How to Replace a Coolant Bypass Hose

by Jack Hathcoat

Car engines need a way to equalize water pump pressure. The engine is cold when it first starts, but the water pump spins even though it isn't needed. Without a bypass route, the coolant would dead-head against a closed thermostat. Should the thermostat stick closed, the same problem would occur and the pump could cavitate, or form air pockets, which could destroy the water pump. Replacing the coolant bypass hose will help ensure this doesn't happen.

Jack up the front of the car and lower it onto jack stands below the two front tires. Place a drain pan under the radiator drain valve. Loosen the valve and partially drain the radiator so the coolant level reaches just below the bypass hose.

Use a screwdriver to loosen the bypass hose clamps and remove the hose. In some cases, the bypass plumbing uses a pipe instead of a rubber hose. These pipes, or hoses, are located near the thermostat housing. Follow the upper radiator hose to the entry port on the engine. Usually the thermostat is located inside the engine, in a housing where the upper hose is attached. The bypass hose is below the housing. The bypass hose is always much smaller than the radiator hose.

Install a new hose and tighten the clamps. If the bypass circuit uses a pipe, inspect the old pipe. These normally rust and routine replacement is vital to maintain the integrity of the cooling system. Tighten the radiator drain valve.

Lower the car from the jack stands and refill the radiator with coolant. Start the vehicle. Allow it to reach operating temperature and check for coolant leaks. Shut off the engine and allow it to cool. Recheck the coolant level and refill as needed.

Items you will need


About the Author

Jack Hathcoat has been a technical writer since 1974. His work includes instruction manuals, lesson plans, technical brochures and service bulletins for the U.S. military, aerospace industries and research companies. Hathcoat is an accredited technical instructor through Kent State University and certified in automotive service excellence.

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