A real Citric Acid flush is primarily a Mercedes Benz "thing" although any engine can benefit from it and many radiator flush products contain some form of acid. For the best results you need to do the involved MB proceedure IMO. It is quite involved and messy however and it will take you most of a day to do. If you want to use Citric Acid to clean the cooling system though this is the way to go.
Below is a quick outline of the proceedure...
1 - do a thorough flush with water then drain the system completely.
Note - you will need to pull the lower radiator hose off the block and radiator AFTER EACH STEP to allow all the rust and debris that is broken free to come out. Just draining through the radiator petcock is not enough.
2 - refill cooling system to proper level with JUST water containing some kind of degreaser and idle for a while( approx an hour I believe? ) to clean any and all oil and grease out of your cooling system.
Note - the acid won't do it's job if there is any oil or grease in there.
3 - do another flush to remove all pockets of the degreaser and any debris. Drain and refill with water and the Citirc Acid. Idle for a while( hour? )to remove scale and deposits.
4 - do another flush then drain again and refill with the appropriate anti freeze and an additive like Royal Purple Purple Ice or Redline Water Weter to help stop it from happening again.
Note - during the degreasing step and the Citric Acid step, while the engine is idling, make sure the heat is on full bore so your cooling system fully circulates the water.
Mercedes offers the Citiric Acid Part# = A.000.989.10.25( older # - still good? ) or you can buy it in bulk at wholesale food/candy supply stores. Maybe even at a pharmacy. Prestone offers a flush that contains Sodium Citrate and Gunk Super Radiator Flush also contains Citric Acid( or they did anyway - still? ).
Mercedes also offers the degreaser Part# = A.001.986.21.71( older # - still good? ). Lots of people use different things though such as dishwashing detergent or degreaser cleaners. Shout was always popular with folks. I have heard/read however that laundry soap is not a good choice as it gets too foamy?
You can find more info on how to do this, complete with the proper amounts of product to add, on most any Mercedes site( it is where I got the MB part #'s ). Only car mfg I know of that really gets into this so the MB forums are your best source for accurate and safe How To info.
