Thanks. Can you elaborate on your setup? I think 5% is probably good. I used 10%. 1.5 lbs acid in 15 lbs reverse osmosis water. I drove for about 20 minutes but I know how it takes a while to bleed the system and wanted to give it a good highway run in daytime. When I came out in morning
There was a ton of water on floor. I tightened upper hose on radiator and it just broke off in my hand. Do you think it was citric acid that did it?

Can that stuff do any damage inside engine and head?

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

The factory service manual advises to use an "open thermostat" SST to do the citric acid flush - I did not have that and used a rubber cork in the bypass port and put the flange back on without the thermostat.

The 5% solution cleaned out the engine nicely. A lot of rusty and milky water came out.

I then neutralized it too with some baking soda just to be safe.

The radiator pipe fitting breaking off is an age issue and probably also caused by the lack of the metal collar reinforcement. Its a known problem and citric acid does not cause it - infact if I were you I would consider it a good luck sign that it broke off in my driveway and not on the highway.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)