Alright here’s some history: 2004 Blazer RWD

Original owner ran the engine with a cracked radiator with a ton of Stop-Leak in 2015. I purchased end of 2015 not knowing and the radiator blew up 2 weeks later so I replaced it. Found out weeks into 2016 that the head gasket was blown so I replaced that myself, and didn’t send any parts to a machine shop (I regret that). Vehicle ran fine with the slightest misfire that didn’t even show up as a DTC. Now fast forward to middle of 2017, the head gasket is possibly bad again, so I take it apart and find it a little torn but in tact.” So I send the heads out to a shop this time. They find 3 inch cracks in both heads (Dammit). So in light of that news I bought a 90k used engine that came out of a smashed blazer. While at it I replaced the radiator again, cool.

fast forward to current: a month ago the vehicle just started randomly overheating. I the upper and lower hoses, the thermostat with a ac delco (but it oddly didn’t work right so I had to go aftermarket), I replaced the rad cap, the water pump, the fan clutch and heater hoses. I even replaced the temp sensor in which I know doesn’t immediately affect the system, only monitors the temp. The coolant is CLEAN and I can’t emphasize that enough. I bled the system like twice til no bubbles came up.

I had it pressure checked and it’s holding! No coolant leaks unless I overheat and it bubbles out of the overflow tank. Besides that There is no loss of fluids at all.

I haven’t replaced the radiator because there is no debris in the system at all, the rad fins are in great shape, and looking inside while engine running it has good flow.

If someone has any idea what I can try I will gladly cash app you $40 if the tip/trick/idea is successful.
im just about ready to sell this thing but I don’t want that cause everything is in great shape and we’ve been through a lot .

 

What are the conditions that it's overheating in? Does it do it when cruising, or only when sitting still as in traffic?

If it's only doing it when sitting still, it sounds like a bad fan clutch.

 

It overheats at idle and driving. I replaced the fan clutch aftermarket and still did the same thing BUT the fan clutch was always high speed so it was loud af. So I replaced it with a factory one and it is now quiet.........BUT STILL OVERHEATING 🥵

 

Do you have an IR thermostat? If not get one, they come in handy. Is the radiator reading hot when this happens? Is the thermostat in correctly and working properly? I mean as in closed when cold? Can you see the change in flow as it heats up and the thermostat opens?

 

If it were me, I’d pull the thermostat and run it without a thermostat for a while and observe results. We are in the middle of summer so you can get away with it, worst case it will throw a code for low temp and it will stay in open loop.

This will rule out a bad thermostat (even though it may be a new one... it does happen).

Your statement of “randomly overheating” , as in sometimes it’s ok and sometimes not? A thermostat that hangs up intermittently is just about the only option. Everything else would make it overheat all the time.

 

I don’t know what type of thermostat that is. Yes the radiator is hot when it happens. Yes the thermostat is in correctly and opens up at operating temp. When reaching operating temp it does hold steady for a short time but after that it starts rising again.

the only thing I haven’t done is use an infrared thermometer to measure each point. I think I’ll get one tomorrow

 

sorry for the confusion. The engine started overheating one day out of the blue...now it over heats every time after 10-15mins of driving. It overheats slowly if I idle in park.

 

Ok, it’s still probably the thermostat. You’ve done everything else. What brand thermostat did you put in it?

 

At first I put a factory AC Delco in and that one was bad. So I went a got a duralast one (not my proudest moment but hey) and after that the temperature would only go up to 3/4 of the way....

I will try to run it without a thermostat but I’m kind of scared to leave it without one

 

All the garages up this way use one of these:
Stant

Like I said, running it without the thermostat in the middle of summer won’t hurt your engine. Remember the goal is to rule out a problem you’re having trouble with.
If the thermostat is the problem, your engine temps would probably stay around 150ish without the thermostat, your engine would stay in open loop, the MIL would set, and the engine would operate off the VE tables and run rich.

In the mean time, the next time the engine temps start to escalate, turn the heat on full.
It wont be pleasant inside the cab, but this will help lower engine temps and prevent overheat damage.