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Question about coolant pressure

6143 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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Assuming you drive the vehicle about the same each day and put her to bed the same time the residual heat in the engine each morning should be the same. If you relieve the pressure one morning and then again the next morning (with similiar ambient outdoor temperature and similiar cool soak of the engine from day before) there really should not be any significant pressure in a closed system.

The Ford TSB for detecting possible blown head gaskets has the technician install a pressure gauge and determine at what pressure the cap relieves; then drive the vehicle and observe the cooling system pressure under heavy throttle (and boost) to see if the pressure is rising to the pressure relief point. This is an indication of a blown head gasket. This can be going on w/o any loss of coolant if the level is low enough in the degas bottle.

These engines don't necessarily blow gaskets apart as is probably more typical with a blown head gasket; when they fail they can let combustion gas get past the gasket into the cooling system w/ no signs of coolant in the oil, etc.

I believe Diesel Doctor's site, IIRC, has a procedure to use a combustion gas analyzer or detector to check for combustion gas in the degas bottle.

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