Ask the Mechanic

Get the Air Out

(Steve Thompson)

READER: When I drain the cooling system on my tractor, what precautions should I take to make sure everything is working properly? The dealer told me that it is possible to get air in the system, which could cause my tractor to overheat.

STEVE: Your dealer told you right. When changing the coolant, changing the thermostat or even changing out a radiator hose, air can get in the system, and that can cause catastrophic problems. This happens when the air locks the coolant from flowing, and the temperature gauge sending unit, which is located around the front of the engine near the thermostat, can't get a coolant temperature reading.

The reason you can usually find it there is because that is where the hottest spot for the coolant is located. This is because coolant flows from the back of the engine to the front of the engine, all the while picking up heat from each cylinder.

After it goes up through the thermostat, the coolant flows into the top of the radiator, down through the radiator and then back through the engine.

The best way to make sure the air is out of the system after service is to bleed the air out of the system. After filling, if the system has a bleeder valve, it will usually be around the thermostat (see photo above). Open the bleeder valve until all the air escapes and tighten.

If you find no bleeder valve, loosen the top radiator hose until the system "burps" out all the air, and then install the hose and tighten the radiator hose clamp. Watch the temperature gauge or light on initial use.


-- Editor's Note: In the February 2022 Ask the Mechanic column "Why the Change," I made a mistake. I stated that a 6-volt system creates "less" corrosion than a 12-volt system. Of course, a 6-volt system creates "more" corrosion than a 12-volt system. Did you catch the mistake? -ST

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


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