ODIOUS

I have 90k miles on my 2011 4matic and will be replacing the thermostat soon. I'm debating on whether to just top off with MB antifreeze or going ahead and doing a complete system flush. From my quick search the rec is to change / flush every 15 years or 150k miles.

I have just a few questions:

How many premixed gallons might I need to refill the system after a quick thermostat change?

Do you good people recommend doing a complete flush at this point? If so, how many gallons would I need to do the whole system flush?

Any special considerations, things to watch out for or ticks / tricks when doing a whole coolant flush?

I haven't actually found the flush procedure for the W251 yet but I think it's bcos most people haven't hit this scheduled service yet. But if anyone knows of a link to a walk thru or write up if appreciate a link.

Thanks in advance

marc hanna

I would just top it up. Last year I did a flush on my '05 E320 and my '08 R320 and the coolant came out nice and clean. No debris no nothing. Clearly, it was unnecessary.

That being said, if you do decide to do it, you will need 2-3 water cooler jugs worth of distilled water to complete the flush and refill if you do it properly - this includes a cleaning stage and flushing out of the cleaner (citric acid/distilled water blend).

There is a red drain at the bottom of the rad - use a large flathead stubby screwdriver to unscrew it. It has an anchor to keep it from falling out completely.

The colour of the replacement coolant doesn't matter - MB changes up the colour every few years - I think as a quick indicator of how old the coolant is.

The replacement volume of coolant will be around 10 litres (2.5 US gallons).

If you do the full clean don't buy the citric acid from the dealer, it will cost a fortune. I just bought 5lbs of citric acid off Amazon for about $5. MB gives you a few ounces for about $25.

Full system clean and flush procedure:
1. Drain old coolant
2. Dissolve 1/2 cup of citric acid into 10L of distilled water
3. Fill with acid/water blend and run the engine for an hour with heater on full
4. Let cool and drain
5. Fill with clean distilled water and run engine for an hour with heater on full
6. Let cool and drain
7. Repeat 5 and 6
8. Refill with MB coolant/distilled water mix
9. and Bob's your uncle

marc hanna

Here is the drain plug in case you need to replace it:

https://www.ecstuning.com/Search/Sit...lug/ES1604414/

marc hanna

Here's a view of where the drain plug is located:

auto385

The method you described is ok but you have to watch your dilution rate. Draining the radiator doesn't drain the block completely as well as the heater core. So you are blending pure water with a blend. If you measure the total drained fluid from the radiator and subtract that from the coolant capacity, then you know how much is left in the system. You add pure coolant to the system until desired ratio is reached. Then you add the premixed amount to top off.
Good luck.

marc hanna

I agree, there is a little that gets left in there, but I have found that as long as you have the car facing downhill on a gentle grade, you get about 90% out. I have found that if you fill it with the mix, that afterward as the air works out of the system and the reservoir level drops, you can top it up with pure coolant and that gets you to the correct dilution.

ODIOUS

@marc Hanna you are half man and half amazing. Thank you for that excellent explanation and my apologies for just now reviewing this response and not replying sooner. I'm going to press the easy button and just drain a little and top her back off.

marc hanna

Thanks for the compliment, but really, it's not necessary. Happy to help.

ODIOUS

I got her done this weekend. Very straight forward. Even though I drained as much coolant from the reservoir as would come out it still wasn't enough to prevent coolant from spilling all over the place when I removed the thermostat. Very weird. Popped the old T-stat off and popped the new one on. Torqued it down to spec and replaced the serpentine belt while I was at it. Used about 1 gallon to refill what was lost. Took about 3-4 days for the CEL to finally go out.

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