Why does my cooling system do that???

This problem seems pretty odd to me. 1986 Dodge D100 truck, manual transmission (OD), with high milage continually fills the overflow bottle with coolant.

This thing starts EVERY time I hit the key. Runs so good there can't be anything seriously wrong with it. (OK, once you get past the blue fog on startup, but I'll get those valve stems seals in there someday..) It doesn't run hot, but every time I look, the radiator is down there and the coolant bottle is full! I recycle it to the radiator and away I go again for a month.

I originally thought it was blow-by (rings), but then I thought how would that put positive pressure in the coolant? Blow-by would show on the breather, but not this one.

I'm thinking head gasket cross-over into the coolant. If it's not that, cracked head maybe? It's been hot once or twice...

What else could it be?

JK mity225@mchsi.com

replace the radiator cap. use a closded system cap without a pressure relief lever.

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replace the radiator cap. use a closded system cap without a pressure relief lever.

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In a closed system with a properly configured coolant recovery bottle [the hose dumping at the bottom of the bottle], it is normal for some coolant to move from the radiator into the overflow bottle as it expands with heat. What might be at play here is that your radiator cap has failed or is of the wrong design. A lever-less radiator cap designed for this type of system will have a small vacuum relief valve in the center of the seat. Once the expanded coolant cools [and contracts], some amount of coolant will be drawn back into the radiator from the overflow bottle.