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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? – Pᴀᴜʟsᴛᴇʀ2 May 17 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 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. – Pᴀᴜʟsᴛᴇʀ2 May 17 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 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. – Pᴀᴜʟsᴛᴇʀ2 May 17 at 21:21
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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 at 15:02

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