Originally Posted by bigscott
I have a 1998 ( I think
) mariner 25hp 2 stroke outboard ( ex RNLI)
Was out on the boat today enjoying the sunshine, spent 4 hours out with no problem. when I returned to the slipway I noticed the Pee pipe was spitting very little water out when in neutral. I popped it into gear and slowly did a lap around the moorings,very little water was coming out the pipe but it was pumping water.
Put the outboard into neutral to check again and this time the water stopped completely with only exhaust fumes coming out so I turned the outboard off. Took the hood off and it was hot to the touch
I check the water flow everytime I start the outboard ( this could be up to 12 times per trip) so im really surprised at how quickly this has happened.
Do the impellers just suddenly go or is it more likely a blockage somewhere in the system?
If it is the impeller I would really like to change it myself, Is it a big job or just a matter of taking the gearbox off, flushing the system and changing the impeller.
Any help gratefully received
Do you know the history of your engine?
Normally impellers are changed every other season, or if it has a lot of hours then it would be prudent to change it annually.
To replace the impeller, drop the leg. To do this, ensure the engine is in neutral. Normally there are 4 bolts under the anti-cavitation plate. Undo but do not remove. The gear shift selector (if it's like the Yamaha 25hp) locknut needs to be undone too. The final bolt is usually hidden under the trim tab. With everything disconnected, it should simply drop down. If there's any resistance, then use a rubber mallet and gently tap the gear box at regular intervals and around the anti cavitation plate. It should drop down now.
The impeller is located in the water-pump housing. Unto the 4 bolts and lift it off the drive shaft. Prize the old impeller off (there will be a crescent shaped woodruff key) on the driveshaft. More often than not it's corroded to the driveshaft. If it is, leave it alone - there's no benefit in trying to remove it.
Check the water-pump housing. If the stainless steel faceplate is scored or you notice any other damage (rubber 'o' gasket on the pump housing for example), then replace. Clean everything up. Apply a suitable marine grease to the inside of the pump housing and lube the impeller. Slide the impeller onto the driveshaft until it locates in the woodruff key.
Now slide on the pump housing. Turn the impeller clockwise and it will fold inside the pump housing. Secure with the 4 bolts (don't over-torque.) Apply grease to the splined driveshaft end. Now refit back to the engine. The driveshaft will locate in the powerhead, also make sure the water-pipe lines up.
Reconnect gear-shift selector, and tighten locknut and bolts. Try engine in forward, reverse and neutral before starting the engine. If you're happy, then start the engine with muffs on.
If you're going to the bother of changing the impeller, then look at changing the gear oil too. I'd also remove the thermostat and clean it up (not unusual to get a build up of salt deposits here). Using some strimmer cord, you can clean the tell-tale pipe if you suspect there's something reducing the tell-tale water flow.
This is a useful description regards the shift-mechanism if it's not a lock-nut as per a Yamaha engine. Good luck.
First Impeller replacement - Tips 25hp outboard , 96