Sludge in Coolant

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.  

I've not seen this on a Rotax either.

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😈