2004 Blazer, doesnt go higher than 210 with the A/C off. Turn it on and within 15 minutes its hitting 240. A/C works great, radiator fan works good, just did a radiator flush to try and help, it didn't. Heat works good and the coolant return line has good flow when warmed up. I also power washed the radiators to try and clean those out. Still over heating, what could be causing this?

 

I was experiencing the same over heating issue when running the AC even after flushing the system and adding fresh Dex-Cool Coolant, so I pulled out the thermostat that I had installed in the winter months and replaced it with an Autozone Part Number: 15119 Thermostat which they claim :

• Part Number: 15119
• Warranty: 1 YR
• Application: OE recommended temperature
• Notes: 195 °F (91 °C)

Features & Benefits

Larger than OEM opening allowing for increase in coolant flow.
Triple bridge opening design for increased stability and strength. Manufactured with OEM quality materials and safety standards by an OE supplier. Stands up to even the most demanding automotive cooling systems. Copper case and wax compound assures rapid response to temperature changes in the coolant.

• Larger opening for increased coolant flow
• Increased strength and durability
• High grade stainless steel and copper construction
• Meets or exceeds OEM standards
• Consistent and precise response to cooling system needs
• 100% tested and calibrated

I haven't had a problem since. The larger opening did the trick.

 

Replaced the thermostat with that one. Only change is that it takes longer to overheat, but only by 15 mins. Still going up to 240.

 

That last post is me, not sure why my phone did this...

 

Also, not sure if this helps, but it's mainly on the highway it does this. And I have also noticed that when left on Max A/C it doesn't overheat as much as it would on regular or head/feat A/C.

 

Does it overheat at all sitting parked, at idle?

How old are the hoses? How old is the fan clutch?

Overheating at higher engine RPM's can be a collapsing lower radiator hose.

The fan clutch is supposed to be changed when the water pump is changed, but that rarely happens. Usually a bad fan clutch will overheat more at low speeds or stop-and-go traffic, though.

I haven't seen it much on these engines (or on small block V8's which use the same water pumps), but sometimes the water pump impeller will get a little loose on the shaft, and it won't pump coolant except at idle. At higher engine speeds, the impeller slips on the shaft and stops. That's common on some other engines I deal with a lot, but like I said, I haven't seen it much on Chevrolet small blocks and Vortec engines.

 

It will overheat at any speed with the A.C. on, parked as well.
I would presume stock, being its an 04
Fan moves fine

 

The fan is mostly to pull air through the radiator when standing still or very load speeds. After your car starts to overheat pull over, stop the engine and feel to see if the radiator is hot on the top portion and a lot cooler on the bottom. It's a sign that crud has clogged the lower portion of the radiator.

 

I'm also suspicious of the radiator on an 04. If the radiator is clogged, that will cause overheating at any speed.

IMO, the fan clutch and lower hose are also questionable, especially if they are still factory original.

A good fan clutch will make lots of fan noise when you have to sit through a long traffic light in hot weather. When I replace the fan clutch in one of our vehicles, I get questions from the wife about "What's wrong with the fan" during the first drive(s) in hot weather. She gets used to quieter take offs from traffic lights, and when it sounds like a helicopter taking off right after the fan clutch is replaced, she thinks that's a problem. It takes some explaining that it's supposed to pull more air, and that when it was quiet, that was a problem.

If you aren't noticing a very noisy fan when it's overheating, either the fan clutch isn't properly engaging or the radiator is clogged, and not enough heat is getting out of the radiator and into the air going through the fan.

The lower hose could be an issue at higher speeds.

When the A/C is running, the condenser is hot, and you're drawing hotter air through the radiator. So if anything isn't up to the task, the overheating issue will be worse with the A/C on.