The previous owner ran tap water in the cooling system for few years, and we have very heavy, mineral rich water locally. When I bought the car about six months ago, I drained the water, and put a standard pre-mixed "green" glycol based coolant in as per the manuals recommendations.

Recently, I replaced the thermostat ( actually there was no thermostat -- eeck ! ) and upon peeking into the engine blocks coolant passages I saw the following:

enter image description here

I had to re-fill with tap water yesterday as one of my hoses sprung a leak ( replaced the hose today ) and I was wondering if one it's worth trying to clean out the system, and two is it even possible.

I've heard of using distilled water and vinegar to get out the mineral deposits, although I'm worried that might react with any remnants of coolant in the system or maybe attack the head gasket.

Quoting some unknown person on this forum:

Search under my name...Ive done the citrus flush twice and both times other components have been taken out, like the radiator, water pump etc. Im not saying dont do it, just be ready - anything that is on the edge of failure will be pushed over. Have your credit card warmed up and parts supplier ready forewarned :-)

In my opinion, if your cooling system is functioning properly, I would not do the citrus flush.

Is it safe, and is it worth doing, or should I just drain the water and fill back up with my normal "green" coolant?

POSTSCRIPT Oct. 6th, 2016

Before I did a vinegar flush my water pump fins looked like this:

enter image description here

Afterwards it looked like this:

enter image description here

All the fins were melted off the impeller! Of course, this may be partially my fault for getting busy and leaving the vinegar in overnight, but you should definitely be careful as there is a real risk!

4 Answers 4


Getting rid of those deposits is tedious but not difficult:

  1. Drain the coolant and flush with tap water (dispose of the coolant safely!)
  2. Fill the cooling system with about two cups of liquid dishwasher detergent dissolved in hot water.
  3. Start the engine and run it until it warms up to regular operating temperature.
  4. Drain the detergent solution and flush with tap water again until it runs clean. (The idea here is to remove all oils from the system.)
  5. Fill the cooling system with two pounds of citric acid powder (available from industrial chemical supply houses, or from Mercedes-Benz dealers at a considerable premium) dissolved in hot distilled water (since your tap water is hard).
  6. Again, start the engine and run it until it warms up to regular operating temperature, or at least fifteen minutes.
  7. Flush with distilled water. At this point you should see all sorts of rust, mineral deposits, and miscellaneous crud loosened by the citric acid come out of the drain hole.
  8. Refill the system with the appropriate mix of coolant and distilled water. Use a modern "long-life" orange coolant if you have an aluminum cylinder head, block, or both--green coolant promotes corrosion in aluminum.
  • I would imagine if there is oil in the cooling system then you've got much more serious problems than mineral deposits :-) Wouldn't any weak acid like vinegar work? Is there any real benefit to this whole process? Have any references? Jun 3, 2015 at 3:42
  • 1
    @RobertS.Barnes Yes, large amounts of oil in the cooling system do indicate a separate problem that needs addressing. Trace amounts of oil will get in and are no cause for concern, but they do interfere with the action of the acid. As for references, this is a factory-recommended regular maintenance procedure for Mercedes cars from the '50s through at least the '90s (hence why Mercedes sells the citric acid at their parts counters). Anecdotal evidence (such as it is) indicates that, as you'd expect, removing deposits increases coolant flow, making the engine run cooler at lower RPMs. Jun 3, 2015 at 5:15
  • So unless someone has noticed an overheating problem, is there any reason to do this? Wouldn't any weak acid work? I use vinegar to clean stone out of my kettle for example. Jun 3, 2015 at 5:34
  • 1
    It's like any other preventive maintenance procedure--it's a matter of how long you want to keep the car, how much you want to spend, and the conditions under which it's driven. Hard water certainly plays a factor! An overworked cooling system shortens the life of the hoses and water pump, and can contribute to a prematurely leaking heater core (which is a bear of a job to replace on most cars). I don't know how well vinegar would work; I do know that citric acid is almost always specifically recommended to clean machinery exposed to untreated water such as dishwashers and boat engines. Jun 3, 2015 at 5:50
  • I'd like to see some clarification on point 2. Filling a coolant system with two cups (i.e. around half a litre) of dishwasher detergent mixed with hot water that usually has, for example 12 litres of coolant (on an E36 M3) and running it up to temperature is going to create some serious problems for the car. You'd effectively be running it almost dry as I doubt two cups would even fill the heater matrix. Oct 6, 2016 at 13:27

My preferred method is to disconnect the top and bottom hoses. Place a garden hose down the top hose, run the water until what's coming out of the bottom hose is clear. Reattach the bottom hose and refill the system as normal.

If the water refuses to run clear after a few minutes, you can reverse flush the system by putting the water hose up the bottom hose. This can however cause blockages in the coolant system if particles become detached during this process when the water is effectively running in reverse.


If flushing with fresh water is not sufficient; sulfamic acid is ( was) the standard cleaner. I think it is in commercial radiator flushes. When you are finished , make sure your radiator cap seals to prevent new rust from forming ( assuming an iron block).


just read to use oxalic acid [sold as wood bleacher-find at GOOD/OLDER paint stores]. It actually attacks the RUST parts!

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