Ask the Mechanic

Steve Thompson, Contributing Columnist

READER: I have a 7800 John Deere tractor that has a plastic container on the left side of the engine that looks like it has coolant in it. When the tractor is cold, it has very little coolant in sight. But, after the engine gets out in the field and is working, it fills about halfway up. Is this spare coolant? What makes it come in and go out?

STEVE: Yes, that is coolant you see in that reservoir (see accompanying photo). The coolant reservoir's function allows your radiator and engine to remain full of coolant, even after the engine's heat creates a gas in the cooling system, expanding the total volume of the liquid space. As the coolant passes through the radiator, the gas portion converts back to a liquid when it gives off its heat. As the coolant gets hotter, the volume of gas and liquid is larger than the volume of the cooling system, and it needs a place to go. So, it opens the pressure cap on the radiator and pushes the extra coolant into the reservoir.

The volume of the reservoir stays constant as long as the engine is hot, but when it cools, negative pressure pulls the coolant in the reservoir back into the radiator until it's completely full. The coolant being pressurized in the radiator is why it's not safe to open a hot radiator. Older cooling systems without a reservoir, when completely filled, just pushed out the excess amount through a drain hose.

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-- Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email mechanic@progressivefarmer.com, and be sure to include your phone number.

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