I recently replaced the head gasket in my Renault K9K (700) 1.5 DCi, due to a cylinder/cooling system type failure. As such, the cooling system is full of partially burned diesel. Despite having flushed the cooling system every few days for a couple of weeks and having used commercial coolant flushing products a couple of times, the coolant continues to turn black after relatively little use.

Has anybody got any tips or tricks on flushing a stubbornly contaminated cooling system? I've read of people having success with using a pre-dissolved low suds washing powder, is that a sensible idea? Do I just need to be more patient?

  • Welcome to Motor Vehicle Maintenance & Repair! Are you sure you still don't have an oil leak into the coolant? May 17, 2019 at 19:50
  • As sure as I can be. I'm not using oil rapidly, and there wasn't any oil/water contamination in the first place as the gasket failed between the cylinder and the cooling system, without breaking into any of the oil passages.
    – Tim Angus
    May 17, 2019 at 19:52
  • Is there an oil cooler? You may want to check it and see if it's spewing oil into the water. If it has one, you could possibly bypass it to see if, after pressure builds in the coolant system if any coolant comes out of it. Long shot, I agree, but really it shouldn't take this long to get rid of any contaminants in the coolant, especially after a flush. May 17, 2019 at 20:07
  • Hmm, that hadn't occurred to me; I guess that's possible, though it would be somewhat of a coincidence. Also, would you not expect more of an emulsion type contamination in that case? As it is the drained water is still very low viscosity, like fresh water, only it's black. I'll give it a look in the morning though, thanks for the idea.
    – Tim Angus
    May 17, 2019 at 20:24
  • Maybe I just don't understand what's going on. Black is contamination, no doubt, but what kind. Some kind of carbon ... not sure. May 17, 2019 at 21:21

2 Answers 2


You'll want to drain the coolant out, of course. Then fill the cooling system with straight water and add dish soap, or better, an industrial degreaser like Purple Power. Run it for about 30 minutes, drain it out, and do another identical flush (water and whatever chemical you used the first time around). After that you'll want to do a third flush, but with straight water (no chemicals). If you are really that worried you can do another water flush. Once the water flushes are done, you can put fresh coolant in, and many guys will recommend vacuum-filling your cooling system to get any air pockets out.

Hope this helps.

How To Perform A Cooling System Flush On Your Diesel Engine

  • 1
    Yeah. This was quite some time ago now, but that's essentially what I did, with a whole bunch of different detergents. I never really got it running completely clear, but certainly better than when I started. Not worth worrying about really, it's a piece of crap 17yo van. Just frustrating how many times I had to flush it to see an improvement.
    – Tim Angus
    Feb 8, 2021 at 20:12

I had a blown head gasket in my Mercedes and I went to AutoZone and bought a bottle of k-seal you don't have to flush radiator just change oil it's been months now and not one problem with it overheating since also you don't have to run the heater when applied just pour it in and crank car for about 30 min to hour let it idle and it should fix it

  • I've already replaced the head gasket. My problem is the cooling system is dirty and I don't know how to clean it effectively.
    – Tim Angus
    May 19, 2019 at 15:02

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