randall L

Who's right? Is my Cat. Converter (brand new) getting cherry red cause of running rich or lean?

Seems to be lots of disagreement.

antipode

More than likely, your cat is clogged. Or you're running rich, fuel will combust in the exhaust. Could be caused by an O2 sensor malfunction, a bad or fouled MAF sensor, fuel injector leakage, etc.

ThatAintStock00

^^Agreed with all. But for more info, what cat did you replace it with? Have a part number or anything? I replaced mine with a Walker (http://www.summitracing.com/parts/wlk-15038) & Flowmaster 50 combo and have had no issues. It could be too small a cat for the 4.0L

randall L

No..it's not clogged. Magnaflo direct fit. Brand new.Doesn't have 10 miles on it.
I think it's running rich. But it could be running lean(and rich). Depending on whether it's hot or cold. Every sensor on her is new. Along with a ton of other stuff. All sensors check OK.
But I'm going through thermal failure when it warms up. And nobody can figure it out.
I only posted the question because every other "mechanic" has a different opinion when it come to the red hot CAT.

Going out to pull the rail & injectors right now to check for leak down.