I have a manual 2 door 2000 4.3 l v6 blazer. It overheats after about 5 minutes of driving. I flushed oil/coolant and replaced it along with a new thermostat but it still seems to over heat. It also seems to not pull any coolant from the overfill reservoir, I put the new coolant straight into the rad. Any ideas on what would cause this?

 

When you start it and run it for a few minutes go check the radiator and hoses. If they get soft and collapse in on them selves that could be the reason. Also if the motor is starting to over heat but the radiator is still cool then your water pump may be out.

 

Thats a start. Post what you find out from this and hopefully can find the issue with minimal cost.

 

Turns out it had a small crack in the rad.

 

I'm willing to bet that yours wasn't an OEM radiator unit. Only the OEM ones from AC Delco seem to last on these trucks. BTW: if I were you I'd replace the lower radiator hose too just for..funsies. In my experience, replacing an old lower rad hose makes a vehicle run a little cooler, but YMMV