1993 Grand Cherokee 4.0

93 Jeep GC with 4.0 engine. Has always passed smog no problems and then this time it failed with High HC only at idle. The high speed 2500 rpm works just fine. NOx is good too. I went home and replace all the plugs with NGK, new set of wires, cap and rotor and then changed the oil and filter. Also had run that fuel additive Guarantee not to fail. Failed again the HC was even higher...went from 131 to 161...max is 100, average about 17. I have never replaced the Cat or O2 sensor (only has one before the cat). I replace the O2 Sensor and now am thinking I need to replace the Cat. The tech said he could smell the unburned fuel. I did a run from San Francisco to LA keeping the speed at 65mph and got 19 mpg which is great for this car as it is all wheel drive. Runs with power and just performs great. Does not use any oil and has 161K miles.
I am thinking the Cat needs replacing...any ones thoughts?

Thanks

This is my post and it WAS the cat...in fact the cat was totally empty...I mean nothing inside and I replaced the O2 sensor just because. All is well as it passed smog as good as a new car!! HC went from 161 to 1.....so many more miles for this old girl. Now to tackle a loose rumbling sound in the rear area when going over a bumpy road. Just put in new shocks too and noise is still there if not worse. Any ideas? Have looked at all suspension bushings and nothing looks worn or loose. Will have to climb in the back while someone drives and see if it is under the car or some sort of thing inside.

Just to finish up the rumbling thing was a loose shock bolt....not really loose but not torqued up to the max. Now to tackle a anti-freeze smell inside that is a must be leaking heater core....no water anywhere but just that smell. Going to try some barsleak silver.

Just to finish up the rumbling thing was a loose shock bolt....not really loose but not torqued up to the max. Now to tackle a anti-freeze smell inside that is a must be leaking heater core....no water anywhere but just that smell. Going to try some barsleak silver.

It's great you are using the 93 Jeep. Hope everything is safe,
wouldn't want anything to go wrong. Well, the strategy you use
is ok but make sure everything is up to snuff. Barsleak? been there
done that, probable needs a radiator. Well, Enjoy.