Ok here's the deal. I have a 2004 2wd blazer s-10 model.I can drive the vehicle approximately 15-20 miles at 60-70mph and it starts to overheat very slowly. If I keep the vehicle around 55mph I can drive it more miles before it starts to overheat. I've replaced the thermostat and tested the fan clutch, made the fan clutch to always be engaged, but it still starts to overheat very slowly. Once it starts to overheat nothing will help to get it back normal operating tempature. I guess my biggest fear, after reading on line, is a cracked head. I have no oil/water mixing but that doesn't mean it hasn't cracked. How can I check this without spending any money on testers as my budget is limited. I'm about to pull the radiator and flush it and disconnect the heater core hoses and flush that. I would hate to waste a bunch of money throwing parts on it. I just can't figure this one out. Any help will be appreciated. Thanks

 

I had this same issue happen with my Venture van. It progressively got worse until it was just not able to be driven.

I took it to a friend who's a good mechanic and he put a tester on the radiator that pressures up the cooling system to find leaks in things like cracked heads. At that point the van would overheat in seconds and the radiator hoses would swell with the RPM's of the engine. He swore it was a cracked head causing the issue yet when we put the tester on the radiator he could pressure it up well beyond the usual running pressure and it would hold that pressure with no problems. He had me get a set of heads for it anyway and went to change them for me.

Turned out the heads were good and the head gasket had a place in it that was allowing cylinder pressure to leak into the cooling system but was holding the (by comparison) much lower pressure of what his pump in his test kit could push the cooling system up to. It was a very small place in the gasket and was almost not even noticeable. I had him go ahead and change the heads (I had already bought them) and it's still going strong. Also, I ended up selling the old heads to him for his daughters car about a year or so later and she's still driving that car with my old heads. I would call that definitive proof that the heads never had an issue.

I'm not saying that's your exact issue but I am saying it doesn't HAVE to be the heads. There's a lot of things you can check first. First and foremost have you checked and tried to clean both the radiator AND the air conditioning condensing coil (I think that's which one it is) that's in front of the radiator? If one or both of those are plugged up that would definitely cause your issue. If you find them clean then the next thing to do would be to pressure test your cooling system and see if it will hold pressure. If you don't know anyone with one of these testers you should be able to borrow one from just about any auto parts store that has a loan-a-tool program. They're listed as tools available in the program at Autozone, Advance Auto Parts and Oreilly Auto Parts for certain. At this point if it holds pressure then either the water isn't circulating correctly (water pump/flow issue) or perhaps you have the same issue I had with the van. I replaced the water pump in my '01 Jimmy when it started running warmer than it should (was also dripping lightly from the weep hole on the water pump). I pulled the top radiator hose off and even revving the engine would barely get any water to run out of it. My thinking there was that it should at least move some water. I replaced the water pump which definitely took care of the small drip and the new pump could easily pump the radiator down. If the pump seems good to you then watch your hoses closely as you rev the engine. A little flex is normal, but the ones on my van would look like a bicycle tube inflating and it was easy to tell that something wasn't right.

I hope all that mess can be of some help to you.