Am I reading this wrong?? (cat question)

[AL Deville]

I`ve been reading up on catalytic convertors, and ran across this.

"If the outlet temperature is a lot hotter (more than 500 degrees F) than the inlet temperature, it indicates unburned fuel in the exhaust. The most likely cause would be ignition misfire (fouled spark plug, shorted or open plug wire, cracked distributor cap, arcing rotor or weak coil), or a compression leak (burned exhaust valve). But other causes may include lean misfire (check for vacuum leaks, leaky EGR valve, low fuel pressure or dirty injectors). A single misfiring spark plug can cause an increase in HC emissions of 2,500 or more parts per million, which can push the converter's operating temperature well above its normal range."..

Now, from the other things I`ve read about catalytic convertors, it seams as though their indicating, that the rear (towards the tail pipe) end of the catalytic convertor, should be/normaly "hotter" then the front (of course, not way hotter as described above) but...whats odd about this is..
When I had the car up on the ramps..I could hold my hand steady on the rear of the cat (towards the tail pipe) even after several minutes of running the car and it was just barely warm.., yet..very soon after starting the car, the front of the cat (near the motor) got pretty hot..could`nt hold my hand on it for very long at all..

Just to add..I`m NOT saying/thinking this is whats wrong with my car (see "ocasional mis fire on cadillac deville")..but just been wondering of all the mis firing ect problem, may have/been damaging my cat..

I plan to pull the oxy sensor, and check the back preasure hopefully soon..and would like to ask a quick question about that..
should I leave the oxy sensor plugged in, and just set it off to the side away from the exaust so as not to melt the wires ect, or unplug it all together?

Thanks for any and all thoughts..

[submariner409]

If your overall unburned hydrocarbon emissions are very low and the cat is functioning properly, even at idle the front of the cat (where the "untreated" exhaust gas enters) will get the hottest because that's the first area where anything that can be treated will burn.

Eventually the cat will heat up fairly uniformly, pretty hot, enough to cause blisters immediately, and the outlet gas will be hotter than the inlet. If the cat continues to heat due to excess unburned hydrocarbons it will glow cherry red. Now the outlet is very much hotter than the inlet. If a cat runs that hot for any length of time the catalytic metal coating on the ceramic honeycomb is ruined.

What makes you suspect a plugged cat ?? Be careful of O2 sensor wiring: one wire is a ground "clean air reference" and does not like to be twisted or kinked, particularly after 18 years in the same position.

I read the cat article to mean that "a single misfiring plug" is totally dead. Much different from an elusive occasional miss.

[AL Deville]

Thanks for the reply..
It seams that about 50% of the time, the car acts like its got a spark plug not firing at all..(except when it cleared up on my highway run a few days ago) anyway, not knowing how long its been running like this, perhaps the cat has became damaged..and just wanted to rule it out as being a problem created by a previous exsisting problem..(if that made any sence..LOL)..

the "occasional mis"..is more like a constant, but then a few miles later (sometimes), its completly gone..its not like it just stumbles now and then, but rather when it starts "missing".. it cointinues to do so, somethimes for 20 miles..yet other times, runs VERY smooth, for 80 miles?
strange as hell..

Thanks again for the reply..hopefully tomaro I`ll have some answers...(or more questions..LOL)

AL