Odd phenomena

[MissahPapaJ]

I’d like to call this a dumb question.. but I’ve sure as hell never seen this. When I open up my radiator cap even after not running for several days ( I walk to work ) it’ll still have slight pressure behind the cap and a little coolant may slightly spill over, dead cold several days.
The entire cooling system is redone except the upper hose water outlet pipe gasket
All OE parts excluding a Land Tank modded blue hub and a rear heater delete.
And the radiator is a CFC ( not in love with that brand but it looked too good ( newish ) not to rock and see if it keeps her cool.
Vehicle never gets past 188-190 after a heat soak and usually runs between 178-181/185 when driving no matter the ambiant temp.

Any thoughts I’ve been wrenching for 20 years and I don’t think I’ve ever popped a rad cap even the next day where it decided there was slightly too much coolant and wanted some out, usually the rad cap and overflow tank swap spit and all is happy.

Side note if this is a CFC radiator inconsistency I’m gonna TYC this hoe so quick.

 

[Irish Reiver]

Just spit balling here with some thoughts. The cap is set to release when pressures in the radiator reach 15psi so in theory once the pressure is, let's say 14psi, the cap will be closed and the cooling system will remain pressurized at 14psi. Obviously, as the temperature of the coolant drops the pressure will also drop but i have no idea how long, if ever for the coolant pressure to drop to 0 psi. So in my mind when you open the cap you release whatever little bit of pressure is remaining. I think you just have a perfectly sealed cooling system.

 

[jonheld]

Any thoughts I’ve been wrenching for 20 years and I don’t think I’ve ever popped a rad cap even the next day where it decided there was slightly too much coolant and wanted some out, usually the rad cap and overflow tank swap spit and all is happy.

FWIW, my LX450 does exactly the same thing with a 100% OEM refreshed system, but it depends on ambient temps.
The radiator cap will only vent excess coolant when system pressure gets over 88kPa or 13 psi, and it will take some negative pressure to draw coolant back from the overflow.
IMO, that means that the coolant system is properly filled and sealed. I always want to see coolant to the top of the filler neck.
I'm calling that good.

 

[TRDdrew]

I would say that this sounds pretty normal to me. The only thing I would maybe check is that the to hose from the rad to the overflow is clear from any obstructions. Just pull it off and blow a little compressed air through it, but honestly this all sounds normal.

 

[MissahPapaJ]

Guess I can chalk it up to a FZJ phenomenon I do cooling system work on nearly every make and model and excluding modern vehicles with expansion tanks or degas bottles / or what ever a manufacturer is calling them that week. Most traditional cap and recovery systems I’ve maybe never noticed this happening I typically like to keep a cooling system refresh or cooling system work over night to recheck dead cold and top off if needed. Then I slap it on the ass and usually never see it again for months or sometimes ever.

 

[LongDuck]

I have a TYC-1918 and it does exactly as you described. I kinda like it, because it's letting me know that the system is perfectly sealed. Run it,

 

[robnicko]

Its normal on a properly sealed cooling system

keep in mind that ambient temp, even on a dead cold engine the coolant will expand / contract slightly. I have noticed the same thing on mine when it hasnt been driven for a couple of days, in cold weather the top hose can be compressed easily yet if its a warm day will feel slightly pressurised, not much but you can feel it.