There seems to be some confusion as to what coolant you can use for your liquid cooled Rotax.  Rotax currently lists only fluids where they have had feedback from the countries Rotax distributor about what is available and what has worked.  The list therefore is not always going to show your favorite brand and perhaps we need to look more at what are the best things to look for in a coolant.  There is a lot of good information from the coolant suppliers in form of materials safety data sheets, MSDS, that will give you what you need to know.  In general, most enough information is on the container, we have to take a few moments to look carefully at that information.  

So what are we looking at with our cooling system?   There is a ceramic seal directly behind the water pump impeller (yes even on the 2 strokes starting with the type 582) We also flow fluid from the radiator into an aluminium housing and impeller, into a rubber hose, to an aluminum head and out with a rubber hose again into a collector and back into the rubber hose to a radiator. We use a pressure cap with a coolant recovery ability and it should use a coolant recovery bottle.  We can look at the failure points after.

Historically the way liquid cooling worked on the Rotax was to treat it like your car.  We would use some diluted ethylene glycol mixed with distilled water.  This prevents freezing and the whole system is under pressure to raise the boiling point.  In 1989 when the 912 came online early engines all used mostly the same coolants.  They had additives to keep the system clean and prevent corrosion internally.  The most common ones used silicates, older fluids used phosphates.  Coolants with phosphates however cause a great deal of corrosion when flowing in an engine with a lot of aluminium so it was not a good match.  The term " aluminum compatible" was used with fluids where the phosphates were removed, silicates were used to take up that task.  The fluids themselves were what we now term inorganic additive technology, IAT.  For the most part other than the number of additives the base will hold the silicates in solution for a period.  (generally, no more than 24 months) These fluids however had some issues. The IAT fluids can break down and release the silicates, they will separate at some point.  As the fluid ages it also becomes more electrically active.  This can produce, with the flowing fluids, electrical-chemical discharge.  The phenomena is called ECD.   You can measure the flowing fluids with the IAT types electrical current when the engine is running with a multimeter.  It was not uncommon to see water pump seal failure due to contamination from the silicate drop out after as little as 500 hours with those coolants.   Since many people don't want to believe someone without a name let us just go to a coolant manufacturer ad see what they can tell us.  The link has nice short explanations of each type of fluid.  I can tell you that any good OAT or HOAT coolant is far better than the old IAT versions.  (my view)  But for sure never mix between the blends or type of coolants. 



note, also check out gates hose web pages for information on hose replacement requirements from a hose manufacturer

10054_1_Gates ECD failure inside hose.jpeg (You do not have access to download this file.)
10054_1_silica dropout rad cap.jpeg (You do not have access to download this file.)
  • Re: Coolants and what you thought you knew about them

    by » 8 months ago

    Vey informative post RW, my thanks.

    The following advice is taken from the Gates website

    "Failure rates on radiator hoses increase dramatically after four years in operation. This is relevant to all coolant hose, but especially the upper and lower radiator hoses

    The primary cause of radiator hose failure is an electrochemical attack on the rubber compound of the hose (called electrochemical degradation, or ECD). This attacks the hose from the inside out, and as such it is not easily visible to the eye until it is too late. 

    Too often, when it is time to replace a hose (usually because it has already failed), only the affected hose is replaced, and this significantly increases the risk of failure on the other hose which remains in place. This is risky practice, and can certainly lead to problems, and unsatisfied vehicle owners. "

    Thank you said by: Rotax Wizard

  • Re: Coolants and what you thought you knew about them

    by » 8 months ago

    Hi Sean

    Thanks for taking the time to read that on the Gates site.  I have worked on the type 912 engines since they came out in 1989.  The most failures in the early days were coolants that had a lot of ECD.  The other side of that problem is in the castings, mostly the impeller housing at the pump, in that we used to see major issues with erosion from the discharge at that point.  Electrical discharge also will be most prevalent just after the exit from the head, normally the hoses will start to fail just after the head nipple exit.  

    Here is the good bit.  Simply get rid of coolants that have electrical activity.  (generally IAT fluids with silicates) I have been preaching for years to use OAT or HOAT fluids.  By design the new fluids are very kind to the water seal and the hoses.  The coolants with silicates are the worst for the water seal, if silicates drop out they start to etch the seal surface which then leaks. Remember that the hoses also are chemical in nature and over time, depending on your environment, they can degrade simply by age.  Another tip for someone if you are unsure of an engines history and what was used in the past.  If you open part of the coolant system note if you see white buildup at the joints, like where the nipples for the hoses at attached to the heads and just inside the nipples at the hose attach points.  Note if you have any signs of erosion at the tips of the coolant nipples.  Those tell us you have ECD in your coolant, generally from someone never changing it or running the older IAT fluids.  If the engine has been using the newer OAT or HOAT coolants they will show, on the castings like at the water pump, a dark grey appearance.  This coating is normal and protects the castings from corrosion. Simply speaking, white is bad, grey is good. Somewhere I have pictures of all of these failures on the Rotax, will post them if I can find them but the library is a big bulky. 


    Thank you said by: Sean Griffin

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