I recently went to vacuum the w124, when i lifted up the passenger-side floormat ,i noticed that the foam padding of the floormat was soaking wet with antifreeze, the driver's side floormat however is dry, i believe that the heater core is leaking into the cabin.

Has anyone done a heater core bypass on a M104/w124?

any advice on how to go about bypassing the heater core, and what materials will be necessary to do the job would much be appreciated.

thanks,
tom

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1993 300E 2.8L M104

I think its the mater of joining up both hoses , one going in to the matrix under the hood and the left firewall ,and one on the other side of the firewall that exits in to the engine .Just like cutting off inlet hose joining up with another length of hose to the exit hose so coolant flows back in to the engine bypassing the heater matrix.

Connecting the heater hoses together gives the same result as partially bypassing the radiator. This would be OK in cold weather but on a very hot day it can lead to higher coolant temps.

Blocking the hoses is a much better solution , you can get rubber caps at most auto parts. At the back of the engine / cylinder head ( left / drivers side behind the oil filter ) , remove the hose and use a rubber cap , this is the supply.

At the front of the engine there is a pipe that goes across the front of head to water pump. Remove the hose from the pipe and cap the pipe off. This is the return. Note, there are a few different M104 water pumps so the pipe I speak of might be in a slightly different location.

You might unplug power to the aux pump so it doesn’t burn out. Maybe the monovalve too.

Sixto
98 E320s sedan and wagon

97SL Thanks for clearing that up .

Here is a cooling system diagram for my 97 SL320 M104. I eliminated the washer and windshield heater since the car never gets driven in the winter. This was done by using plain hose not the molded " T " hoses.

I also added a ball valve to the supply at the back of the head. I found that when the car is off, the mono valve opens and allows coolant to thermo siphon increasing interior temps.

Attached Files

COOLING_SYS_FLOW.pdf (39.2 KB, 241 views)

a big thanks to optimus, 97 and sixto.

made a trip to a couple of local autopart stores yesterday and managed to find everything i needed to bypass the heater core.

items used for bypass:
1x 1m automotive heater hose (inside diameter 19mm/the stock benz hoses seem like they are 18mm)

2x 19mm(diameter) straight brass hose fittings

and
4x hose clamps


i basically used the new hose and brass fittings to connect to the hose which comes out of the cylinder head at the back of the engine on the driver's side of the engine bay and ran that behind the engine all the way to the hose which comes from the auxiliary pump and monovalve. i disconnected the monovalve and auxiliary pump and gave it a try .

so far,so good, no leaks, and am not noticing any increase in engine temps.


initially , my plan was to just bypass the heatercore and NOT the auxiliary pump and monovalve, but from the looks of it, it seems like heatercore output has two pipes, so a Y-brass fitting would have been necessary , which i did not purchase.

hopefully this fix holds up until october when i hope to replace the heater core

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1993 300E 2.8L M104