Weird raw water pump issue (Universal 40)

[EricFox]

For the past two years my raw pump (Sherwood G-6) has had sporadic episodes where it fails to pump at engine start-up. I won't go through all of the debugging that I have done, but there are no blockages upstream or downstream, there are no water leaks, and the impellor is fine (swapped it out with no impact).

In fact, when it's failing to pump, I can detach the input hose to the pump from the strainer and put it in a bucket of water, but get nothing out from the pump even with the output hose disconnected - the pump can't pump hard enough to overcome what I expect is a small air lock in the input hose from the bucket (yes, I have swapped out the input hose too, and it's not kinked). If I wait several minutes, sometimes it can clear what I expect is the air lock, but only if I temporarily detach the output hose so that there is no back pressure. It typically fails after I have been sailing with some good heel - my hypothesis is that the water drops out of the through-hull when I'm heeled and the pump can't overcome the airlock when I do to start the engine. What I do in those situations is to detach the output hose from the pump to minimize back pressure and it usually starts pumping after a few seconds.

All of that so say that everything points to the pump itself, but when I have it apart I can't see any obvious problems (pics below) - maybe some light scoring if I squint just right, but that's it. Also, I'm almost positive that the shaft from the engine is turning based on the position of the key when I open the pump (and the fact that it does work most of time). There is no play rotationally or in-out on that shaft when I have the pump off the shaft.

I'm quite willing to replace it with a new Oberdorfer (N202-M16 for 1/2" NPT fittings from pumpsandpartsonline.com), but what is confusing me is that in all of the posts about bad Sherwood's seem to be about leaks, not about an inability to pull. Is the failure mode that I'm seeing consistent with a bad pump? Is there anything internal to the pump that might be bad that I'm overlooking? There doesn't appear to ne a whole heck of a lot to go wrong with pump if it's sealing, the impellor is good, and it's turning.

 

[G Kiba]

Possible that the impeller rubber is spinning on the impeller hub? Your second picture shows some strange bubbling where the hub connects to the rubber. I had a brand new impeller do that.

 

[EricFox]

Thanks for that suggestion, Grant. I read about that happening in another post, but I can't get the centre part of turn independently from the rubber part. And the problem has shown up with two different impellors. Not impossible, but it seems unlikely at this point. Maybe I'm underestimating how much force would be required to achieve the independent rotation.

 

[ConchyDug]

Is the face plate stepped from wear? Kinda looks like it in the last 2 photos.

 

[Joliba]

Eric,
Have you tried slathering a little PTFE grease around the impeller blades and face plate? This often solves a problem like that if there is no air leak elsewhere in the system.
Mike Jacker

 

[EricFox]

Doug, there is a small deviation from perfect planarity there - that's why I took those pictures. It's what I referred to as "light scoring if I squint the right way" in the OP. I wouldn't have thought it was severe enough to cause the problem, but maybe I'm wrong - that's why I wrote the post.

Mike, I tried o-ring grease, but at the time was mainly focused on the o-ring - not sure that I put any on the impellor. Will that last, or would it wear off the impellor after moderate use? Even if it does wear off, it would be a good test to see if allows the pump to provide enough draw, even if only temporilly (i.e. indicate that the seal between impellor and pump internals is lacking).

 

[Joliba]

Exactly. If it allows the pump to temporarily work (who knows for how long) at least it will help rule out one possible source of your problem.
I have been happy with an Oberdorfer pump.

 

[ConchyDug]

If it's smooth to the finger no big deal. If you catch a fingernail on it or feel a ridge it might be keeping you from getting the proper sealing pressure between each "chamber" of the impeller. I dunno just a theory.

 

[Afrakes]

How many operational hours on that pump?

 

[Dave G.]

Have you checked the mechanical drive coupling & roll pin ? Could be spinning and when it heats up and expands it grabs....maybe ? You shouldn't be able to turn the pump shaft if the coupling/pin are good.

 

[driftless]

Eric I have the same pump, and the similar issue. It's sometimes quite stubborn to start pumping in the spring when we flush the system and when we launch.
What worked for me this spring was to pull the input line to the pump and give it a little suction to start pulling water through. I figure that got any air out of the intake and strainer. Hooked the hose back up quick. With the pump primed, all good to go for the rest of the season.

 

[EricFox]

Al, unfortunately I have no idea how old the pump is.

Doug, I can't rotate the shaft by hand, and it's turning even when cold (if the key slot is oriented down whenever I reassemble the pump I can get the shaft to rotate by pulsing the starter so that the slot is oriented such that the key won't fall out).

Tyler, your description of the problem is exactly what I am seeing - pump just doesn't seem to have the "oomf" to overcome an air lock in the input line. Except that my problem reasserts itself when water drops out of through-hull due to healing so I'm having the detach hoses to get the pump to go on about half of my sails. I posted yesterday on this because I spent a frustrating 1.5 hours the day before trying to coax the pump to pull from a jug of antifreeze - ended up having to gravity feed it through a funnel to get the pump to start.

It sounds like I'm not overlooking anything obvious, so I think I'll bite the bullet and replace the pump next spring.

 

[Tooluser]

It sounds like I'm not overlooking anything obvious, so I think I'll bite the bullet and replace the pump next spring.

I’d love it if you’d report back anything you learn; this is a real head-scratcher.

 

[EricFox]

I'm headed out to the boat this week for one last time before winter - thinking that I'll bring the pump back with me so that I can play with it at home. I'm assuming that I can rig a way to run it using a drill to rotate the shaft.

 

[Christian Williams]

I had a Sherwood on the M25 (different model pump). Sometimes when reinstalled it wouldn't pump. The issue was a spacer, which had a slot in it designed to mate with the engine drive, but often didn't. I would see that no water was coming out the exhaust, and have to take the pump off, fiddle with the connecting spacer, and reinstall it so the impeller spun. The replacement Oberdorfer didn't need a drive spacer.

 

[EricFox]

I'm fairly certain it's spinning, but of course I can't be 100% sure. Removing the pump from the block may give me some further insight. BTW, I haven't checked, but I'm expecting that the pump sits above the resting level of the oil, i.e. I won't need to drain the oil before removing pump - is that correct?

 

[EricFox]

To further explain, if the spacer isn't connecting, then it's an intermittent failure to connect - must be not engaging at startup, then when I remove back pressure it somehow decreases resistance to turning enough that it re-engages. Christian, based on what you saw, is it something that could be "one the edge" like that?

 

[Christian Williams]

Yes. If you have the spacer, failure to seat was a fairly common issue. It's easy to remove the pump, no oil will come out if the engine isn't running.

My install looked like this--the Sherwood pump model was not a good fit for the M25, and you can see why a spacer was needed. May not correspond to your issue.

 

[K2MSmith]

Thanks for that suggestion, Grant. I read about that happening in another post, but I can't get the centre part of turn independently from the rubber part. And the problem has shown up with two different impellors. Not impossible, but it seems unlikely at this point. Maybe I'm underestimating how much force would be required to achieve the independent rotation.

This happened to mine . Took me days to figure out . I discovered it by leaving the pump cover off and having someone turn the engine over .

 

[EricFox]

But it needs to a failure mode that is intermittent. Tt's definitely turning most of the time - it would have to be a failure that can come and go, i.e. sometimes engaged and sometimes not, with the ability to get it re-engage most times by simply reducing back pressure at the output of the pump. Perhaps I'm wrong, but I'm guessing that for most who have had a problem with the spacer not engaging that it's always unengaged, i.e. it is never turning. Please correct me if I'm misinterpreting.