I had a similar problem on an Onan. If we were running we would loose the raw water prime. I would get it running fine at the dock but as soon as we ran with the generator off I would loose the prime.
Finally I replaced the pump and all was well. My theory is that the pump shaft seal was beginnig to fail and under way and the suction pulled the water out of the lines.
<snip>
More likely that the housing or cover plate became worn to the extent
that the pump performance was impacted. It's remarkable how a seemingly
minor amount of wear will cause the performance to fall off to the
extent that the pump won't build head, or as you've discovered, won't
overcome just a minor bit of air ingestion on the suction side.
If a replacement pump is costly, the pump may be salvageable. If the cam
is replaceable, remove the old one, and fabricate a flat piece of
stainless shim stock to fit the recess on the shaft side of the housing.
A hole of the proper diameter would have to be punched, and the die
clearances would have to be fairly close to make a clean hole, a task
for a capable metal shop. Trim the OD to match the recess, making sure
the shim is burr-free and perfectly flat, and re-assemble the pump using
a new cam and cover plate. The old cover plate could also be re-faced to
coax some additional life out of it, I've done that with a pump that was
not performing and I didn't have a replacement cover. Flipping the cover
is also an option. I've successfully used this method on impeller pumps
with satisfactory results. You just have to determine if the cost of the
pump offsets the tedium of reworking the old one.
As always, YMMV, and this information does not come from one who has
professed an oath. ;-)
--
Steve Sipe
Solo 4303 "Maerin"
At sea, Onslow Bay
On 5/22/2011 6:54 AM, djp@poorvu.com wrote:
> I had a similar problem on an Onan. If we were running we would loose the raw water prime. I would get it running fine at the dock but as soon as we ran with the generator off I would loose the prime.
>
> Finally I replaced the pump and all was well. My theory is that the pump shaft seal was beginnig to fail and under way and the suction pulled the water out of the lines.
<snip>
More likely that the housing or cover plate became worn to the extent
that the pump performance was impacted. It's remarkable how a seemingly
minor amount of wear will cause the performance to fall off to the
extent that the pump won't build head, or as you've discovered, won't
overcome just a minor bit of air ingestion on the suction side.
If a replacement pump is costly, the pump may be salvageable. If the cam
is replaceable, remove the old one, and fabricate a flat piece of
stainless shim stock to fit the recess on the shaft side of the housing.
A hole of the proper diameter would have to be punched, and the die
clearances would have to be fairly close to make a clean hole, a task
for a capable metal shop. Trim the OD to match the recess, making sure
the shim is burr-free and perfectly flat, and re-assemble the pump using
a new cam and cover plate. The old cover plate could also be re-faced to
coax some additional life out of it, I've done that with a pump that was
not performing and I didn't have a replacement cover. Flipping the cover
is also an option. I've successfully used this method on impeller pumps
with satisfactory results. You just have to determine if the cost of the
pump offsets the tedium of reworking the old one.
As always, YMMV, and this information does not come from one who has
professed an oath. ;-)
--
Steve Sipe
Solo 4303 "Maerin"
At sea, Onslow Bay
