kohler 7.3 impeller change

[CaptBob]

A little over a year. Last episode was July 4th 2015 weekend, we were on the hook in Downtown Nashville for 24 hours. Impeller gave up on the way home. (Actually very good timing!) This time was Labor day weekend, end of 48 hours anchored. On the end of the last day, I noticed the noise. Apparently I have a very understanding genny, only fails at the end of the weekend.

With that being said,(cause I know I've jinxed it now!) I will now return to replacement every spring with the mains. In hindsight, I seem to remember thinking.... I just replaced that not too long ago....

Always something to do...........

[mjk1040]

Ok guys riddle me this, why are these impellers going to pieces so soon? Is it the water they are in, the anti-freeze, what?
And Bud just an after thought, which is not good, you probably should have tried to back flush the system if you were able to, to see if yours had any more pieces laying in there! Mike

[bud37]

Source of the post Ok guys riddle me this, why are these impellers going to pieces so soon? Is it the water they are in, the anti-freeze, what?
And Bud just an after thought, which is not good, you probably should have tried to back flush the system if you were able to, to see if yours had any more pieces laying in there! Mike

I was going to post the same question,seems they don't last long for just pushing water thru a heat exchanger , and not cheap either, would be interesting to see where they are sourced from......(sherwood pump)...iv'e had impellers in outboards last for years and come out intact, and to add the last kohler I had was a flathead four 6.5 and the impeller was in there fine for 5 years.
Thanks, I did back flush a bit,interesting there is only 40hrs on this thing, just want to get to haul out which will be first week in Oct...

[mjk1040]

Bud, only one thing I can conclude is lack of use. This rubber item sitting still in water all the time and over time harding and then out of the blue gets fired up in its stiff state and bang/broken!
Mike

[tomschauer]

I don't know about other boats, but on my 355 when the genny is off the water level drops below the impeller. It has a couple seconds of dry run time with each start. I am sure that doesn't extend the life at all.

[bud37]

Source of the post I don't know about other boats, but on my 355 when the genny is off the water level drops below the impeller. It has a couple seconds of dry run time with each start. I am sure that doesn't extend the life at all.

You could be on to something there,mine is also just above the water line, there is also that syphon break. One thing though the perko stays full though.....also these machines are 3600 rpm also where the older ones were 1800 rpm.

[tomschauer]

My strainer also stays full. The impeller is about 20" above the water line, so it has to be a 5- 10 seconds at each start it is running dry. Plus it sits dry and hot in the engine compartment when not in use.

[Viper]

A few things at play;

- pretty typical because of placement for genny pumps to run dry for a bit at start up unlike a typical engine pump which for more modern engines are usually always wet.
- typical recommendation is impeller replacement every two years for engines (and they're always wet)
- non-tox causes rubber to expand so they'll sit in the pump tighter and the fins will bend further than they would otherwise
- impellers take a set when sitting in the same position for too long (so use your genny!!)

When laid up for the winter, I like to bump my engines and genny over periodically so engine hardware and impellers don't sit in the same position for months, I'm talking a very quick/fraction of a second bump just until/as soon as the starter engages. Some guys will remove the impellers for the winter but that's overkill IMO. I'd rather leave them in and have a better peace of mind by replacing them every other year.

Outboard impellers typically last longer because of the impeller design. Same goes for the newer Alpha drives for example. The impeller is always wet. The fins are much longer and have more give. They produce less pressure but more volume and are more forgiving. And they don't sit in non-tox for six months where applicable. The impellers on engines have shorter, stiffer fins for higher pressure but have less give, are less forging, sit in non-tox for six months (where applicable), take on a set easier and thus don't typically last as long.

Now there are people that will say they've had their impellers for "10 years." Okay good for you, but they're boating on borrowed time and they should be knocking on wood as it's just a matter of time before they fly apart. Of course that will be at the worse possible moment, trying to get back home on a Sunday night, with the whole family on board, out in the middle of nowhere!

[bud37]

Ok, I have a question for you guys that are in the business, you guys are the ones that change these more than anyone on a daily basis.........do you see or have you seen the same impeller failures in brass style pumps as opposed to the plastic housings, is there a difference in the fail...just worn out vs torn off blades.
To qualify a bit, I have had boats of various sizes for a long, long time and never had plastic pumps before, all I remember is worn out,scuffed,vanes that have set,cracked but not torn apart in pieces....time for a pole maybe...

[Viper]

I haven't experienced any major difference. All the plastic pumps I can think of at this time are typically submerged so always wet at start-up unless something is preventing water from getting in so in some apps, plastic is fine. Barring an unforeseen occurrence such as ingesting debris, the most common impeller failure based on work histories is likely due to the failure of the customer replacing them on a regular basis. That usually results in the impeller flying apart no matter the pump design.