Identifying a worn raw water pump cam

[iamtjc]

I have had problems with my raw water pump (a Johnson F4B-9 pump on a Nanni 2.14 engine).
It won't pump unless I fully prime the system and gun the engine.
After that it works at idle, but if I stop the engine it won't then start pumping in idle.
I have checked for air leaks, replaced pipework, new clamps etc. and replaced the face plate and impeller.
I suspect the cam is worn but realised I didn't know how to recognise a worn cam.
The wear pattern doesn't look even. Is this what a worn cam looks like?

 

[kieron riley]

I have never tried it , but a friend of mine was told by an engineer in a marina he visited in Ireland that if his cam was worn then a temporary fix was to put a packing of gasket paper under his cam and if it then worked ok it would get him home , but would need a new cam asap . So you could give it a try , nothing to loose ( I don’t think)

 

[RunAgroundHard]

If the face plate (the brass cover plate) wears down, it will not prime. The pump requires close tolerances between the face plate and the impeller to allow the vanes to pull and push the water through the ports. If the gap is too big it just spins in space and can't generate the head needed to suck up water.

Read through this web site, it may help you Water pumps

 

[LittleSister]

I think a failure to prime and to pump at low revs is most unlikely to be the result of a worn cam (*see below).

You say you have changed the face plate and impeller, but have you tried changing the 'wear plate' disc behind the impeller and cam? (Your wear plate looks a bit knocked about, too - lots of random scratches, as well as normal wear from the impeller.)

I also wonder whether a worn shaft seal (yours looks rusty - maybe its spring has failed?) might allow enough air in to cause problems, but presumably would also be noticeable for dribbling a lot of water out off the back of the pump when it is pumping.

*A worn cam would, I believe, just result in less water being pumped at any given rpm, rather than problems priming. Significant cam wear might result in engine overheating when under very heavy loads, but wouldn't stop it pumping/priming at all, i would have thought . For example, my own engine has two variants, both have exactly the same raw water pump except two different cams are available. One cam delivers 24-30 litres per minute at 3,600rpm, the other cam delivers 36-42 litres per mute at 3,600 rpm (which is used depends on whether the engine is direct seawater cooled or indirect via a heat exchanger.).

 

[iamtjc]

You say you have changed the face plate and impeller, but have you tried changing the 'wear plate' disc behind the impeller and cam? (Your wear plate looks a bit knocked about, too - lots of random scratches, as well as normal wear from the impeller.)

I also wonder whether a worn shaft seal (yours looks rusty - maybe its spring has failed?) might allow enough air in to cause problems, but presumably would also be noticeable for dribbling a lot of water out off the back of the pump when it is pumping.

That's an interesting question about the wear plate. The exploded parts diagram in the Nanni manual doesn't show one. I understand that this version of the pump is specific to Nanni. Is it possible there may not be one?

On the shaft seals. They do look rust coloured which is not encouraging. I renewed then in late October when the boat came out of the water so they have done less than 1hr. I bought ones that specifically claimed to have stainless steel springs but they don't look it. Anyone got recommendations for a source of quality seals?

 

[Tillana]

The back of my pump was worn on my md11c. There was no wear plate, but a local engineer milled out the back of the pump to accept a wear plate from a newer version of my pump. Might be an option.

 

[snowbird30ds]

The back of my pump was worn on my md11c. There was no wear plate, but a local engineer milled out the back of the pump to accept a wear plate from a newer version of my pump. Might be an option.

I did the same on my tmd41a pumps to take a jabsco wear plate.

 

[iamtjc]

The Nanni distributer A R Peachment confirmed (when I ordered a replacement pump) that this pump has no wear plate at the back making it a somewhat disposable part.
With the new pump in I will pursue @Tillana's suggestion in #6 and refurb the old pump as a spare.
Thanks for the suggestions