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Doing a 5 year rubber on a client's Tecnam and found this gray sludge throughout the cooling system.  In the expansion tank and deposits in most of the hoses.  First time I've come across this issue.

Borescoped the water pump impeller, but it looked fine with no residue or wear.  Pulled the radiator to have it flushed and plan to flush the water jackets on the heads, perhaps with a solvent like mineral spirits.

Anyone else ever come across this?  Was concerned it might be oil in the coolant, but I didn't see evidence of coolant in the oil when I drained it.  Could this happen if Evans was used and then not properly flushed before changing to DexCool?

10926_1_IMG_7054.JPEG (You do not have access to download this file.)
  • Re: Sludge in Coolant

    by » 3 weeks ago


    I’ve not seen this on a Rotax 912 myself, but on an automotive engine I have seen this when oil has infiltrated the cooling system. Most often due to a blown head gasket or cracked cylinder head.  


  • Re: Sludge in Coolant

    by » 3 weeks ago


    I've not seen this on a Rotax either.


    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Sludge in Coolant

    by » 3 weeks ago


    Hi John,

    I am with RL & JB on the oil in coolant idea.

    "Could this happen if Evans was used and then not properly flushed before changing to DexCool?"

    No actual experince however having heeded the warnings, on coolant labels, about mixing diffrent brands/types, it seems to me, that your statement above is the most likly cause of the "sludge". You may also consider that the coolant concentrate, may have been mixed with tap water, rather than pure and the sludge is mineral residue.

    Time for a really good flush and refill with the correct coolant/pure water😈


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