Here we go, did a set of head gaskets on a 99 4.0, the poor guy has only driven it 4 miles since his son brought it home after purchase. When he drove it the head gasket went , popping the top hose. So I started it up, wasn't happy with the throttle response, very sluggish, like an old diesel. Left it running 30 minutes, hoses all hot, no pressure, I then notice a whoosh of smoke from l/h side, then again, I dived underneath, to find the heat shield above the left cat, looks like I've sat there with a blow torch, circle of orange glowing metal, would a blocked cat cause this ? Hmmm , I'm not sure as it was so localised, I managed to cool it. Was worried about the carpet catching fire if it's that hot,, any opinions would be appreciated on this one,, (apart from setting fire to it, lol, it's trying that on its own)

Must be something causing it to get hot enough to make the heat shield glow - plugged nanocom in to see what that side is doing?

It's likely to be either running very weak or the cat is clogged. If it is sluggish revving that could be caused by either. As Sloth says, what does Nano say the lambdas are showing? How does it rev with the lambdas removed?

Haven't got a nano, and my step fathers away who has diagnostics.. I was concerned that the heat was so localised, as though something at that point alone was hot, the orange glow area was about 2 inches

the sheilds on mine are burnt on the passenger side and the driverside one is completely missing.

I think if the engines throwing unburnt fuel into the cat, the cat can get REALLY hot.