The following procedure (found in the service literature available online) details the coolant system flush for a couple of different engines.
http://mb.braingears.com/126_DISC2/program/Engine/107/M117_56/20-015.pdf

My engine being the 116.965 in an '87 420 SEL, I want to know where in the engine compartment (or otherwise) I might be able to find the "vent valve of the supplementary heater" mentioned in steps 7, 12, 16, 21 and 22 of the procedure. Does this specific vehicle/engine have a supplementary heater and associated vent valve?

Also, when the tool "flushing pipe connection with hose piece" (which I've created from fittings and hose pieces I bought at ace hardware) is attached as described in step 3 of that same procedure, should it connect between the radiator and the lower coolant hose or the radiator and upper coolant hose? Does it actually matter that much? I had imagined that it could be easier to operate if connected to the upper hose, but it also might flush better if connected to the lower hose. The photo shown in the procedure does not show enough area or detail that I can easily distinguish the location.

Any thoughts regarding the matter?

Not familiar with this model - those who are will chime in. If the braingears doc didn't say so, also remove the t-stat B4 you start the flush. Allows flush water to get thru more of the coolant circuit.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I've actually ordered one of the permanently opened thermostats to allow flow to the radiator and plug the bypass across the whole temperature range. It's supposed to be here later this week.

Good luck with that.

Engines are designed to run at specific temps. If your 420 runs TOO cool, you will run rich.

Just my 2 cents.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

The positively opened thermostat is for flushing only. It's described in the flushing procedure that I linked to in the original post. It's needed to make sure that all of the citric acid solution flows through the radiator and doesn't bypass it through the bypass valve instead. It's used considering running tap water through the engine won't let the engine heat up enough to run anything through the radiator with a normal thermostat.

Sounds like you're wired up and ready to go. Happy flushing.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

So...

"vent valve of the supplementary heater"

anyone??

A Webasto(?) gasoline fired supplementary heater was available in the 126 cars in non-US markets. It permitted one to preset a time to automatically warm up the passenger compartment and engine cooling system before starting the engine. I don't believe this option was available in the US due to fire hazard concerns but many grey market cars arrived here with the heater installed. Mark

Can you also order the flushing pipe connection with hose described in the braingears link you provided? Sounds a little easier since all the water is discharged out the radiator drain.

hookedon210s: Thank you for that description. If that's the case, I'm fairly certain my car doesn't have that option and will ignore those steps in the procedure.

Educaid: Yes, the flushing pipe connection can also be ordered. Since it's pretty much just a tee fitting, I went to the hardware store and made up a tee connection out of a bunch of fittings with a valve between the tap water hose and coolant line that I could screw directly onto the end of a garden hose. This way I can use the valve to regulate water flow and don't have to run back and forth to adjust the tap water flow from the wall. I'll take a photo of it and post it when I get a chance.

As for that connection to the car, do you think it should be connected at the top of the radiator? After a closer inspection of the photo while I was outside with my car yesterday afternoon I decided that it was probably connected to the upper connection in the photo. I also don't think it matters that much as long as the water goes in at the same rate that it comes out. Any thoughts?

...drain and refill the system a couple times with fresh water, then...

Install the T-fitting (& cap) on the inlet (hose going to) the heater. Then flush a couple times with the fresh water hose running into the T-fitting, radiator drain plug open, motor running.

That should do it. Out with the old, in with the fresh...

Cheers!

__________________
'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435

I assume from the photo on braingears that the device is attached to the upper radiator hose which is then attached to the thermostat housing. As the water enters the device, it pushes both through the thermostat into the engine and also through the radiator. The old coolant is thereby replaced by the water and is eventually expelled through the radiator drain plug.

Did you order the open thermostat and coupling from the dealer and if so how much did they cost? Thanks.

I ordered the opened thermostat from a mercedes parts warehouse in St. Louis. The coupling I made myself at the hardware store (as described in my previous post). The opened thermostat cost me around $65. I'd be willing to sell it for less when I am done with it if you are interested in using it.