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Question about coolant pressure

6150 Views 4 Replies 3 Participants Last post by  lexicat17
Does anybody know if it is normal for the coolant resivior to still have some pressure when you unscrew the cap after the truck has not been used overnight. when I remove the cap it lets some pressure out after the truck is cold over night. The truck does not lose any coolant and does not puke any. It only puked coolant once about a year ago and the level is now an inch below full cold and has stayed there for a year. Coolant is clean in the resivior also.
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In a closed system (with a relief valve in the cap, true, but it's a closed system), it's not uncommon for there to be a little residual pressure even after an overnight cold soak. It's not much, maybe a PSI or two, but nothing really worth any sort of worry.

The system pressure is a result of thermal expansion of the liquid inside the fixed volume of the closed system. So if the last time you had the system open to the atmosphere was at a temperature of, say, 30F, and you checked it this morning at 50F, there will be a little bit of pressure. Also keep in mind that it can take anywhere from 8 to 12 hours or more for everything to cold-soak and equalize internal temperatures with ambient conditions, so the actual coolant temp inside the engine might be 60 or 70F.

Again, entirely normal, nothing to worry about.
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