Yeah, my pressure doesn't drop unless I shut-off the engine, or release it at the degas bottle cap. In either case the pressure goes back up fairly quickly. This discussion had me hoping I had dodged a bullet. Better luck next time!

Can someone please explain how ambient temps affect thermal expansion of engine coolant?

 

Just as with tire pressure, when ambient temperature drops your tire pressure is reduced, when temperatures rise your tire pressure increases. The same is true with your engine coolant. Going from coolant temperature of 50 degrees and then warming your truck up the coolant temperature increases 140-150 degrees and the coolant pressure rises. Once at that temperature with the truck running and you release the pressure through the degas cap there should not be much more expansion because your coolant is at operating temperature. If it comes back up to 10-15psi after that and you have an EGR delete that pressure is LIKELY coming from combustion gases jumping from cylinder bores to coolant passages. A slight area of head gasket sealing surface is all it takes. There’s only about 3/16” of sealing surface separating combustion gas from coolant passages. I drove my truck for about 35-40k with leaking head gaskets and it would only puke if I really romped it hard. Otherwise when I got on the throttle I would occasionally get the tea kettle sound and see steam on my windshield as the heat and coolant vapor escaped my degas cap. With moderate driving my coolant pressure would maintain just under venting pressure.
One important thing I learned in my field is when compressing natural gas with a compressor to drive gas down the pipeline is that gas can be compressed and liquid cannot. I think the heat is expanding the coolant and compressing the gas in the cooling system.

 

Thanks for the thorough explanation Mack. Funny thing is, I romp on it all the time and I've never heard the tea kettle or seen steam. It did puke minimally a couple times but that doesn't happen since I started keeping the coolant level low.

I still can't get my head around the temps and pressures thing. In a closed system with gas and liquid (pressure does not exceed degas cap limit in this example) as it heats up the liquid expands and the gas compresses, raising overall system pressure. Say the system is opened, topped off, and closed in the summer when ambient temp is 100 degrees. As ambient temps drop wouldn't you end up with a vacuum in the system, which would offset the delta between the lower ambient temp and engine operating temp, and thereby resulting in no difference in pressure at said operating temp?

The only thing that makes sense is that air makes its way into the system as the system cools so vacuum is never present.

 

It may only seal in one direction.