if this is in the wrong section please move my post, my delema is i'm working on my 89 K5 with a SBC 350. now i had taken this truck on a few trips towing my pop-up camper and it was running fine at 190 degrees, well one day i was coming home and it sky rocketed to 280 so i shut it down, i was low on coolant, so i filled it up and was fine for a bit, well long story short this keeps overheating now after i drive for say a half hour or so, i tried flushing the heater core, radiator and the blcok with a hose and there is no resistance, i put a new thermostat in, and tested it by putting it in a pot of boiling water, yet when my truck is running and hits 190 i noticed my t-stat doesnt open.... is it supposed to open when water boils or just before?... i used a snif tester to test the fumes from my radiator for exhaust fumes and got nothing, and there really isnt any smoke from the tail pipe, once and a while it will. i checked the oil numerous times and its not in there either... i go through prolly a gallon after it over heats like that i'm think ing maybe my t-stat isnt opening soon enough?

 

few other things you can do is check the front of the radiator, make sure it isnt full of cotton woods mud or anything else. also i had a similar issue in a k5 that would drive around find but soon as i took it offroad it would heat up really quick. ended up being the fan clutch was bad.

one ghetto way of checking the thermostat is leave the cap off the radiator and let it run, when the tstat opens you will see the water inside the radiator start to flow around and possibly drop.

 

yeah i tried that and it never seemed to drop,

my radiator is clean and i got the stock solid metal fan that doesnt flex,

 

i was goig to try a new t-stat, i mean mine isnt stuck or anything but why does it open when the water boils? shouldnt it open just before not after? i also checked and my water pump is circulating and spining, but from what ive heard sumtimes the impeller can rust to nothing and still circulate just not enough