Originally Posted by billball
Ok, I understand how the cap works, etc. I am wondering if the overflow tank has some sort of valve on it, and if so, is it on the bottom, or built in? It looks like the coolant in my overflow tank never runs out, and nothing ever runs into the tank. How can I check it? Surely, somewhere along the way, there would be movement in or out, especially after driving a long way in stop n go traffic. Thanks.
It gets into the nitty gritty of things.
I don't know about other motors but on my 300/6 the rule of thumb when you were refilling the radiator was to leave the level 1 inch low.
As long as the cooling system is working the way it's supposed to that 1" air space at the top of the closed system acts a compressible "pillow" to accomodate normal expansion and contraction due to changes in temperature.
The radiator cap really is more of a safety devise than anything else for when something isn't right.
ie: You didn't leave that air space & overfilled your radiator. The cap allows the extra coolant to flow into the reservoir.
ie: You've got a small leak somewhere in the system and the engine is consuming 1/2 pint every 5,000 miles.
The cap allows the cooling system to draw in some "make up" up coolant from the reservoir to top it off to the correct level as it's needed.
It's kind of a neat design really. More for safety and environmental protection than anything else. Back in the day most vehicles didn't have a reservoir so if you did happen to overfill your radiator the excess would just dump on the ground.
Your dog or cat would come along and lick up that sweet tasting puddle under the truck and get poisoned.