I have a 2002 blazer 4.3l. It runs 210'f on interstate, and 230'f in town. Replaced both radiator cap and thermostat to no avail. Heater only blows hot air when above 50mph. Never had a coolant flush to my knowledge. Checked for coolant leaks and found nothing.

 

Its WAY past time for clean and flush. Theres a chemical to pour in to clean the system. It may take several rounds of that.

Disconnect the lower radiator hose at the water pump, pour water in the top hose ( into the radiator) to flush.

Its possibe the radiator is plugged.

Its possible on an old engine to have a water pump impeller thats rusted away so it wont pump.

Also check the thermostatic fan thing to see if the pin is stuck. I just put a post with pictures on how to free that pin on the S10 forum. Check by trying to rotate the spring CCW.

 

Only thing I can add is that if you are not up for doing it yourself, you might have a trusted shop do a proper full flush.

Depending on the history of the maintenance on this engine, you could have lots of sludge in the bottom of the block along with the overflow tank, radiator, and heater core.

And daveca, that is a good thought on the effect of a rusted out water pump impeller. BTW I am kicking myself for not thinking to check the thermostatic spring on my cooling fan when I had it out recently.

 

yeah i learned the hard way about the pump. the impellers arent cast iron anymore!