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Old 05-23-2003, 09:30 AM
Registered User
Join Date: Apr 2003
Location: Boston, MA - USA
Posts: 17
Citric Acid - Leave in car for How Long?

1989 300SE - M103 - 136,000 miles

Cooling flush with MB Citric Acid:

1) How much of the bottle (500 grams?) should be used for the M103 Motor in a W126 ?

2) How long should this flush remain in the motor (by days or hours or driving time) ?

3) How many times should the system be flushed out with clean water?

I don't have any practical way to undo all the drain plugs and will have the mechanics drain the system through the right lower radiator hose. It's a lot easier for me to have them do it several times (flush, refill with water, drive for a day) than to get them to open other drain passages in the block.

Thank for your comments and help!
Michael Stoic

1979 450SLC
1989 300SE
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Old 05-23-2003, 10:38 AM
Mr. BILL's Avatar
Ghoulardi Rules!
Join Date: Aug 2000
Location: Austin, TX
Posts: 576
1.) Doesn't the container have the instructions on it? 2.) I'm thinking running the citrus solution for 15-30 minutes would suffice. 3.) You should flush the system until the water coming out runs clear. Don't drive around with just water in the sysyem.

Why is it not practical for you to not be able to open the drain on the block? It's right there on the passenger side by #5 spark plug. A 19mm wrench will do the trick. Of course, if you don't have the tools or a suitable location to do this I understand.

However, I really don't understand why your mechanic can't do it. If he can't, you need a different tech. Draining the block is one of the easiest things to do. You simply loosen the plug and let it drain. The radiator is the same proceedure

91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)
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Old 05-23-2003, 10:39 AM
I told you so!
Join Date: Jun 2002
Location: Motor City, MI
Posts: 2,804
I routinely use a 5-10% citric acid solution (in water) where I work to remove rust from small samples so I can examine fracture surfaces.

I wouldn't leave the solution in the system any more than an hour. Even that may be too long. Citric acid is a mild acid that can removes corrosion without much affecting the base metal. But if left too long, it can start corroding healthy metal like any other acid. This is speaking from experience where I forgot about my samples and left them in solution overnight.

Ultrasonic cleaning in the solution - as I use in the lab - "melts" the corrosion off the surface. Unfortunately, we don't have that luxury with our cooling systems.
95 E320 Cabriolet, 140K
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Old 05-23-2003, 08:06 PM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
Hi Michael,
I did a flush on my '87 420 last year and I recall that the procedure needed about 1.5 kg i.e. 3 containers at 500 gr. at $16 from MB. I believe the container mentions the amount of water or the CD. You have to let it run for 15 minutes at 2500-3000 rpms. Then drain and flush with fresh water and eventually fill with MB coolant (at least 40% solution for anticorrosion protection).
During the flush you want the heating on so the stuff goes also through the heater.
Reinhard Kreutzer
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Old 05-24-2003, 11:42 AM
Join Date: Jan 2000
Location: Maryland. USA
Posts: 180
Recommend you deoil (basiclly nonsudsing detergent [bottle of SHOUT spot remover] ) first. It only adds two steps since you'll be decalcifing afterwards. ANY residual scum/petroleum will interefere with the decalcifying solution.


Removing the thermostat can speed things. HOWEVER, you can crack a block in a heart beat so be mindful of the flushing and filling.

Concur with previous posts about opening the black drain plug.

Unsolicited thoughts: Fill with your favorite Anti-Freeze. I'm a fan of Mercedes-Benz anti-freeze and distilled (de mineralized) water with Red-Line water wetter. Most go 50/50 but I don't live in extreme winter area so I reduce the antifreeze side of the mix per the info on the bottle. Water is a better heat transfer medium than antifreeze. For a few extra bucks spent on MB fluid, I worry less (it has their anticorrosion package, neutral pH (almost)...).
S, J.R. Brown
2000 G500 LWB Obsedian Black
2005 Toyota Tacoma Access Cab Off Road Sport
1993 Volvo 240 Sedan Anthracite
1980 450SEL Champange (owned it for 15 years. Great car)
1986 280GE LWB Anthracite (Sold it and kinda wish I hadn't)
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