Always felt a good bit of heat coming up through the passengers side in the engine bay on my '90 351w 4x4, and lost a few vacuum lines to melting. Last weekend though I was driving at highway speeds when a got a rush of smoke from that side, pulled over and had a small fire going in the engine bay on that side. After extinguishing, washing the extinguisher crap out I realized some of the insulation around the HVAC area (I think) had hung down and made contact with my Cat causing the fire. Looked up specs on the exhaust and realized I was missing a heat shield which may have contributed to this. Any suggestions on replacement, the part from Ford seems overpriced, can I just use the generic Walker shielding and wrap with large hose clamps?

 

The cat is far enough down that it shouldn't melt anything up top. If the cat is getting hot enough to go that far up it's cooking the side of your trans along with fluid in it. Your exhaust needs to be getting red hot to melt that stuff. Just having the heat shield missing will not cause these issues. There's a bigger problem somewhere.

 

There is a bigger issue and I believe my trans fluid overheated a bit and spit out of the seals too, though I haven't been able to duplicate the incident. It's possible the cat is clogged, it rattles when tapped with a hammer and smokes a little when very warm. This is the factory setup with two inline cats before the muffler. I'm open to suggestions on how to proceed.

 

The heatshield is only on the 2nd cat right? That one is like 8'' behind the first Y Cat on my truck. I don't think the first one has a heat shield but I could be wrong.

 

If there is rattling inside the cat then it's coming apart. Chance are that pieces have now clogged the 2nd cat. You need to pull that Y pipe off and clean it out ASAP.

 

In the diagram it looked like the one closest to the manifolds had the shield. Of course if the cat is rotted out and cooking it's no good anyway. I could just splice in a Walker universal cat? Hoping I'm not the first ever to have this issue. Engine and tranny (E4od) run great otherwise. Truck has 151K miles.

 

you're not the first one to have this issue. I've fixed this issue on all types of vehicles. and yes you can replace the cat. Any exhaust shop can cut out the two cats and put in a new universal one. There's really no need to put 2 back in.

 

Oddly enough, my 351 has the exhaust set up mentioned in the Walker catalog as the 5.0. The first cat has no heat shield, but the second one did. It's gone now on my truck however

http://www.walkerexhaust.com/catalog...catalog-lookup

 

Unless your state has emissions inspections, I'd say get rid of 'em entirely.
Your o2 sensor bung(s) should be fairly close to your exhaust manifolds, so that shouldn't be an issue as far as the ECU is concerned.

 

Actually no emissions or inspection at all, historic tags. I'm considering it, the O2 sensor is before the first cat.