusted to rich to the point where they almost would smoke black soot. It doesn't smell like rotten eggs at all. Will it run o.k. without the cat?

 

It is illegal not to have a cat. Some people gut them out anyways if they don't have state inspections. When they do this, they have to get a O2 simulator so the check engine light doesn't come on.

 

Golly, I didn't know you could have a 4.0 without oil leaks

I have a 90 cherokee with 4.0 and I struggle to get 17 mpg, I average about 15 mpg on winding/hilly rural roads.

Your mileage would indicate it may be time to replace the cat convertor. There is a test you can preform to check the cat but I can't remember what it was. A plugged cat will also result in loss of power.

 

Well if there is a leak it doesn't make it to the ground, maybe it blows back and smears (ha).........Anyway the jeep has plenty of power, that's why I never considered the cat as the problem. My county doesn't have inspections so i may just take it out of the system and see what that does.

 

Remember, if your Jeep is fed/cali emissions equipped, and you remove or gut cat, you will most likely have to get a O2 similator for that sensor that was downstream of the cat or else your CEL will stay on.

 

When was the last tune up done ?
Could be simple maintenance.

 

would start by checking codes cycle your key on and off 3 times leaving it on and count your check engine light flashes should end with a 55.
a 91 model shouldnt have an o2 sensor downstream from the convertor but it will have one upstream from the cat and if it has a code set for it it may be all thats wrong dont really think your cat is causing your problem.