With some trepidation, I decided to go ahead and do the citric flush. Upon flushing/backflushing the various connections I noticed that my system was in amazing shape (no rust/sludge) which was no real surprise as my '82 300SD was well cared for by PO and me for the past 5 years. I mixed one pound of citric acid to one gallon of warm water and added to system and then followed with another gallon of warm water and then some and let idle with cap off for about 12 minutes and then flushed all connections numerous times. I did notice that my block drain had some rust/scale on the end which was gone after the flush -this stuff works fast! My advice is if you have a clean well maintained system, I would not worry too much and just go ahead and do the flush. If when you flush with water (before the acid) and notice lots of sludge and rust -BEWARE! I would pass on the citric acid and just flush with water and refill. Also, I suspect that some of you who have had immediate leaking issues are either using high concentrations of acid or letting acid sit in your system too long -don't do this as the citric acid does work fast as evidenced by my block drain bolt.

A few tips - I found that a hose nozzle that was tapered (cone like) with a shut off valve was great as the nozzle would self seal in the smaller hoses and provide great pressure. I found a large rubber plug with a small hole that fit in my upper radiator hose so I could backflush with good pressure without water backing up and coming out quickly. I did order a block drain plug for a 201 motor (looks like a large brake bleeder) and can confirm that this will not work in the OM617 motor block drain -too large. I would like to make a petcock for the block drain as this would be much easier for the draining flushing process than reaching up and having hot citric acid splash out on me as I am laying off to the side -

I need to do the TD wagon next and will take my own advice as its history is somewhat questionable -if I see evidence of much sludge/rust -I will forgo the flush and just flush with water . Have fun, this project is very satisfying, I will do this every two years in this particular car and will use MB coolant, good stuff!

Why hold off on the critic flush is there is rust or sludge present? Isn't that the purpose of the flush?

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1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k

Is it not safe to use the citric acid when there is rust/sludge present?

__________________
1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k

My thoughts are now this after having read the many posts where members developed leaks in either the radiator or heater core - if you have a system with a lot of rust and sludge, it is highly likely that you already have a lot of corrosion and weak areas in your cooling system -notably the thin walls of the heater core and radiator. When you flush with citric acid, the acid etches both the good metal and weak metal and sometimes the weak metal is just intact enough to keep your system from leaking so best not to take chances in stripping away more structural metal with acid unless you want to replace a radiator (not a big deal) or worse, the hard to access heater core! Most Mercedes seem to receive pretty good care over the years by maintenance minded owners -if this is your car, go for it, if not and you see lots of sludge and rust come out BEWARE! Good luck!

Thanks for the info. If there is only rust present(no sludge) will the standard Preston radiator flush be sufficient?

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1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k

I think you have the concept backwards. There is no point doing an acid flush on a well-maintained system.

??? I was thinking the same thing. What should be used to remove rust and/or sludge? When is critic acid suppose to be used?

__________________
1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k

I think the point is, on a well maintained system, the scale that develops is easily cleaned with the citric flush. on a poorly maintained, scaled up and full of garbage system, a flush is still a good idea, but plan on needing a new heater core, or radiator...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Got it.

__________________
1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k

According to the FSM, the acid flush is intended for "derusting and decalcification." Apparently, deoiling is a necessary prerequisite in order for the acid treatment to be effective.

Preston is strong stuff too -just flush and backflush with lots of water, my advice -

No, we are in agreement on this. If your system has been well maintained, no real need to do the citric flush though scale will develop naturally - I think I would descale maybe every 3-5 years. Using MB coolant consistently helps -stuff is great or perhaps Zerex G5

Yes, I used Shout first before the citric acid to remove oils per DG's excellent tutorial. Another thing -MB sells citric acid for 20 bucks and more -I bought a pound of it for 3.10 at a winemaking supply house. Citric acid is basically a food grade product as my MB parts guy will attest as he tasted theirs -

Just completed the wagon also, same method. The wagon has a newer radiator and upon pulling heater hoses block drain, etc. I noticed no evidence of rust but did notice that someone had filled with green coolant with a very high concentration so I went ahead and flushed with ctric acid for about 12 minutes or so and rinsed/flushed all connections several times until water ran clean. A good feeling to have both cars flushed clean and ready to go for two more years with the right MB coolant and clean system! Note, I dropped one of the thermostat housing bolts and could not find it with mirror, light, compressor, etc! So I decided to work on and then got the idea to start up the engine and let vibration do the rest -voila! dropped to the floor and I proceeded to finish the job -love these cars!