My 220D w115 has an overheating problem at high load, so I decided to try a citric acid flush. You all previously warned me of the risks, so I feel I went into it reasonably well informed.

I did the following:
1) remove thermostat
2) remove old coolant by opening radiator and block drain, close again.
3) refill with degreaser (shout and water), drive until hot, let cool, drain. Rinse out degreaser from system.
4) refill with 10% citric acid solution (MBs formula), drive until hot, let cool, drain. For this one step, I kept the heater system closed to avoid damaging the heater core. Other rinsing steps had the heater flow open.
5) refill with hose water, drive until hot, let cool, drain.
6) repeat step 5 at least 5 times...... I also removed the hoses several times and flushed with a hose through every available opening

After all this, the water coming out of the system was still rusty colored. So it occurred to me that perhaps the rust was coming from the heater system, which never saw the citric acid. So I decided to flush the entire system again, but this time using Prestone Radiator Flush. I used the same methodology as above, but with the heater system open the entire time. I used Prestone's instructions for a "heavy" treatment which means I kept the stuff in the car for 3 days of moderate driving.

After doing step 5 several more times, I decided to try distilled water for another flush. The results of that flush are in the attached photo. You can see the water is still pretty murky. So I guess my question is, when can I stop this madness? When is it "good enough"? I honestly didn't expect to be at this for a week with probably 15 total flushes of the entire system. Am I just shooting myself in the foot if I do my final fill now (40% Zerex G-05/60% distilled)? Initial tests with pure water indicate that the car is actually running cooler now(yea!), but I just don't understand why the water coming out of the system is still so rusty colored. What would you do at this point? Thanks for your help.

(as an aside, if at all possible, please don't advise me to use Evans. I have read about it carefully and prefer a water/G-05 mixture in my car)

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).