Ahhh, Impellers

[minuteman62-64]

The latest in my 10 year saga of fighting impeller issues with my 82 Mariner 30A. Was running it in a barrel yesterday and noticed tell-tale flow at idle was way down from what I last remembered. OK at higher RPMs but actually set off my overheat alarm after a few minutes at idle. My first thought was to blame the Sierra impeller quality.

Pulled the impeller (one year old, almost to the day) and this is what I found (see photo - one vane reversed). I don't see any way the vane could have reversed while running (it actually had a pretty good set to it), so, must have been a re-assembly screw up on my part. My theory is that I somehow got the vane reversed during re-assembly and it (the impeller) was flexible enough to do a decent job of pumping till it just got too stiff to work on only five vanes.

So, now being extra careful installing the new impeller by the numbers. We'll see how it goes.

For those of you that subscribe to the theory that the impeller blades automatically align once the engine turns over, take another look at the photo. I've got another photo on here from a few years back showing a similar situation.

 

[racerone]

Sounds like there was not enuff water in your test barrel.----Water must be 6" ABOVE the pump.

 

[Texasmark]

Put some Vaseline or soap on the blades and turn the drive shaft CW looking down on the LU, while pushing the WP cover down on it.....don't forget to ensure your drive shaft to impeller KEY is solidly in place.

Merc theory has it that at low rpms and low boat speeds, the WP is a displacement pump, pushing a measured volume of water every rotation. When at high rpms and underway with incoming water pressure (on LUs so equipped) the blades fold back and it becomes a centrifugal pump.

My guess is that either it took a set before it had the chance to fold back in the CP mode, or your engines water pump didn't conform to the theory about Merc pumps and even though it may have tried, it was too long to go past straight out and flip back in the normal direction. Looking at your photo, I don't see room for it to straighten out fully so I guess it just remained in the DP position. Who knows.

 

[minuteman62-64]

Sounds like there was not enuff water in your test barrel.----Water must be 6" ABOVE the pump.

Nope, easily a foot above the pump - I keep the barrel full to the top with water running in it while operating the motor.