Hello all,

I have been rsearching the proper technique to do a coolant change on my 87 300d, but I can't find the answer to one question I have...I have tried looking a lot!

The FSM calls for a forcibly opened thermostat when flushing out the system. Infor I have read, such as (Pelican Technical Article - Mercedes-Benz W124 - Coolant Flush and Replacement) state to just remove the thermostat when flushing, but this info doesn't say if it will work for the 300d...

So...should I just remove the thermostat when flushing out the system? Or is there some sort of trick/step that I need to know about.

Thank you and sorry for such a basic question!

I cant think of anyway to "forcibly open the thermostat" without first opening up the thermostat housing to get at it. At that point it'd be easy enough to just take it out. probably wouldn't want to or need to expose the thermostat to any flushing chemicals or unusually high concentration of muck that could be stirred up during the flush.

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

I recall reading a while back on here that the thermostat being in the path affects the fluid dynamics in the system. Something about running with an empty thermostat chamber is bad for the flow and it can cause hot coolant to bypass the radiator and cause a rapid overheat condition.

The "forcibly opened thermostat" is a special MB tool with a part number. You're supposed to open the thermostat chamber and install this special thermostat for flushing.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Hmmm...then how have people done a coolant flush on the 603 without the special tool??? Is there something I am missing? Thanks for the help!

Looked up the part and it is $200...I am sure that people have flushed the 603 system without using this part, just trying to figure out how

Drain, refill, repeat.

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CENSORED due to not family friendly words

All motors have some way of allowing coolant to circulate around the block when the thermostat is closed. The reasons should be obvious: avoiding localized boiling as the engine warms up, having circulation around the thermostat so it senses that the engine is hot, and avoiding pump cavitation.

Mercedes engines have what's known as a "balanced flow" system, which means that coolant circulates at the same rate, regardless of whether the thermostat is open, closed or in between. To make that happen, there is a "bypass passage" which allows coolant to loop around from the head to pump. The thermostat is located between the engine inlet (where cooled water enters the engine) and a bypass port. The thermostat is "dual poppet", meaning that it has two disks, which move in tandem. One disk controls the bypass port, the other disk controls the inlet passage. As one closes, the other opens and vv. So when the thermostat is fully closed, coolant flows around the block without passing through the radiator. When the thermostat is fully open, nothing circulates through the bypass passage.

I would guess that the standard flush procedure doesn't last long enough to guarantee the thermostat will open, so there must be a special tool which takes the place of the thermostat and allows it to partially open regardless of temperature. Simply removing the thermostat is practical, cheap, and simple. But, 'cmon, this is a Mercedes service procedure. Those are three adjectives that don't apply.

@mxfrank...thank you! What you wrote is exactly what I have pieced together from research over the past few weeks...

But I still don't understand if removing the thermostat will provide a solid recirculation of coolant between the expansion tank, radiator, and block.

I have read, and you hint, that running the engine, to perform a flush, with the thermostat removed, can lead to overheating...another poster said the same...

With the "forcibly opened thermostat" costing $200 (the proper procedure for a 603 coolant flush), I wonder what other people have done to flush the coolant on a 603 without buying the specialty part??? F

If you're going to replace the thermostat, have an old one or don't mind sacrificing a McParts store thermostat, cut away the closing plate to fashion an always open thermostat for flushing.

Sixto
83 300SD

Without a thermostat, both the bypass and the cool water return are open to the pump inlet. Both sides will flow, it's like turning on both the hot and cold taps. So you would expect the car to run warmer. On a 603, that will be the worst you'd see. Some cars will overheat (don't try this on a V6 GM car).

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