Radiator cap pressure when cold?

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Mar 26, 2018
Atlanta, GA
How much coolant should exit the radiator when you pull the cap on a cold system?

I recently replaced my radiator (Denso), both hoses and the t-stat. Retained radiator cap from OEM unit. Big burp after install. Coolant topped just below the cap line. No leaks. Truck is running perfectly, heats up well and holds dead center when warm. Prior to, I had a leak in my original radiator that I believe was caught before the engine ran hot for too long. OEM radiatior had about a gallon of coolant left by the time it started to boil over and steam as I pulled into my driveway.

Few days after install I proceeded to do another burp on the system when cold. With engine off, I noticed coolant spilling from the cap line upon removal as if it were under slight pressure. Reservoir level at this point was just above the cold mark. This alarmed me as I'd never removed a cold cap and experienced a spill like this. I tossed the cap back on and parked it.

Flash forward to yesterday I started researching cold cooling system pressure and many general conclusions allude to the presence leftover of combustion gases in the system due to a blown head gasket, leaving the system over pressurized after cool down - that was a great feeling.

Last night I pulled the cap again and same scenario, slight pressure and spillage at the cap. This time I left the cap off, turned heater on full blast and let the engine get up to temp. Revved up to 2-2500 a few times, no fluctuation of the coolant line throughout the revs and warm up but coolant held right at the brim and slowly exited the top. Zero bubbles present over the 15min test, no gas fumes either. Coolant level in tank barely rose, if at all. I felt like it was odd that the coolant wouldnt be right at the tank tube line as the coolant warmed up and exited the system as designed, it was actually sitting steady above the hole.

I then proceeded to perform a couple of shade tree tricks to test for a HG leak- dabbed some coolant on a paper towel to see if I could spot a residual oil ring once dry, coolant looks perfect. Lit the dipstick on fire and listened for popping that would point towards the presence of water in the oil. Oil is perfect.

So, is the pressure I'm experiencing on the cold system normal? Am I just paranoid? Only other option is to have my indy do a pressure test and perhaps a coolant and oil analysis to eliminate the HG.

I also have a new OEM cap on order, could my original cap be faulty and thus not depressurizing the system properly during cool down?

Another area would the reservoir and tube, I have not touched either, how would I inspect for proper function? Was a 15min warm up enough for the tank level to rise?

Other reading has pointed towards a t-stat that is stuck closed, wouldn't the system be overheating if this were the case?

At this point Im leaving the rig in the garage until Im confident that Im not risking another blown radiator due to excess pressure.

Any and all advice much appreciated!
I can't definitively answer your question the as the radiator heats some coolant goes to the over flow bottle and as it cools some coolant goes from the bottle back to the radiator...
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Clean the reservoir cap and hoses, those might be clogged

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