So here's the deal, the other day I was driving and I noticed the truck was taking a long time to warm up. 3 miles down the road the heater still wasn't really blowing warm air. About a mile later the temp gauge dropped to completely cool.

I pulled over and popped the hood and the upper rad hose was rock solid and still mostly cool. Definitely not hot. I cracked the degass bottle and coolant bubbled up like the cooling system was pressurized.

I limped it home and swapped out the thermostat and water pump. Same problem. Coolant system seems to pressurize, thermostat doesn't ever seem to open, etc. I should note that I did test the thermostat by boiling it and it opened around 195 just fine.

I figured if there was pressure in the cooling system it had to be combustion gasses so I used a combustion leak tester to see if I had a cracked head/bad head gasket. The fluid color did change a bit but barely. I had to compare the fluid in the tester to fluid straight out of the bottle to be sure it had changed at all.

So I'm at a loss. Bad head/hg? Clogged rad? Something else entirely?

 

The radiator wouldn't clog up that bad all at once, and even if the radiator was clogged the heater would still work.

I would say the water pump failed if you hadn't already replaced the water pump. The only logical explanation at this point would be there isn't enough coolant in the system. SO are you sure there is enough coolant in your truck?

 

I'm fairly certain. I've done plenty of coolant changes before so I think I have a firm grasp on how to burp the system properly.

That being said, the thermostat never seems to open so I'm not sure how to get the engine to drink a full load of coolant.