Hi all,

I have a 2012 E550 (M278 engine). Car has been leaking coolant at a slow rate and I was unable to figure out where it was coming from. It now leaks at a much faster rate from more towards the rear. I can see the drip from the bottom and it only leaks while the car is running, not while off. There doesn't seem to be any oil mixed in the coolant and the heater core looks fine as well. I have added distilled water (to not waste coolant) and see if I can get a better idea of the leak location. After a few minutes of the car running the fan started to run as well and I saw steam coming out of the back of the engine. Here is a video link
I've done every repair and maintenance on this car myself but it looks like I need to remove a lot to get to what I want to see so I am debating on taking this to a mechanic. Any ideas on where this leak can be from? Any instructions on removing items on top of engine? Otherwise anyone know a good trustworthy mechanic in Orange County California?

Thanks!

 

That leaking region is a plastic pipe assy called heater valve.
Depending on your M278 variant, where older engine does not have the small aftercooler coolant reservoir bottle, newer model has it.
See red arrow below : From https://www.benzworld.org/threads/m2...rvoir.3081144/






The plastic pipe with electric valve in discussion is this one : Version where it is directly bolted to Bank 2 ( LEFT )




If I recall correctly, newest Part A278 200 21 52 will no more have the electrical connection or basically no more valve, just plastic straight thru.
See here : https://www.autohausaz.com/pn/MB-2782002152

The video still showed older Part Number A278 200 08 31 where it has the electric valve and it is discontinued.
You can use the lastest P/N but your HVAC coldness will be reduced, as hot coolant from your engine will forever be circulating inside your heater core and only separated
from the EVAPORATOR cool side using air-flap. Where the old plastic pipe with water stop valve simply does not allow hot coolant to flow inside your heater core, which is the best way.


=====================

In M157 engine which is the bigger AMG brother of M278, the heater core water valve is different, see here :

I am not familiar with M278 and M157 like I am familiar with my M276.820 3.0 Turbo.
But I do pay attention to all these 3 engines cooling system.

The other plastic sh-iets you must also replace while you are in there will be :
The 2 hoses as shown on this video below .............


Don't smile yet....more shi-et coming.
These parts WILL BREAK 100% guaranteed too, soon if yours has not broken yet.

....



Next will be best you do it as preventive : Oil cooler gasket





Oky doky..... you may cry now.

I am teasing + encouraging you .... because I am doing the same preventive works and much much more than yours, as part of my E400 10th Bday .

 

Well I certainly came to the right place. Sorry for the delayed response. So my car doesn't have the reservoir. If it is the valve and I can access it by moving the firewall, that would be a lot easier (hopefully that is possible with this car). Everything under the hood is just so densely filled there isn't room to put a hand in anywhere. I'm worried it might come down to removing the the intake manifold since the steam was coming from underneath that, and that alone requires removing a thousand and one pieces. I will check again to see if the firewall is moveable and if it will give me easier access to viewing the problem.

I did see on a different thread someone poured in a mixture into their coolant to make to glow under UV light to see where the leak is coming from. Might give that a shot as well. Any opinions on that?

As for the turbo coolant hoses, I have already changed all (most?) of them, and that for sure was a huge pain in the ***

The oil gasket is one I am definitely not looking forward to haha.

Thanks for your help!

 

Yes, M278 engine top part is so high density with components. I feel you.
No need UV dye, just get a boroscope camera. Some are very low cost.

At the back of the engine, there is only 1 coolant output path as far as I know. That is to heater core.
What I do not know is the coolant system of the charge-air cooler on top of your engine. I never yet took a deep look at it as I don't have access to an M278

But by position shown on your video, that is heater core related engine coolant out plumbing.

 

I have an M278 that I can look at, but I have the coolant reservoir on top of the engine, so I don't know if it would be of much help...

As Prihadi mentioned, there is a plastic piece that goes from the back of the engine to another plastic piece on the firewall. The hose goes through the false firewall, and working in that area is a huge PITA. There is a youtube video of a guy fixing the piece at the firewall. During the process, he also breaks the part on the back of the engine so he replaces that as well. Unfortunately, he doesn't really show it, but he does it by unscrewing the fasteners holding on the false firewall and kind of shoving it slightly out of the way.

This part is located on the back side of the engine and not the top under the intake manifold, but because everything is so packed in, the steam might be appearing in a different place than where it's escaping...

 

For all the coolant leak detectives, our engine V-valley has a drainage outlet that dumps out on the side at the rear, near starter.

So you may have a leak up front that looks like its from the rear

Also our coolant circuit is not built to be high-pressure. Replacing the plastic thermostat is really easy on non-turbo and great cooling insurance.

Old Tstat hardened shaft seals make it lazy to keep reliable temps. Don't only focus on the current leak location but its root cause else every seal is gonna keep venting pressure.

• radiator leaky side boxes
• radiator mixing ATF... mayo
• Coolant/oil heat exchanger.... mayo
• timing chain cover seals... mayo

My car has consistently smelled hot coolant since factory new... the best MB factory warranty + MB extended warranty could never deliver a non leaking Mercedes engine.

I've had time to rationalize it was only a small leak until I tested removing extreme heat stored in dry-lubed pistons.


Now my engine no longer leaks coolant and surprisingly my garage stays at room temperature when I return home.

 

Did you put mating connectors on the unplugged oil solenoid connectors or did you just put boots of some kind on them. I would like to add connectors but I am not 100% sure what kind fit.

 

I simply unplugged and didn't look back... everything is still left airing out uncapped!

Some ppl figured the pump solenoid uses the exact same 2x wire connector as the VVT Solenoids (not the 3X wires CPS!).

If you're gonna go that route with a "pigtail" .... then patch in a dummy load on the harness side to dismiss ECU fault. absolutely not necessary without CEL but a cheap effective fix.

 

Follow this thread... this is where Master Surya gets into exact connectors to help us plug loose ends.


U can trust... divers are not jokers !!

 

I actually had a feeling those were the same connector but wasn't sure. Thanks!

 

uQuik ler-ner uhm sai hahe zaiha