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I've been having a slow leak in the cooling system and having to top up frequently.
I've located a leak but it was not easily noticeable. The leak is from a heater hose that goes from the firewall to the washer fluid reservoir.
There is another hose that is in close contact to it but I haven't traced down to where it goes yet.
In the pict, the leak is circled in green. The location of the picture is on the drivers side between the brake fluid reservoir and the strut tower behind the 'fake' firewall (where the heat shield keeps engine heat from spreading.
Has anyone changed either hose ? How involved did you find it ?
I've changed a similar hose behind a V8 BMW engine years ago. It was tight but doable. I needed someone who had smaller hands than mine.
Sorry this is a cold country feature. I've never seen this system.
How much coolant is your MB leaking ??
Check the crimps all around your radiator for active pressure relief leaks.
In addition the coolant hose that brings back hot coolant from the oil exchanger has an "O-ring" on the list of known leakers.
It is surprising at first how many coolant leaks a low mileage W212 has.
I'm losing about a quart every 250-300 miles.
There is a small leak on the driver's side of the rad as well on top near the rad intake where the bump stop from the hood lands. Is small as well. Can't see an active leak there; just crusty dried up rad fluid. I might try to seal that up instead of getting a new rad as its really small as well. I doubt the small leak from the rad makes it that far back to the firewall. The entire hoses back there would be coated I would think. The 'wet' pattern on the 2 hoses is always in the same spot..
The oil cooler gasket has already been changed. No leaks there anymore.
Yes, its freezing part of the year and we do get hammered with snow not to mention the road salt.
Give us your VIN, I will help you find the hose. I think ur engine is M273 yes ? Looking at the engine oil cooler.
In M278 or M276 with "heated coolant hose " for wiper fluid, the 1st hose is the one from engine to firewall and then another 2nd hose to HVAC heater core and
this 2nd hose behind firewall has a small tee for washer fluid.
But me no experience on M273, however M273 I believe have HVAC heater core coolant flow shut off valve too.
You have a real leak:
1Qt/300Mi. is not real small driver side windshield heater circuit
Thd cooling system is supposed to hold pressure and use zero coolant.
Yes, it is wet down there as well which I should have pointed out. M273 engine.
The wetness on that little 'ledge' extends forward towards the front of the car just infront of the strut tower and drivers cylinder bank where that line is more easily visable. At first I thought it was the rad spraying back toward the firewall but I don't think thats the case. The hoses have never been all wet and there is somewhat of a shield from the strut tower there.
It looks like its coming from the line from the HVAC control valve going to the washer fluid reservoir. Its item 110 in the attached.
Last edited by - Mover -; Jun 4, 2025 at 10:16 AM.
I had that issue. I have no idea why they didn't use an electric heating element.
It's a ***** to get to that piece. Search this forum-- @S-Prihadi made a writeup about it and linked a video showing a guy doing the job without pulling the fake firewall... The dealer quoted me a price to do it which was lower than my threshold for doing this myself--which is very rare. The guy in the video broke another part while doing it. Replace all the plastic stuff you can while you're back there, or pay someone else to.
Yes, it is wet down there as well which I should have pointed out. M273 engine.
The wetness on that little 'ledge' extends forward towards the front of the car just infront of the strut tower and drivers cylinder bank where that line is more easily visable. At first I thought it was the rad spraying back toward the firewall but I don't think thats the case. The hoses have never been all wet and there is somewhat of a shield from the strut tower there.
It looks like its coming from the line from the HVAC control valve going to the washer fluid reservoir. Its item 110 in the attached.
The coolant drips down, and becomes visible on, item #110, but I can almost guarantee that it's item #150 that is your issue. Replace everything you can while you're in there.
Good luck man. If you can get an assistant and take lots of pics we would appreciate it. If the assistant also has small hands, you might appreciate it.
Not sure why I couldn't upload a picture but got the parts..
Its engineered so that the male end of the hose coming from the engine simply pushes into the female end of the hole in the plastic and locks in. Very well engineered and convenient.
The hose hangs off the end of that barb and vibrates around putting a lot of cyclic stress on the green line where the nipple extension meets the body of the part. There is hardly any radius there which means you have a stress riser. Combine vibration, long lever arm, stress risers, extreme temperature cycles, and crappy recycled plastic, and that thing is guaranteed to fail. It's also in the second most annoying place to have to deal with a busted coolant connection.
The hose hangs off the end of that barb and vibrates around putting a lot of cyclic stress on the green line where the nipple extension meets the body of the part. There is hardly any radius there which means you have a stress riser. Combine vibration, long lever arm, stress risers, extreme temperature cycles, and crappy recycled plastic, and that thing is guaranteed to fail. It's also in the second most annoying place to have to deal with a busted coolant connection.
Unfortunately, nothing lasts forever. )) I do get your point; this part should have been made robust enough to outlast the car.
Just like planes, a car needs maintenance.
Unfortunately, nothing lasts forever. )) I do get your point; this part should have been made robust enough to outlast the car.
Just like planes, a car needs maintenance.
It's a mixed bag. Overall, the W212 is a great car. One of the greatest sedans ever made.
But it does have some areas which could easily have been made much better. With the number of parts and systems on the car, it's natural that there would be some "mistakes" compared to an older sedan that just didn't require as many separate subsystems and components. Hence the mixed bag...
Planes are decades behind cars in tech because of the amount of validation that is required. Cars are decades behind consumer electronics...
I will say one thing though. It would never be acceptable on an airplane to combine a critical engine circuit (its coolant) with ancillary functions in this way, with or without properly designing and testing the components.
The weather was getting way to hot and humid to do any work so I kept putting it off.
There isn't much to this other than getting the wiper arms off and wiper motor off under the plastic cowl.
Heres a summary.
- 3 nuts to remove the wiper arms. 2 on the passenger wiper and 1 for the drivers. The nuts are under the round plastic caps that cover them.
If your wiper arms are stuck to the wiper motor posts, you'll need a puller to remove them. Ask me how I know. Mine were stuck.
- 4 small plastic philips 'bolts' to remove that help keep the plastic cowl in place. Once they are removed, you can pry off the plastic cowl from one corner all the way to the other.
- 1 large nut to remove the arm going from the firewall to the drivers side strut. The arm is about 2 - 2.5 ft in length. Its there for support.
- 2 nuts and 1 bolt to remove the wiper motor. 1 nut on the drivers side. 1 bolt facing down half way on the wiper motor assembly; 1 nut on the passenger side.
- 2 electrical connectors to the wiper motor to remove.
I couldn't for the life of me get one of the electrical connectors uncliped from the firewall. See below. After spending a few minutes on it, I stopped.
Stripping it down to this point doesn't take long at all. I had to be somewhere later in the day so I popped on the cowl again without bolting it down,
Does anyone have any idea of how to unclip this from the firewall ?
I know its from the back end of the connector where the coloured wires enter the connector. I just couldn't trip it to release.
The rest looks straight forward once I get to it.
Last edited by - Mover -; Jul 23, 2025 at 10:57 PM.
Does anyone have any idea of how to unclip this from the firewall ?
I know its from the back end of the connector where the coloured wires enter the connector. I just couldn't trip it to release.
The rest looks straight forward once I get to it.
Here u go.....
I think for LHD car, I should have wrote PUSH DOWN the latch. Your brown wire is on top, for mine being a RHD intall orientation my red wire is on top.
If the connector is the same ( which I am sure ) , the locking latch teeth is pointing down to brown wire, thus it has to be pushed to RED color wire and DOWN for you and UP for me.
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Let me know how it goes, small thing like this is often irritating...I know.
My install version, RHD car.
Last edited by S-Prihadi; Jul 24, 2025 at 05:35 AM.
Reason: ADD INFO
I think for LHD car, I should have wrote PUSH DOWN the latch. Your brown wire is on top, for mine being a RHD intall orientation my red wire is on top.
If the connector is the same ( which I am sure ) , the locking latch teeth is pointing down to brown wire, thus it has to be pushed to RED color wire and DOWN for you and UP for me.
WIS has all the torque data for them, as per MB requirement and they are much lower than typical torque based on bolt strength.
Follow MB WIS for torque data, they are really proper.
I've had this undetectable coolant leak for YEARS. I think I may have the culprit here. What parts do I need?
You may want to strip it down to take a look and confirm whats happening in there, otherwise, you'll be spending time doing returns.
It doesn't take to long. It maybe a simple fix.
Last edited by - Mover -; Jul 29, 2025 at 09:36 AM.
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)) I do get your point; this part should have been made robust enough to outlast the car. 
