Raw water pump issues

[henry.blanchette]

Hi all,

A month ago I ran my A4 with the raw water intake closed and overheated the engine and fried the impeller. I replaced the impeller and still overheated. I assumed it was a piece (only two small ones unaccounted for) of the old impeller clogging up the heat exchanger. However after more thorough investigation, it seems I haven't had an opportunity to have that problem yet. I am not getting water to the heat exchanger. I am getting water in through the thru hull fine. when I take off the inline strainer and open the thru hull water comes in. there is no blockage in the hose from the strainer to the water pump.

I disconnected the out flow hose to see if water was coming out of the pump itself.
I filled the intake hose with water (holding the disconnected end in the air) from the end i disconnected from the strainer and ran the engine. water at first dribbled out but then stopped with water still remaining in the intake hose. The impeller looks fine when I take the back plate off and take a look. the brass fitting on the outflow side is clear. The belt is in good shape and the drive seems to be turning.

I don't know what else to check. any thoughts on what could be going wrong would be greatly appreciated. Thank you!

[ndutton]

The first thing I would do is measure the thickness of the new impeller and the depth of the impeller cavity in the pump. Make sure the impeller is slightly larger than the cavity depth. A tape measure isn't accurate enough, must use a caliper.

[Wrsteinesq]

Is it possible that a piece of the fried impeller did in fact make it to the heat exchanger? Clogging it, such that now, you have an airlock between the heat exchanger and the pump?

[Wrsteinesq]

Sorry; I missed the part about disconnecting the discharge/outlet hose from the pump. So, what I’m hearing is: No impediment between the seacock, i.e. the seawater intake, and the strainer. Water’s getting thru the strainer unimpeded. But it’s not flowing thru the pump, which appears to be turning as it should. Now, I’m assuming that all this is below the waterline, i.e. if you removed the intake hose from the pump, but held it at the same elevation, water would naturally flow, and eventually sink you. So if it’s not coming out the pump outlet, then either there’s a problem with the pump, OR there’s some impediment, maybe an airlock, between the strainer and the pump. What am I missing?

[henry.blanchette]

no worries, I appreciate the follow up. A picture is worth a thousand words, but oh well. water flows in when I disconnect the strainer which is just about the seacock. I think the water pump must be a tad higher. if I disconnect the intake hose from the pump water does not start to pour out of the hose. However to test if it was a water intake issue I disconnected the intake hose from the strainer side, still connected to the pump, flipped it up so i could fill it with water from the sink and know i was running water with the help of gravity to the pump, then ran the engine. at first some water drippled a bit, then stopped, leaving water in the hose.

all tests were done with the outflow hose disconnected, ruling out any issue with downstream occlusions.

[henry.blanchette]

also, thank you neil, I will double check the dimensions. I cross referenced things when I bought it, but I could have gotten it wrong i suppose. to the eye it looks right

[ndutton]

Where I'm headed is your testing suggests the pump is not priming and a poor seal around the impeller face due to pump cavity wear will do it every time.

[Dave Neptune]

Good seals

The info you are getting is good and I would like to mention one more thing.

Unless the "hose fittings" are good and snug AND the "seal" on the strainer usually an "O-ring" is properly sealed it can leak air drastically reducing water flow.

A easy check on the impeller thickness is to check with a straight edge across the front of the pump with the impeller contacting the back of the cavity you should feel interference of the impeller against the straight edge which would confirm the impeller sealing.

Another is be sure the "shoe" for the pump is not badly worn or missing. There are various shoes available for most pumps to adjust volume of the pump.

Dave Neptune

[henry.blanchette]

Ok, this is all very helpful. If the cavity is too large due to wear does that mean the solution is basically just to replace the pump?

I will also check my strainer seal

[ndutton]

Ok, this is all very helpful. If the cavity is too large due to wear does that mean the solution is basically just to replace the pump?

Sadly, yes. On the plus side, the Moyer Marine 502 pump is a first class pump.

[Easy Rider]

It seems to me that you're pump could be just sucking air. You might just need to prime your strainer by making sure it's full and making sure the seal is tight. I've had your situation in the past and priming the strainer was all it needed and you were good to go.

[edwardc]

Another possibility: if you have an elbow directly on the pump outlet, it’s a prime place to get clogged by broken vane bits.

[henry.blanchette]

Thank you all. I am afraid that the answer was (as per usual with me it seems) simpler and more embarrassing than everyone was thinking. I took the pump off the boat so I could bring it home and get the stripped bolt out of the backing plate and take the back plate off. It seems that I displaced the keyway when installing the new impeller and so of course the impeller was not turning. got a new keyway in there and im pumping raw water beautifully! now to fix the engine coolant system...