Hey guys, longtime lurker here. Here is one for you cooling system experts:

1991, non-ic, all stock, except the full power screw is turned in and the fuel pin is rotated. The truck all of a sudden started getting hot, like the thermostat was stuck closed. It would puke coolant out of the overflow bottle, and slowly get hotter on the temp gauge. So I changed the thermostat, and it still does the same thing, slowly warms up, and starts to boil in the overflow tank. System was flushed about 6 months ago, and coolant is clean. Any ideas?

 

Did you let all the air bleed out of the system? Was the radiator full of crud when it was flushed out? Is the water pump pumping? If so, maybe the radiator is all plugged up, which is one of the reasons that I run 100% anti-freeze...Mark

 

Is there a petcock or something to drain the radiator? How can I bleed air out of the system if that is the issue? The water pump is dry, it's not leaking From the pump. I personally did not flush the system, it was done when the engine was rebuilt 8 months ago.

 

I would probably try a water pump next.

 

There is a petcock on the lower drivers side of the radiator. Try draining enough from the rad to see the tubes inside. Sometimes you can see that there is corrosion clogging the tubes. If they are clear, check to see that both rad hoses are pressurized, when it is at operating temp, if not, the thermostat may not be opening. If there is no pressure, or you cannot see the water flowing in the rad, at operating temp, the pump may be bad. If you did not just do the flush, air is probably not the problem...Mark Also, is the front of the rad blocked by bugs or other debris?...

 

There is also an air bleed plug above the thermostat housing.
Check on a cold start, feel your upper hose, if the cooling system builds pressure immediately, you have a head gasket leaking gasses into the cooling system.

 

Also is the fan clutch working right?

 

Im thinking joeytechman has it right. I'm suspecting that the system is being pressurized, and can't circulate properly.

 

Could be the T-stat was installed wrong. The BB could be in the wrong position causing fluid to not enter the motor for pre-cycle. It should be south so that coolant can bypass it at all times. You can feel the hose let go when you rev it up. If it stays hard then your T-stat is having issues.

 

It is a very simple system, go down the list of things it can be, simplest first, until you find the culprit. Check that t-stat is working, hoses are good, pump is working, radiator is clean, inside and out, and work your way to the more complicated, less common problems, like pressure problem. It doesn't have to be a guessing game, just be systematic...Mark

 

Also, air in the system, something common and simple, can cause your symptoms...Mark

 

Forgot the most simple of all...New Rad cap?

 

Mark

 

I run a Mac Tools truck, and every mechanic I've talked to today, the first words out of their mouths were "Head Gasket"

 

While that is unanimous, it still is better to rule out the basics first. I put head gasket at the end of the list to check, but I'd check it after I ruled everything else out. my opinion...Mark