Some 2008-2010 Ford F-Super Duty vehicles equipped with the 6.4L diesel engine may experience an external coolant leak at the radiator. This may be caused by a hyper-extended thermostat. If the bypass thermostat hyper-extends, it causes the primary thermostat to oscillate. This oscillation causes thermal spikes throughout the cooling system and places added stress on the radiator tubes.
1. Remove the radiator and thermostat.
2. Inspect the bypass thermostat for hyper-extension. Measuring from the base of the housing to the top of the bypass thermostat, normal height is less than 45mm (1.77 inches). If this distance is greater than 45mm, the bypass thermostat is hyper-extended.
NOTE: Thermostat rubber seal damage is not related to radiator failure. A missing or damaged rubber seal may impact cold weather can warm-up only and should be repaired by replacing the thermostat.
3. If the bypass thermostat is hyper-extended, replace the thermostat and the radiator. Follow service manual procedures for replacement and system bleeding. According to Ford, hyper-extension is the only thermostat failure mode that causes radiator damage.
According to Ford, this repair is warrantable for the thermostat up to 5 years or 100,000 miles. Hyper-extension of the thermostat will provide related damage coverage for the radiator beyond the existing 3 years and 36,000 miles radiator coverage.