TOPIC: How long should you "really" run an impeller

OK, I just got back from the coast,put my 75 hp Johnson in a barrel to flush it one last time and it's still pumping out water great. So how long can you run an impeller before it will go bad? I've always been told change it once every other year. What say ye fine folks?

Every other year is very safe and good advice.

That being said, there are things that shorten impeller life. Motors sitting for a long time, being started dry, and salt water use, shorten the life considerably. Back in the day, we used to go 7-10 years, or until they failed, sometimes with disasterous results. I care more about my vintage motors now than I did then with slightly used motors. I will usually go around 3 years depending on use but have no qualms with changing it every other season. It's really cheap insurance and once you have done it on a specific motor, it's easy to do it the next time.

As good as it's doing now, I might just run it the rest of the season.

A water pressure gage is an excellent indicator of pump efficiency.

Once you have a good read on how much pressure your outboard develops at speed & at idle, any drop in readings from that will be a sure sign of a declining pump.

Plus it's a much faster sign of sudden loss of cooling than a temp warning horn. If you're at speed, by the time the horn goes off, the engine's toast.

If you notice water pressure going down fast, at least you'd have a chance to shut 'er down before the powerhead fries.

That being said, impellers are cheap insurance, better to pay for a new impeller rather than a new powerhead!

HTH.......ed

As good as it's doing now, I might just run it the rest of the season.
Still trying to figure out posting pics on this phonehttp://youtu.be/TpqdyzpIKYs

Ed,where would you tie into the cooling system with the sending unit or gauge feed hose?

...on the subject of impellers, I just went through my Dad's '56 3hp Evinrude , which has been sitting for over 30 years. New coils, tune up kit, head gasket, carb kit... I fired it up first time in the barrel, it was pumping just fine. But, since I don't ever remember the impeller being replaced, (at least since the mid 70's). I pulled it out, and I was amazed how flexible it still was... I did install a new one, and it showed a little better flow... but I really expected it to be in pieces...

Jim L wrote:

Ed,where would you tie into the cooling system with the sending unit or gauge feed hose?

That's a V4, yes? You'd have to study the block's water passages and then pick a good place to drill & tap a 3/8" NPT hole for a fitting to feed the pressure gage. I'll try and find some good disassembled block pics that show potential tap-in points.

Aftermarket temp sensors exist that mount on a tab, just pull a head bolt, instll sensor tab and re-torque. But as I recall doesn't that V4 already have a temp sensor? Maybe it doesn't work but there should be a sensor on one or both heads. I'll have to look that up, too.

Yeah Ed,it has one on the head.

OK, have had some time to cogitate on the matter!

2 different thoughts:

(1) There is a cover on the top of the block that looks like it's for the common water chamber that would feed the exhaust manifold. So I'd think that if you can tap a fitting into that cover, it should be a nice point to sense water pressure. I'd imagine the cover is a bit on the thin side, so you'd have to see if it'd hold threads OK. Otherwise you could drill a thru-hole and use some sort of compression fitting. See attached pics.

(2) Lots of meat at the top of the block, forward of the head gaskets. You could easily drill into there and tap for a fitting. The one thought that did occur to me about doing it that way is that, with 2 thermostats, the Port and Starboard cyl head water passages would act like 2 separate cooling systems, in a way. So maybe both sides would need a fitting, which you'd attach with a tee to the single line for the water pressure gage. The pressure readings should balance out each other, so that if there was a drastic pressure drop in one side and not the other, you'd see it.

Or maybe the common water chamber tends to equalize water pressure in the system, I dunno! Never studied water flow thru a V4 to that degree!!

HTH.......ed

Note that in the last pic of the 75hp block, there's a fitting sticking out of the side of the Stbd-bank, aft of the intake port covers. Is that a water tap for the automatic choke function?

If so, that might be a good place to add a tee & pressure gage line, if it doesn't matter if you take pressure input from one-side only.

Yeah Ed, I think that goes to the water choke. I haven't noticed it in a while. I did install a loud buzzer last year in addition to the factory HOT light.I ground the wire going to the sender on the head frequently to "be sure" that it's working properly. Thanks for your insight on this. karma

I had the impeller changed on my recently purchased boat -- '60 Johnson 75 hp. It had been sitting for years and years -- 10? 12? -- in a warehouse. Old impeller shows no signs of wear or cracking. Hope the new one is as stout as the old one!

OMC impellers last almost forever, Mercs only a few years (except the V6 & MerCruisers, they last longer).

How long is a variable like stated earlier in this thread. I check for good water flow at idle on my old stringer drives every time I take them out.