The BMW 325Is is designed to be a high performance vehicle designed for those who enjoy speed and luxury. With performance vehicles, it's common to have issues with mechanical failures like the thermostat. Unfortunately, a part that costs just a few dollars can cause thousands of dollars in damage if it is not replaced when it fails. With a part so easy to replace, taking your BMW to the garage to have it repaired will cost a lot more than the small amount of time it will take to repair it on your own.
Locate the thermostat cover by tracing the radiator hose from the radiator to where it connects to the engine. Remove the two bolts that secure it to the intake manifold with a 10 mm socket and ratchet. Lift the housing to drain the remaining fluid from the radiator hose into the radiator. Push the hose off to the side to access the thermostat gasket and thermostat.
Pull the old thermostat out of the intake manifold and replace it with a new one. The thermostat is directional. Place the end of the thermostat that is not marked "top" into the intake manifold.
Pull the old thermostat gasket off the intake manifold and discard it. Wipe the intake manifold so it is clean of any debris.
Place a bead of RTV gasket maker onto the intake manifold then place the thermostat gasket on top of the RTV gasket maker.
Place a bead of RTV gasket maker on the bottom edge of the thermostat cover then place the thermostat cover on top of the thermostat gasket. Bolt the cover back into place.
