Update 10/26/15 

Mishimoto has listened to many about the constantly cracking plastic expansion coolant tanks and took it serious. they have begun design and R&D on an aluminum replacement! check it out!

From Mishimoto:

“Now that we have a final design that fits perfectly and operates exactly like the stock expansion tank, it is time to start manufacturing our first batch of tanks! Soon we will be launching a discounted pre-sale for those interested in picking up this product.

Keep an eye out for more updates on upcoming Mishimoto BMW performance parts.”

Thanks for reading!

-John

MM-Front-6

Mishimoto site with Expansion tank info

Update 7/3/14

The E46 is known to need a repair here and there, the cooling system is no exception. Most commonly is the coolant expansion tank, this part is known to crack at the worst time and if not replaced promptly you can have catastrophic engine damage from overheating.

I developed what seems to be an internal crack in the expansion tank as I have coolant slowly dripping past the coolant level sensor. Usually this part of the tank is dry and isolated from coolant but if there is a crack inside it coolant will make its way to that cavity and you’ll get coolant dripping down the sensor.

I confirmed it, the tank had a tiny pinhole on the side which when pressurized leaked bad. It also had cracked inside where the tank separates at the level sensor.

The picture below shows what the expansion tank looks like:

Expansion Tank

I repaired the expansion tank with an OEM part so it should be good for another 122k miles. While there I also replaced the hoses and sensor.

The E46 has a few problem areas in the cooling system, These are the Coolant expansion tank, electrical thermostat, Radiator and the Water pumps.

Most problematic is the coolant expansion tank as this one usually happens eventually to all cars over time, My OEM unit lasted 122k miles so they are good as long as the car doesn’t overheat from another hidden problem.

The electrical thermostat is, well, electrical and can fail in the open position so the car does not get to operating temperature. Worst case is the thermostat getting stuck in the closed position so as the car gets up to temp the coolant is not allowed to flow through the radiator and the car starts overheating. The thermostat is basically the housing with electrical thermostat in it. The housing is also known to crack and causes leaks.

thermostat 2

The radiator in the E46 has plastic end tanks and tend to separate and leak. you will notice the smell of coolant and low coolant on the reservoir. Sometimes overheating may happen. As of July 3, 2014 Mishimoto is considering making a bolt on aluminum radiator for the non-M E46.

Now on to the water pump, the water pump has had issues over the time with many revisions done to the impeller and shaft, the plastic impeller was known to crack and the shafts were known to snap, also the water pump pulley being plastic is known to crack. Although BMW has remedied some issues and has prolonged the life of the OEM there are aftermarket units now that have metal impellers and better shafts. If you look around you can also find several water pump pulleys made of aluminum instead of the brittle plastic.

Coolant hoses

There are a few how-to instructions on BMW forums already, here are a few:

E46 Expansion coolant tank DIY #1

E46 Expansion coolant tank DIY #2

E46 Expansion coolant tank DIY #3

E46 Expansion coolant tank DIY Video

This is an Upgraded water pump for the E46:

Stewart EMP Water Pump  at BimmerZone

Stewart EMP Water Pump at OEM Bimmer Parts