Cooling system pressure testing

[WinDancer]

My truck runs great and trouble free... EXCEPT when coming home up our two mile long rough driveway. About a mile up the temp climbs noticeably but still OK. By the time I get to out place, opend the gate, drive through, re lock the gate and pull into the driveway to park the truck is badly overheated. Right on the edge of blowing coolant.

I have been working to try to find the cause before it get seriously summer here.

Last night I pumped my cooling system to 15 pounds [engine completely cold] and left it for an hour. The pressure had dropped about a pound, and there was a single drop of water between the thermostat housing and the intake manifold. I slightly tightened both bolts and pumped it back to 15 pounds.
An hour later it had again dropped about a pound, but everything was completely dry.

When I loosened the tester the pressure was dropping and I went ahead and took it completely off- and there was a huge gush of air and water like happens when an engine is overheated [engine still completely cold].

How did pressurizing the system get the air into a position to bring water out along with the air?

Is this maybe a clue to what is happening with the heating up at long slow speed?

What [and how] can I check next?

Thanks,

[Longhorn Man]

The coolant gushing out when you undo the pressure tester is normal, it will always do that.
Low speed overheating usually comes from poor airflow.
fan clutch not working properly, no shroud, but first thing I would do, is pull the rad and take it to a coin op car wash or use a pressure washer at home, and flush the rad externally, also get the A/C condensor if you have one there. Don't do this when it is installed if you have a condensor there, you'll never know if it is clean.

[LONGHAIR]

It happens because you are pressurizing the system by pumping air in. What happens when the pressure is released is anybodys guess, it could splash nearly any direction.
Holding pressure is great, but that is no indication of actual flow. The coolant has to be able to move through the radiator and the air has to be free to move through the fins too.
Fan? Shroud? Clean radiator? inside and out?
It never actually boils over? or loses coolant?
It never does this any other time? Would it get hot just sitting at idle?

I'm thinking fan and/or shroud. Low speed when natural air-flow is not doing enough.
How slow are you moving? RPM? It could be in the timing. If you are just barely off idle and there is not enough RPM to bring up some centrifugal advance, you could get some addtional heat from that.

[Sammy]

I think folks are going to want to know what type of fan system, & # of cores, what kind of shroud etc. With what info you provided I wonder about the condition of the rad has it been professionally cleaned? Sometimes they lime up & plug themselves.

[Longhorn Man]

a rad that is clogged internally will normally show signs of poor flow, which normally shows up at higher RPMs when there is lots of air to keep it cool.

[WinDancer]

Radiator is new alum 4 core, dual electric fans, new thermostat...

[LONGHAIR]

Shroud?

[Longhorn Man]

where is the fan switch mounted, engine or rad?

[72BLUZ]

if there is alot of miles on your truck, check the air flow thru the rad for being restricted with dirt and debris.