I'm trying to save my daughters 1996 Blazer. It only has 58k miles. She brought me the truck saying it's over heating. Sure enough, the truck was running right up to 260degrees and it would also have no heat. So, I flush the coolant system, use a cleaning fluid, flush the system again, put in new thermostat and top off the cooling cooling system.

I ran the truck for about 30mins in the garage and temp seemed to be fine 180ish. Then I waited a couple of days and decided to take it to the gas station(getting low on gas). First thing I notice, no heat and then in about 5 mins the temp guage climbs right up to about 250(still no heat). I pull over and check both the upper and lower radiator hoses and they are both cool to the touch, so I very carefully crack the radiator cap and the coolant is comes pumping out, but not scolding hot. I close the cap and wait for the engine to cool down. I start it back up, the engine temp says 210 but I take off anyways, and low and behold it drops down to like 160. I keep driving and it climbs to 210 then drops down again, AND I HAVE HEAT now.

I keep driving around and finally the engine is holding at about 180. I pull over just for kicks to see what happens at idle, and sure enough the temp climbs up 240-250, but if I hit the excelerator and hold it above 2500rpms the temperature immediately(like 4-5 seconds) drops back down to 180ish.

What the hell is going on? Is there air in the system? Is the sensor bad? Water pump is weak and works at higher rpms?

It might be easier to diagnose if the damn temp wasn't so sporadic.

 

Sure sounds to me that the water pump has gone wacky on you. I've never seen a water pump do this but it would explain everything going on. I'd suggest looking at the belt closely but you'd have other problem other than the temp if things were slipping.

Maybe pull the belt and see if the water pump is spinning freely? Check the tensioner? Check the fan clutch? Try to eliminate the obvious first and move on to something next.

 

Did you "Burp" the system by raising the front with the cap off after you put new coolant in? Sounds like you're pushing an air pocket around.

 

Yeah, been filling it on an incline, though I think you might be right. Fluid is still going in, but at this point I think there might be a leak(head gasket?) and the fluid just keeps getting burnt up.

Once I can ensure all the air is out, I'll take it for a long ride and see if the fluid drops. Took the truck out yesterday for an hour and temp seemed to hold fine around 180ish. Started to get nervous because as the engine warmed up, the temp went to 210 but then backed off once the thermostat opened(at least thats what I think happened).