OEM vs non OEM impeller Yamaha FT60 2009 - lessons learnt

[ocular]

Came to ausfish forum when started to get overheat alarm after DIY service on this 4 stroke motor. Owned this motor since new - 189hrs in salt water. Replaced impeller every 12-18 months with OEM water pump kit. Motor been running well- always flushed with muffs 5-10mins after . This service I took down the thermostat housing as well ( something I dont usually do), as my brother had recently done the same on is motor and was surprised how much material was around the thermostat and inside the housing. My thermostat was caked with material and corrosion and was stuck open at 3-4mm. Put it in pan of water and took it to 100 degrees and the thermostat didnt move. How long I had been using the motor with thermostat like this was diffcult to know.

Local yamaha dealer said the thermostat wasnt on the shelf and there were 10 on back order and parts are being shipped out by sea from Japan and there was a 6-8 weeks delay. Tried another yamaha shop and the same story. So I was stuck, so went to a non oem supplier and got a water pump kit and new thermostat. Installed this and took boat out, idle tell tale didnt look as strong at idle but picked up at revs. Then I noticed I would get high temp alarm after 5-10mins of idling. Took new non oem thermostat out, dropped lower leg and took apart pump housing everything looked OK. Ran motor with out thermostat and same issue with overheating at idle. Could confirm by touching the thermostat housing. Cooled down straight away by stopping the motor and then restarting and increasing revs.

Ordered OEM water pump kit ( plus upper plastic housing and new outlet grommet - $20 just for grommet!), then made a setup where I put the lower leg in a drum with water level just above the pump and made a connection between drive shaft and electric drill. Used a 16" long 1/2" copper pipe to connect to the pump housing outlet and then 26" length of clear PVC tubing to the end of the copper pipe. The drill is rated at 2 speeds 400 rpm and 1200 rpm (not sure if this is under load).

With new pump housing tested the pump outlet head at 400 and 1200 rpm with the drill, with each impeller being the only difference.
At 1200 rpm with the drill the non oem impeller had a vertical spurt of about an inch of above the end of the 42" rise. At 1200 rpm the oem impeller had a vertical spurt of about 3 inches above the end of the tubing.

At 400 rpm the non oem impeller could only take the head to a 24" rise ( about level to the top of the driveshaft). At 400 rpm the oem impeller could take the head to above 42" with water coming out the top of the tubing.

Examining the impellers closely, the non oem didnt have as flexible fins, but also the outside diameter was 0.8mm less. This seemed a make a big difference at low revs - remembering that idling is about 800-900rpm not 400 rpm as tested. so I presume that at idle water was not getting to the top of the powerhead/thermostat housing.

So the lessons learnt are that poor quality impellers can make a huge difference to water pump efficiency (particularly at low rpm), the tell tale doesnt always give the full picture and thermostats need regular checking. Off to put the boat in the water and hopefully all better (though have a non oem thermostat in place until the OEM arrives).

The other suprising thing is how low pressure the pump circulation pressure is. Certainly it suggests that anything you can do to increase pressure/flow through the motor at flushing time, water bag/bath rather than muffs, allow thermostat to open and increase motor rpm would give a better mechanical flush and use oem impellers!

[Noelm]

Some non genuine parts are OK, but, water pumps, filters and so on I use genuine only, just as a side note, be careful buying cheap spark plugs on line, there is lots of cheap copies being sold (especially NGK) and they fail, break or snap off easily.