This is a spin off from this thread : https://mbworld.org/forums/e-class-w...rainstorm.html

I finally figured out to why M278 can have Y16/2 heater shut off valve for its hvac heater core, vs M276 which never get a Y16/2
The M278 Y16/2 shut off valve which later on due to often physical breakage of this valve, the newer parts has no more the electrical water solenoid in that pipe at the rear of the engine....just simple pipe.

I been wondering, how can M278 CLOSED its Y16/2 heater shut off valve when thermostat is also still CLOSED during cold start, how does the coolant circ. pump can get liquid coolant to suck ?
There is no gallery or coolant port in engine block for coolant circ. pump to suck, it has to be from a hose of the radiator return/output.

The answer lies on how M278 coolant circ. pump is different than M276 3.0 turbo or 3.5 NA.
M278 turbocharger coolant lines, its RETURN is sent back to coolant circ. pump suction side at the FRONT of body of the pump itself, unlike M276 3.0TT which uses a Tee fitting at the suction HOSE of the coolant circ.pump.


M278







This could also be the reason ( aside higher plastic percentage), that the turbo coolant lines of M278 has higher failure rate than say M276 3.0 because during a thermostat CLOSED and Y16/2 valve CLOSED, the full power/pressure of the coolant circ pump is experienced by ONLY the turbocharger coolant lines/hoses and no other coolant pipes. If user of M278 is wild and do WOT before the thermostat partially open ( pressure reduced ), those coolant hoses will suffer more.

Do not under estimate the power/flow of a coolant circ pump. https://asecertificationtraining.com...olant-systems/
Lets just use 24,000 liters per hour, that is equal to a 2 HP swimming pool pump which I use and the head can be as high at 30ish PSI due to filter system and pipe head.
On marine engine, the sea water pump with rubber impeller is even more powerful than coolant circ. pump.


M276 3.0 turbo or 3.5 non-turbo* ( *simply remove the turbo cooling circuit )




If M276 3.0 gets a Y16/2 heater shut oof valve , it is still possible, and the turbo coolant lines will have increase coolant flow and pressure too like M278.
Maybe there then the plastic ends of the M276 3.0 will suffer shorter live like M278.

Now, what if M276 3.5 non-turbo gets Y16/2 ?? .............. IT IS NOT POSSIBLE
If the rear coolant output where Y16/2 is at, on M276 3.5 NA is fully closed, its coolant circ. pump will have nothing to suck and will cavitate and probably damage its suction hoses....when and if thermostat is still closed.
So, I think since M276 3.0TT needed to be as little different to M276 3.5 NA as possible..... to save development $$, hence the M276 3.0TT get that stupid looking turbo coolant line return like a last minute after-thought.


This is an M276 3.0 TT
1 in green is the coolant return line from Bank 2 turbo into a tee ( can't see from photo, it is behind ) joining Bank 1 turbo and 2 in green is the return joining the HVAC heater core return into 3 in red also a Tee.



Below EPC parts list : Item 40 is the turbo coolant return combo/Tee-ded Bank 1 and Bank 2 or item 1 in green for photo above. Item 2 in green of above photo which is Tee to Tee interconnect is not drawn at below's EPC parts list.
Item 80 and 100 are FEED coolant line for turbos and are plumbed from the rear of the engine block, unlike M278 which gets its turbo feed coolant lines direct at its coolant circ pump.





In theory, M278 when its thermostat is fully/partially open, its turbos get fresh cooled coolant from radiator output ( via coolant circ. pump ).
In M276 3.0 case, the turbos are forever getting "spent" hot coolant as feed supply even at thermostat fully opening because its feed posistion is after the coolant has cooled 3 cylinders ( Bank 1 and Bank 2 of a V6 ).
NOT GOOD and is BAD for me in the tropics. The price paid for afterthought engineering

There is a unique video of someone modifying coolant path to save the last most rear cylinder from killed early due to getting "hot-spent" coolant due to its unfortunate position.



This hot spot problem also occurs on a MAN V12 marine diesel most distant cylinder.

 

Keep in mind that there are 2 routing versions of cooling for the M278. Really more like 6 if you include the S,SL, GL,ML, and G class, but it's mostly the plumbing routing, and not the actual layout.

There is one that has it's own turbo coolant loop, and another that does not.

The around 2013 MB changed it to the independent turbo coolant loop for both the M278, and M157.

 

Thanks S.M ,

If the water pump design has not change, I mean those connection 4 holes for turbo coolant lines feed and return and only having the additional small coolant recovery bottle like M276 3.0TT, it is the aftercooler coolant circuit which is separated, not the turbo.
The cooling system image M278 I have is older version where aftercooler circuit shared the same big coolant recovery bottle belonging to engine block coolant circuit.

I sometime got "blinded" too and thought that the small coolant recovery bottle is for turbo ... LOL, while it is actually dedicated to aftercooler only.
Soon I will install a thermocouple to the metal skin of the currently installed coolant pressure sensor hose adapter, hoping to see how hot does the aftercooler's coolant circuit can be.
EPC called the radiator for aftercooler which is dedicated on my engine for aftercooler only, as Low Temperature.
Hottest air intake I seen is 57C at the track, so I supposed assuming heat removal as good as say 20C delta, maybe the coolant for aftercooler can be up to 77C.

 

Interesting stuff as always Prihadi!