Red Hot Cat

I've got a 1990 xj. It started running really rough a couple of days ago and there is an odd smell coming from the exhaust. The cat appears to be getting extremly hot aswell as the rest of the exhaust system. I checked for vacuum leaks and I did not find any. I did find that the rpm would increase if I created a large vacuum leak, i.e unplugging the vacuum hose that goes to the brake booster. I cleaned all of the spark plugs and checked the cap and rotor. The spark plugs were all black and sooty with a couple of them being wet with gas. I noticed an odour on the wet plugs similar to the odour at the tail pipe. I'm thinking i've got some bad gas. This vehicle has been sitting awhile with only starting and moving in the driveway.
Well, I put in some fresh gas and I took it for a drive hoping to remove any bad gas it the line but the engine lost power after about 5 minutes. I turned around and headed for home and when I looked under the jeep, the cat was red and the undercoating was on fire.I threw snow on it until the fire was out.
I have changed the cat and muffler. I started it and the cat was red in about 4-5 minutes. I'm sure it's running rich. If I were to unplug the O2 sensor, would that help or hinder. Is there any other sensors that could cause this problem? It is also backfiring through the exhaust a bit. This could be a timing issue but would timing cause the exhaust to get that hot?

Bad MAP sensor?

I'd go with MAP too, I....I mean someone (lol) disconnected mine(long story) and my cat got hot as hell, in fact it was so hot it made the carpet steam on the pass. and and was blowing black smoke. fixed my....I mean their mistake and all was good again.

Stuck open injector will also dump raw fuel in the cat.

Bad MAP signal or O2 sensor.

Kyung

Thanks for the input.

A plugged up cat will do it also, had a 89 Grand Am that the cat was plugged and it looked like it just came out of a steel blast furnace, cherry red.

McCoy, you can test the O2 sensor to confirm or rule it out.
Titania O2 sensor thread
This thread explains the procedure.

Bad pre cat exhaust leaks will allow air to enter the system and let the 02 sensor think its running to lean. this will cause it to richen up and load the cat excessivly causing abnormal heating and failures.

Or a miss created by anything else can cause this condition for example burned valve, bosch platinum plugs, plug wire or wires, or a number of other things. Sounds like unburned fuel goin down the pipe to me.

If you could smell and see fuel on some of the plugs, then those plugs most likely are not firing. Check the wires and cap.

check the throttle body positing sensor i had that happen and it made my jeep run really rich and the cat got super hot...... my 2cents

McCoy, you can test the O2 sensor to confirm or rule it out.

Click to expand...

My voltage doesn't switch, It stays around 5v. The engine probably didn't reach operating temp though, because the cat was glowing and I shut it off.

It turned out to be two bad spark plug wires. Thanks for the help, I learned a lot about the XJ.

Who said missfire first huh? who had plugwires in thier post? anybody keepin tabs ? pretty good info comes out of the redbeer laboratry..................You are very welcome....its just part of what I do...........

Yep, you nailed that one Majic_Tech. Thanks again.