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Tuesday, April 23, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Saturday, February 7, 2009

Thermostat and Housing

Several weeks ago the OBC alerted me to a low coolant level. I didn't think much of it at the time because these cars tend to lose coolant slowly over time and it had been six months since my most recent coolant flush, so I just filled it with some spare 50/50 mix of coolant and distilled water and went about my day.

(Image: Closeup of flange of BMW plastic thermostat housing with profile 
	    gasket installedA couple weeks ago, as I approached the car one Monday morning to go to work, I noticed some wetness on the ground under the car at a point just behind the front bumper. Since I had washed the car the prior afternoon I figured it was just some water that had made its way out of a drain hole somewhere, but the fact that the OBC reported a temperature of 24 degrees implied that the fluid was something other than water.

I got out of the car to do a simple "finger test" but couldn't smell any coolant. Not surprising...as my sniffer doesn't tend to work as well when it's frozen. I checked the coolant level and found it a bit lower than expected so I grabbed my LED mag light to take a closer look under the hood. Not 15 seconds into my examination I saw the problem -- coolant appeared to be very slowly leaking from the top of the thermostat housing and dripping down the front of the engine into the waiting puddle on the ground. I quickly concluded I would need to replace the thermostat housing and gaskets, and while was in the neighborhood, the thermostat as well.

BMW parts are generally of far better quality than what is available in the aftermarket, but the BMW cooling system components (thermostat housing, radiator, and water pump) are notoriously short-lived, largely due to their use of plastic rather than some form of metal. For this reason I decided to replace the plastic thermostat housing with an aluminum equivalent from Zionsville -- a vendor known for providing all-aluminum replacement radiators based on the OE parts. Before I ordered the part earlier this week I sent an email to confirm that the housing would ship with the gaskets required to do the job and they responded in the affirmative. I received the housing with what BMW refers to as the profile gasket...an oblong rubber piece that fits into a recess machined into the housing flange. I thought I was in good shape with the housing and stopped by the dealer to pick up a 92 degree thermostat and o-ring.

Before tackling the job I read a few DIY articles that involved the use of an aluminum housing. The bimmerdiy article piqued my interest as the author installed both the profile gasket and a second flat gasket that mirrors the shape of the flange. BMW used to provide a gasket like this but they later bonded a thin sealing material to the face of the plastic thermostat housing (check out the picture) so they no longer carry the gasket. That left me wondering if the aluminum thermostat housing would require both gaskets in addition to the o-ring for the thermostat to thoroughly seal the system. If I dared install it without all necessary gaskets I knew I'd have to do the job over again and I just didn't have the luxury of time to do that. I tried to call Zionsville to clarify the issue but they were closed, so I took the safest bet and picked up another OE thermostat housing at the dealer along with a gallon of coolant before heading back to the homestead to start the work.

Roughly two hours later I finished the job. I decided to do a coolant flush since I knew I had to drain most of the coolant anyway and that too went as planned. I found I had to remove the engine driven fan, and that in turn forced me to remove the front radiator cover and alternator cooling duct. I would have suffered fewer scrapes on my wrists had I chosen to remove the fan shroud from the radiator but that would have required removal of the expansion tank and thus the overflow hose that was secured to the radiator with an OE crimp-type clamp. I didn't know for sure whether I had the correct adjustable hose clamp in my ever-growing drawer of miscellaneous parts so I decided to avoid the extra work. The system took a exactly a gallon of 50/50 mixed coolant before coming up to the proper level and before long the test drive was complete with no leaks noted.

Later in the day I finally received some feedback from a question I posted on bimmerforums. I learned that most people install the aluminum housings with a small amount of O2 sensor safe RTV to prevent leaks and at least one person reported no leaks without the RTV. I don't like the idea of using RTV in this application because they make gaskets for a reason but at the same time I'm not sure using a silicone flat gasket would be smart unless it was very thin because it could interfere with proper sealing of the profile gasket. If I had an opportunity to install the aluminum housing again I'd probably wipe an extremely thin (barely-there) coating of RTV on the housing flange and hope for the best.

Book labor for a thermostat replacement is 1.5 hours, or $180 at current dealer rates. A coolant flush is 1 hour or $120. Parts at full retail for both jobs were an even $100, but I got them for $92 including tax with my discount level. This means I saved $300 in labor doing this work myself. Not a bad payday.

Mileage: 172220, Labor Saved: $300, Parts: $92, Parts Saved: $8