A catalytic converter that is working right gets pretty darn hot, and the ceramic substrate will hold the heat for a while. In my experience with a plugged catalytic converter, the engine would start and idle more or less normally, but the engine absolutely would not rev up to any RPMs and was way down on power. The car would not attain highway speeds and the engine got hot when I tried to go fast at all. When I decided it was a plugged converter, I unbolted the converter from the exhaust system and then tried to run water through it. The flow through the converter was almost nothing when I filled the intake end with my garden hose, and I wondered how the car had run at all. I "improved" the flow through the converter with a large screwdriver that I poked through the ceramic inside, and almost all the ceramic ended up falling out. Then I bolted the empty case back into place in the exhaust system and drove the car another 2 years. It picked up several miles per gallon gas mileage and would rev freely.
However I live in an area where so far we do not have to have our vehicles tested for emissions. I would not have dared to remove the guts from my catalytic converter if I thought it would have to be examined, since the case became nothing more than a large tube. We were really bucks down in those days, too.
Your 91 GMC probably has throttle body fuel injection. After all those miles, the injectors may be worn out. Or you may have a vacuum leak somewhere that is leaning out a cylinder or two. Do you get any trouble codes? It might pay to have the problem analyzed professionally. I sure would not advise removing the converter and/or destroying it unless a plugged converter is what the experts come up with.
By the way, 240K miles is kind of amazing. You must be doing something right...most folks get lots less before they have to put an engine in almost any gas pickup. Good luck!
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