[marinmcgreevy]

Hi all,

Wanted to share my coolant system experience. I took delivery of my 335i (manual trans, sport package) in Sept 2007 from Sonnen BMW in San Rafael, CA. The first water pump failed in August 2013 at 71k miles. The water pump only was replaced (not the thermostat) by Bobby Rahal BMW in Cannonsburg PA.

The car has been driven in high temperatures (>100F) and low temperatures (< -20F) and from below sea level to over 10,000 ft. Lots of temperature cycles over the years.

I decided to preemptively replace the water pump and thermostat in December 2020 at 135k miles. I was experiencing slightly more cooling fan cycling.

The 7 year old water pump with 65k miles was still working but upon disassembly of the pump and associated electric motor I found the seal had failed and coolant was inside the electric motor.

I replaced most hoses and even upgraded to some of the silicone hoses and alloy nipples available from ECS Tuning. I used new hose clamps and used glycerine to assemble all o-ring, snap ring connections. After three heat cycles, all the new silicone hoses with hose clamps had relaxed considerably with one coming completely off the nipple. This was only the second time in over thirteen years that the car had left me standing. Ironically, the first time was when the first water pump failed in 2013.

I also replaced the expansion tank and added a water wetting agent to the coolant.

Replacing the water pump and thermostat was a cramped and tedious process. Orientation of your hands and arms and order of assembly are key variables to keep in mind.

The built in coolant bleed feature is great and one of the advantages of the electric water pump. Temp on high, lowest fan setting, turn on ignition, and then hold the accelerator pedal down for 15 seconds starts the cycle. I did this with a battery charger connected. Cycle lasts around ten minutes.

Reading other’s DYI posts and FCP Euros guide was a great help. Thanks to all!

Photos show the engine compartment and the water pump cycling coolant back to the expansion tank. It helps to keep things clean as it makes things run cooler and easier to work on.

Still such a GREAT car!