Citric acid flush -- a caution
Drained the coolant from a high-mile '83 380SL last month. The PO had suffered from the all too common can't-read-instructions syndrome (either that or his mechanic had it), and the cooling system had been serviced with ordinary tap water and green Prestone.
Presto, lots of scale in the system. On the first service I did, I flushed completely clear and installed Dex-cool and distilled water. This made the car run somewhat less hot, but it is still an overheater.
When I got started last month, I drained all Dex solution and refilled with tap water plus 100g/liter of citric acid. Ran that up to operating temp for a few cycles and drained it.
The citric solution shook loose a LOT of scale, which showed up in the reservoir. I suctioned and drained the system as best I could and then put tap water back in for a first flush to dilute out the remaining citric.
At that point I discovered a heater core leak, and put the job on hold for a few weeks while I considered my options.
Now I'm back to working on it and I am finding that there is *still* scale coming out -- flush after flush after flush. I'll drain, refill with tap, take a cruise to get the car hot (it gets to 100C just with casual driving), bring it back, and when I look in the reservoir, there are visible scale chips floating around. Plus black rubber bits from the insides of hoses.
It's coming down but it looks like at least 15 to 20 cycles will be needed to clear out the last of the scale. Amazing. I think that the use of the Dex-cool for a few months softened the scale, or at least it did in the reservoir where I could probe at it. The citric then began to let the softened scale break up and peel away, and now more of the stuff is doing so every time the coolant passages get up to boiling temp. Cavitation from boiling water is probably assisting.
Anyway, I am going to keep flushing until the system stays clear. At that point I'll pull the rad and straighen the fins, to see what that does for cooling. A fair number of them are bent. Might have to have the rad rodded and tanked, though.
So, citric users, the moral is that in a heavily scaled cooling system, scale doesn't all come free on the first flush after the citric. Plan accordingly.