KevinH2000

I replaced my thermostat today, and I am sharing the information I gathered that made this process go smoothly in the hope this will benefit someone else.

The reason I changed the thermostat is that I had a CEL and the code was P0128. A quick search showed this code means that the engine's control module detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. This happened a few times when the weather was below freezing. I've had thermostats fail on other cars. Each time the thermostat was stuck fully open and I became aware of the problem because I had little or no heat in the passenger compartment. There was no CEL in my other cars. The C230 had plenty of heat when the CEL illuminated, so I would not have known about the problem without the CEL. One of the CEL's had two codes - in addition to P0128 I also had code P0172 which means that the system is running too rich. I saw some information that suggested the two codes were related because the system may be attempting to adapt for the low operating temperature by increasing the fuel to air ratio.

I also learned that Mercedes may have changed the specification on the thermostat after they manufactured the car. Some sources suggest that the cars were originally equipped with 85 degree Centigrade thermostats (or possible 87 degree C), but the current part is a 90 degree C unit. I know my car usually ran with the needle right on the 80 degree C mark on the gauge.

There are is a good YouTube videos showing the process for replacing the thermostat on the M271 engine.


The second video has errors. It misstates the model years for the M271 engine (at least for U.S models) and they identified a broken thermostat housing is a spacer. However, watching both videos prepares you for any glitches you may encounter.

I wanted an OEM thermostat and I found posts suggesting that Wahler was the OEM manufacturer. I ordered a Wahler thermostat, but it arrived in a BorgWarner box.

BorgWarner Box

Wahler on the thermostat

It appears that BorgWarner bought Wahler and the branding on the box changed as a result.

The most difficult part of the job for me was removing the old thermostat. It appears to have been changed before and the person who performed the service smeared sealant on the parts.

Sealant made removing the old thermostat a chore.

I found that rotating the housing back and forth about a quarter turn a few times broke the bond of the sealant and eventually allowed me to pull it off. I tried to use as little force as possible since it appears easy to break the thermostat housing. Instead of using a screwdriver to pry off the housing, I used a plastic trim tool. I cleaned up the housing and installed the new thermostat without sealant. I've driven the car about 12 miles without signs of a leak. It seems to me that the O rings that seal the housing are sufficient.

The part number on the thermostat I removed was A271 203 0375. It was also made by Wahler and a search indicates it is a 90 degrees C thermostat.

Thermostat I removed. Not the sealant may be interfering with the operation.

The car is now running slightly warmer than before I replaced the thermostat. The needle on the temperature gauge is clearly above the 80 degrees C mark and there is a slight gap between the 80 degree mark and the needle. I'm not sure whether the aging of the thermostat or gobs of sealant fouling the mechanism of the thermostat were responsible for the problem. Time will tell whether this has solved the CEL problem.

slammer111

For reference there is already a DIY thread for the M271 thermostat.

The 2nd video is a hack job with SEVERAL major errors and should NOT be followed. In fact I would recommend removing that video from your post.
1) The large plastic piece past the outer o-ring is not a spacer - the pipe is actually broken in half.
2) Blue (non permanent) threadlocker is the correct product for the screw threads. Why is he using caulking?
3) The thermostat o-ring is on the wrong side of the thermostat during reinstallation. It goes between the pipe and thermostat, not the thermostat and engine block!
4) The thermostat installation method is completely wrong. The thermostat and inner o-ring pop into clips embedded inside the pipe, then the whole assembly is inserted back into engine. By installing it backwards like in the video, he is going to break off the pipe clips. Now there will be pieces of plastic floating inside the coolant system.
5) Dex Cool is most definitely NOT for this engine. Not only is it not MB approved, but is that stuff even formulated for aluminum engines?

Yikes. What a mess with all that sealant. Not sure why the previous owner had to do that. The OEM setup does not leak at all. You may want to replace the pipe and/or o-rings. There is clearly something wrong.

Wahler is the only (reputable) company I know of who makes the M271 thermostat. In fact I'm pretty sure they make the OE thermostats as well. Every aftermarket thermostat I've purchased (2 so far) was a Wahler, regardless of the brand on the box it came in. In fact most aftermarket thermostat photos clearly show the Wahler markings.

Not sure why MB made 2 versions (87C and 90C) for these things. A 3C difference is negligible. Typical German overengineering. Obviously they realized it was pointless as well.

KevinH2000

I’m not sure how I missed that thread. I have attached some before and after photos. The “after” photo isn’t the best, but the needle on the gauge previously pointed directly at the 80 degree C mark. Now there is a small gap between the 80 degree mark and the car is running slightly hotter. Jake sent me a photo of the gauge on his car and it appears identical.


Before replacement of the thermostat




After replacement of the thermostat

F18Lumpy

I just changed the thermostat on my 2007 C280 yesterday and had the same exact experience. Car ran at 80C when the old thermostat was functional. Installed a Borg Warner from FCPEuro and the car now runs at about 90C. I had a 50/50 coolant mix in the car, and replaced what I lost during the change with 50/50.

I’m guessing that the thermostat design changed since these cars were produced. That’s the only thing that makes any sense to me.

KevinH2000

I appreciate your feedback.

I deleted the link to the second video. I think you are right - there are just too many mistakes.

I've driven the car about 30 miles since I replaced the thermostat and there are no leaks. My thermostat included both O-rings in the box and they seem to be doing the job of keeping the system water tight. Fortunately, it was not difficult to clean off the old sealant.

The replacement thermostat is a Wahler even though it was in a BorgWarner box.

I collected the coolant I drained in a clean jug. I probably could have re-used it. I ultimately decided to replace the coolant since cutting corner to save a few buck on the Mercedes seems unwise. I used Peak coolant for European vehicles. It is labeled as a HOAT coolant - Hybrid Organic Acid Technology.



Picking the correct coolant was a bit of a challenge. This was the best article I could find. Another brand that should work is Zerex G-05 which is what the person who wrote the DIY used. I replaced my water pump last summer and used Peak, so I decided to stick with it. I noticed that the coolant appeared slightly green in color when I drained it from the system. I probably didn't get all the old coolant out when I replaced the water pump. There may have been Zerex in the car when I bought it. I believe Zerex is yellow and the combination of the yellow Zerex and the blue Peak may have resulted in the green look to my coolant.