Good evening folks. I’ve been having an over pressure issue. Coolant is being blown out the overflow hose.

I have about 10k on my rebuilt heads and gaskets, 15k on the fan clutch, fan, thermostat, radiator and hoses. I have no perceivable coolant in the oil, no codes, no white smoke. The thing runs great currently, and other than an evap leak several thousand miles ago, it’s been good.

I noticed the issue when I hit 217* on my ultra gauge, pulled over and realized what was happening. Also of note, on the highway, it’ll stay at 183-188, but after exiting that seems to be when it decides to purge.

I’m likely going to dig into the heads again next weekend, but figured I’d ask the experts here if there’s something else I should be looking at.

Thank you!

 

I would do a pressure test on cooling system
replace your expansion tank cap

 

An easy and decisive test is to check your coolant for combustion gas with a kit, that will give you a direction to go. It sure sounds like a blown head gasket, but it would be pretty frustrating to misdiagnose such a major repair.

 

Yes sir I did that last week.

 

Good call, I just ordered a kit on Amazon. If it’s not the HG, I’m assuming some sort of blockage? Heater core seems to be the common issue, but my heat has been working fine.

 

Either a blockage that the pump is fighting against and building up pressure, or you are boiling your coolant for some reason.

 

Yea that is exactly how mine was with the crack in the gasket, fine at highway speed then it would purge as the engine heated up. You may get no good result with combustion kit, I did not but the gasket was in fact shot.
One other thing my heads were seriously warped when I pulled them they could not be trued. I replaced them and had the new set trued.

• You can bypass the heater core to rule that out.
• Check the return line from the rad to the expansion tank make sure it is clear
• You can do a cooling system flush with plain water from a hose that should show up any issues with blockage
• Also thermostat check that it actually opens

For the combustion test I would try this:

• Have the test kit ready to go
• Loosen the rad cap so the system will not pressurize.
• Go for a short drive around the block to get the system up to temp
• Pull the driveway
• Pull the cap and test

Maybe you will have better luck than I did with test kit

 

95% chance you have a small hg leak. It will get bigger over time.

If you run the inline thermostat mod with the cap one turn loose and it is full of coolant, it will not overheat (assuming the radiator is fine). It will drink coolant, and you will have to top it up occasionally.

I have had probably a dozen trucks with blown hg. The gas test does not always show it is blown. More reliable test is coolant leakage over time, or pressurization of the cooling system. If it is being pressurized, hg is blown.

Good luck, let me know if you have specific questions.