[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.

 

[AutoJim]

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.

 

[lexicat17]

Thanks AutoJim , I was hopeing you would answer my question. /ubbthreads/images/graemlins/biggrin.gif

 

[bnmccoy]

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.

Bob

 

[lexicat17]

There does'nt seem to be any significant pressure when I open the cap , just a little whisp or gasp of air. I was wondering if that was normal or if the cap was bad etc. It seems its normal according to both your answers. I'll check it next time I don't use the truck for a day, whenever that may be. It rarely gets a day off. Thanks again