Catalytic Converter question

[Scott1975]

I have been having some problems with my 2000 jeep GC Laredo overheating. I have been getting some crappy gas mileage since it overheated a few weeks ago and I replaced the the fan relay switch along with flushing the radiator out, pulling the water pump and checking it, replacing the thermostat, and today I pulled the head to check the head gasket, the head gasket turned out fine, I couldnt find any cracks. For some reason the coolant holes on the head and block were stopped up with some crud,(probally from the stop leak I put in it a while back). My questions are, could the stopped up holes be the problem with it overheating? Are these cyl heads prone to cracking or warping?(they are cast iron) and could the catalytic converter be stopped up and causing it to overheat. The only way I can get it cool off is by turning the heater on full blast and put it in neutral. It heats up when I am at idle, when I am going up hills, when I am going down the interstate with the A/C on or off but faster when the A/C is on. Any help would be great, I am getting ready to torch this thing.

[dksob81]

If the water passages were clogged up this is deffinately the cause of your overheating, with those being plugged it wasn't allowing the coolant to flow through the engine properly. Antifreeze doesn't actually cool the engine what it does is pull heat from the engine and transfers it to the radiator and as the air passes through the radiator it pulls the heat from the coolant. This is why having the right mixture of coolant and water is important, too much coolant - then the coolant will burn when it touches a hot surface and cause a film build up on that surface which lowers the effeciency of the cooling system. Too Much water - water will evaporate and lower the pressure of the cooling system which will lower the effeciency of the cooling system.