Here's the problem...

The truck will warm up normally and heater will work fine, putting out plenty of warm air. Without warning, the heater will start blowing cold air for about a minute, then the temp gauge will shoot up into the red and will stay there for a minute or two. Again without warning, the heater will start to blow hot air, and the temp gauge will drop back to normal. This scenario may repeat itself again in another minute or so...or may not repeat itself. And occasionally, it won't bother to overheat at all. There seems to be no rhyme or reason to when these events will occur.

I've replaced the thermostat (not with a failsafe one tho), the upper and lower heater hoses (just cuz they looked like they might need it), and the radiator cap in case it was defective. And though I have a new water pump to put in, I haven't yet just to see if any of the other less expensive fixes did the trick.

I'm thinking that either the radiator is being intermittantly clogged, or the pump is intermittanly failing. Can either of these things happen? Any other thoughts as to what might be causing this problem?

I'm going to do a flush of the system soon. Any suggestions as to what to use? It's a bit rusty looking and I want to clean it up as good as I can. After that, the pump is going to be replaced just for general principle.

I'm getting tired of this thing overheating...any help is always and greatly appreciated.

 

if it is rusty looking you waited to long to flush the system.

did it do this before you replaced the stat?

because it sounds so random I would guess that it is a bunch a debris in the system clogging up everything.