Impeller replacement. OMC - can I check for water flow?

[KM7]

1992 FourWinns Horizon 200 20 ft - Impeller replacement. Engine: 5.8 L V8, 1991 OMC Stern Drive.

I replaced the impeller a few months ago but when I started it up recently the impeller did not pump water and then melted from the heat of spinning dry. Cracked the housing as well. I had the hose connected but there was no flow through the motor. I shut the motor off pretty quickly, maybe 1 minute but I was too late !!. Melted the impeller. Am I right in assuming that it melted from spinning dry? Why didn't I get water flow to it to "Prime" the impeller? What can I check?

When the hose is connected should water enter the impeller through that small hose?

 

[bruceb58]

How were you applying the water. Was the boat in the water or were you using muffs?

 

[Scott Danforth]

to expand on what Bruce is asking. if you simply have the lower unit in a bucket, the pump will not prime. either use the muffs or put the whole boat in the water.

 

[Lou C]

If you're using muffs, you have to make sure they fit right and do not get loose. For the Cobra pump to prime, the water has to rise about 18" from the lower unit to the impeller housing. If you have trouble getting the pump to prime here is a sure fire way to make it prime easily:
Hook up your muffs to the lower unit and to the hose spigot. Then go up in the boat, with a funnel and a gallon of water in a container. Disconnect the raw water intake hose at the thermostat housing. Then fill this hose with water, till it won't take any more. Re-connect the hose and then turn on the water and start the engine. Make sure that water is exiting under the transom mount. This method takes some time but by filling the passages from the transom mount to the water muffs with water, it makes it easier for the pump to prime and lubes the impeller if it has not been run for a while it could be dry. When I start mine up after winter storage, I do the same but I use marine antifreeze. This really lubes the impeller well and I've never had trouble with it priming if I do this first.

BTW, I had some trouble with muffs a couple of years back. I had to replace my large rectangular Tempo muffs and the new ones, were made of stiffer rubber and just did not seal well. I switched to the Merc/Quicksilver muffs with the wire clamp and they are much better. And there is no way they can slip off.

 

[KM7]

I was using a hose and muffs. Now I know that the muffs were the problem. They did not seal well enough and water was not getting to the impeller. I know that now because I took off the trashed impeller housing exposing the water In and Out ports. I connected the hose and same muffs again and.....The water would *not* rise up to the impeller. That confirmed my suspicion: cheap muffs fried my impeller and housing, a $50 lesson.

These were cheap plastic muffs. I have another pair that has a stiff metal bar between the 2 muffs. They holds them tighter against the OD. With the water on, it comes up the the intake port!! So lesson learned. Lou C had it right. The muffs can't be loose.

I've read that you don't want the hose on full blast. Why is that? Is it because you can over pressurize the impeller housing and cause a leak? Until the motor starts there is no where for the water to go so pressure will build up in the impeller housing. Could that cause a leak if too high??

And what is the function of the small hose coming off the impeller housing? Pressure relief?

 

[bruceb58]

II've read that you don't want the hose on full blast. Why is that? Is it because you can over pressurize the impeller housing and cause a leak? Until the motor starts there is no where for the water to go so pressure will build up in the impeller housing. Could that cause a leak if too high??

Use full pressure. Any excess pressure will just bleed off around the muffs. In fact, until you start the engine, all the water will bleeding off from around the muffs.

The function of the small hose is to allow the air to leave while the pump is trying to prime.

These are the best style muffs to get:
https://www.academy.com/webapp/wcs/...9OSJv87X-mlFDrQgDG5bjAHEyWWJfcoEaAounEALw_wcB

 

[KM7]

BruceB Thanks. I will use full pressure from now on. And also thanks re the small hose. That makes sense allowing the air to escape. The boat mechanic I use a lot is also Bruce B, but he is not an electrical engineer.

I have to start another post re: Not starting and no fuel getting to the carb. ( Fuel pump?)

 

[KM7]

So the small hose is to allow air to escape and allow the impeller to be "primed" with water before it starts spinning. After the impeller housing is full of water shouldn't water start coming out of that hose? And if so, can you see where that hose goes ? If you can see where the water is coming out, then you would know that the impeller is primed and there is no risk of spinning dry.

 

[bruceb58]

Been awhile since I owned my OMC. Can't even remember how much water comes out of the little hose.

 

[Lou C]

You don't see the water coming out of the hose because it is routed inside the upper gear housing. If you pull the hose off the nipple on the impeller housing it puts out a good squirt.

About the muffs, Bruce you might want to take a look at that style being sold now. I had to get a new set last year and whoever makes them for Tempo/West Marine etc over in Taiwan or wherever, is making them out of rubber that is much too hard to seal well. The Merc ones I posted are a much softer rubber that conforms to the shape of the lower unit better.

 

[kenny nunez]

The Mercruiser style are the best . If you have to use the other style take a rubber strap and wrap it around the gear housing to keep the muffs tight and not slip down.

 

[bruceb58]

About the muffs, Bruce you might want to take a look at that style being sold now. I had to get a new set last year and whoever makes them for Tempo/West Marine etc over in Taiwan or wherever, is making them out of rubber that is much too hard to seal well. The Merc ones I posted are a much softer rubber that conforms to the shape of the lower unit better.

I just bought a brand new set. The problem the newer ones have is actually the plastic pipes are cheaper and if you drop them, they will crack.

If your OMC does not have it's screen anymore over it's inlets, then you can use the Mercury ones. If your drive does have the screen like its supposed to, don't poke a hole through it. It is there for a reason.

As far as the hose is concerned, it just drops around a foot or so into the cavity below the pump. It's not routed anywhere special.

 

[Lou C]

The ones I have seen, still have the rubber hoses but the rubber cuff part, it much stiffer than the one I bought 10 years before. Must be a few companies making these.

About that water screen. I had to remove it about 5 years ago, because I had a chronic problem with barnacles growing on it, (boat is moored in salt water 6 months/season). Even after painting it with Trilux, etc. For a trailered boat it is fine, but in my application it was causing mild overheating. When I took it out, all my slightly elevated temp problems went away. I have not had an issue with it clogging but that may vary by your location. The later model Cobras had removable screens like some outboards do. The Merc drives and the older Volvo SXs did not have screens either but they had smaller, and more holes vs the Cobra.

I think even if you wanted to keep the screen, drilling a 1/8" hole in it will not hurt anything. They get broken up anyway. OK on outboards, not so good on I/Os in salt water moorage. I keep a stiff wire so I can tilt the drive up and rod out those holes with the boat on the mooring.....

 

[KM7]

Thanks for all the advice. Here is what I learned:
All plastic muffs (Flush adapter) don't hold on tight enough to produce enough pressure to get the water UP to the impeller. I now have an adapter that has a stiff round metal rod that them holds on tight.

 

[Lou C]

the next time you replace the impeller, try this:
after you remove the housing and before you install the new one, try hooking up your muffs and turning on the water. They you'll see if they really put a good flow of water to the impeller housing area. With the Tempo style dual inlet ones I had to hold them tight against the lower unit sometimes to get it to prime. Not so with the Merc/Quicksilver ones.
I actually tested both muffs back to back and looked at the flow. I think its partly due to manufacturing differences with some companies that make these using better rubber that flexes more and a tighter spring that holds them tight.

 

[KM7]

Thanks Lue. That's exactly how I determined that the plastic muffs were not creating enough pressure, no flow from the intake port. With the metal ones, lots of flow.