Can someone enlighten me a little on the science of this pressure?  It seems to me that there could be some pressure build up on the positive side of the pump, but it would go negative on the suction side, so the net effect of the pump should be close to zero??

The volume in the system expands with heating and hydraulic locks against the cap. Some is forced past the cap at that pressure. A rather small lowering of coolant temp, such as reduced power, drops the pressure quickly, and in an amount that seems alarming. This is because the system has a small total volume. So the system looses a bit of volume, and the pressure drops to zero. Since there is no cushion or accumulator feature.

 

So, it reacts suddenly to small temp changes.

 

Many modern car systems have a recovery bottle with a cap that seals tight with no relief feature at all.

 

 A specified air volume is left above the coolant level in the bottle, (fill to level line) and expansion produces a linier increase in pressure, without exotic changes in pressure, with small temp changes, but always on the positive side (always has pressure when heated).

 

The other advantage is that with the hose in the bottom of the bottle, only liquid can return to the engine during pressure changes. This soon removes all of the unwanted air from the system after a few heat cycles.

 

This was the stock Mazda system from the 70s.

 

Measuring pressure right after the pump just shows dynamic changes based on pump speed and temperature. And dynamic pressure may be higher than the cap setting, and dump coolant without regard to temperature. Pressure between the pump and the radiator will be the highest. Pressure just on the intake side of the pump will be the lowest.

 

I use the early Mazda system with a bottom fed bottle and a 22 pound cap on that. The bottle is kept about 1/3 full of coolant. Never a cooling problem on even the hottest days.

  

 

Lynn E. Hanover