Dear Tom and Ray:

I have an '85 Dodge Omni. I can fill the radiator itself to the top while the overflow bottle remains at MIN. After I drive and let the car cool off, the overflow bottle is at MAX, and the radiator is short the amount it took to fill the overflow bottle. If I drain the overflow bottle from MAX to MIN, the same thing happens next time. It seems coolant will flow from the radiator to the overflow bottle, but not the other way around. No leaks, no smoke, no coolant in the oil. The car has only 35,000 miles on it. Any idea why the radiator won't re-take from the over flow bottle?
Robert

TOM: Four possibilities, Robert. Actually, only three if we exclude my brother's "space aliens" theory.

RAY: The way the system works is that when the coolant heats up, it expands. And that expansion builds up pressure in the radiator. Eventually, that pressure compresses the spring in the radiator cap and forces some of the coolant into the overflow bottle, which relieves the pressure. Then, when the engine cools down, the pressure in the radiator drops. And that "negative pressure" or vacuum sucks the coolant from the overflow bottle back into the radiator. In your car, my guess is that something else is getting sucked into the radiator, namely air.

TOM: So one possibility is that you have a tiny leak, like a loose hose clamp somewhere at the top of your cooling system. It may not be big enough to cause a puddle on the garage floor yet, but it's big enough to allow air to get sucked into the system as the radiator cools off. And if air gets sucked in to equalize the negative pressure, the coolant from the overflow bottle just stays right where it is...in the overflow bottle.

RAY: Another possibility is that one of your radiator hoses is collapsing. When those hoses get old, they get soft. And it's possible that the "negative pressure" in the radiator is being used to flatten out the hoses instead of to suck in coolant from the overflow bottle. You can look for that yourself. After the car has cooled off for an hour, examine all the hoses and see if any of them are as flat as a floogee (whatever that is).

TOM: But before you put on your Sherlock Holmes hat and go sleuthing around the engine, just for kicks I'd replace the radiator cap; that's the third possibility.

RAY: The radiator cap houses the mechanism that controls the flow of coolant back and forth between the radiator and the overflow container. And every so often, those caps go bad. And since they only cost five bucks and take thirty seconds to install, that's always a good place to start.