My mind is boggled....I have a 305 w/150,000 miles, 92 1/2 ton. I put a new water pump on it in October. After that it started getting really hot, past 230, then the gauge would fall back to 150 or so, then start over. I changed thermostat....same thing. Took the thermostat and ran it for a couple of months w/o the thermo, and all was fine except of course no heater. Got sick of freezing on the way to work, so I put a new thermostat back in it, and now it is getting really hot again, then dropping to 150-160. When the gauge reads about 220-230, I can hear the motor clattering like one does when they get hot. I have a coolant leak in the front of the intake. When the motor is hot it will spit at you and sizzle right at the leak. What is going on? This is right above the timing cover, where intake meets block. I cant really afford a mechanic at the moment. Another thing, I had the radiator cap off while ago with it running and it never got past 215....put the cap back and it went up to 220 on the next thermo opening.

 

try drilling a 1/8 hole in the thermostat sounds like its vapor lockin

 

i dont know what brand of thermostate you are buying ,but it sound like they are defective to me .i had one doing exactly the same thing to me it was sticking i replaced it and it worked fine .you can test them when out ,in a hot pan of water or use a heat gun ,but dont drill a hole in it .also check the pressure you are building in the cooling system with a pressure tester . and replace or test the cap also (you can usually test them with the same tester also ) Home > Stant Products > Testers > Cooling System Tester
Stant Manufacturing Products
Cooling System Tester and Adapters
The Stant 12270 tester allows you to test today's cars higher pressure cooling systems and pressure caps up to 30 pounds.

 

I agree with Old Dog. If you absolutely need to replace you intake gaskets, it's not bad to do. I might be able to get the procedure if you're are not sure how to do it. Piece of cake, did it to my 92' with a 305. Had to replace my intake gaskets at about the same mileage too lol.

 

I think today I'm going to take the thermo back out. I hate to band-aid up a 17 year old motor.
See because without the thermostat in it, the leaking intake wouldn't leak as bad. Thanks for the replys.

 

sounds like your getting too much pressure in there. You may have a head gasket leaking.