How to Bypass a Heater Core

Disconnect the heater hoses from the firewall side. Both heater hoses will be connected to nipples, which go through the firewall and into the heater core. To bypass the coolant around the heater core, it is necessary to divert the heater hoses coming from the engine to somewhere other than the core itself. Most of the time the heater hoses are connected with hose clamps that require the use of a flat blade screwdriver, but some companies installed pinch-style clamps, which are squeezed with pliers.

Connect the 2 hoses together using small diameter plastic tubing. This step requires you to have the fix on hand in case of an emergency, but if just bypassing the core in your driveway then it can be purchased after the incident. The recommendation is to get small-diameter plastic PVC tubing (the same diameter as the inside of the heater hose). Cut one 3-to-4 inch piece of the tubing and file the ends to ensure they are smooth. Then connect the 2 hoses together and tighten the clamps over the tubing. This will bypass the heater core without impeding cooling effectively. Be advised that the heater and defrost system in the vehicle will no longer work until the heater core is replaced and properly connected again. Auto parts stores also sell heater hose repair kits that come with pre-formed plastic connectors that will work well also.

Start the engine and check for leaks. If the heater hoses are connected properly and clamps are tightened down, the system should contain no leaks. Allow the vehicle to reach operating temperature (so that the thermostat is allowed to open) and closely watch the connection points. Feel the hoses to ensure they are getting hot if no leaks are detected. If there is a leak and a slight tightening down the hose clamp doesn't stop it, a small amount of RTV silicone may be used on the plastic connector to help seal the connection. Allow the system to cool before working on any heater hoses.