onan gen pump not working

[hatterass]

update on my gen. last year I could not get it to pump water. Changed the impeller, nothing. the cover looked warn but not to bad. yesterday I tried again. it did not pump. took the hose off the pump and put in the bilge water flows fine. took the hose off the sea strainer end and hooked up a water hose. fired up the genny and turned on the water. genny ran fine and pumped water. Ok maybe the pump needed to be primed because I just changed the impeller. that did not work either once I reconnected everything it did not work.
Any thoughts besides it a bad pump.
PS I also took off the cover and had my wife crank the motor to see if the impeller was turning and it was.
Dave

 

[(Nobody You Know)]

Dave,

when you hooked up the "hose" you may have been forcing fresh water through the system which gave the false impression that the pump was working (I'm almost sure it's a self priming pump).

Assuming that you are not getting a flow of raw water out of the through hull (and the sea cock is not blocked), loosen the back end hose - the one that leads from the pump to the engine. Crank the genny. If the pump is working it should spill (spew) a fair amount of water. If not, you have a bad pump.

It may be possible that the sea cock is obstructed just enough to allow some water, but not enough to serve the genset?

 

[Jaxfishgyd]

clogged thru hull was my problem a few years ago... Strainer was clean, "seemed" like plenty of water coming thru when I opened the seacock for a few seconds with the strainer open.

Diver was out the next week do clean the hull and said most of my thru hull clamshells were totally enclosed due to barnicles.

Genset ran good after that.

Bad thing right now is, the barnicles are already here in numbers.. And just now turned June, usually it's not till mid July before they get bad.

 

[67hat34c]

I am betting on warn wear ring and warn cover. Most likely cavitating in there. Had the same problem a year or so ago.

 

[Nonchalant1]

I'm not sure there's a wear ring in that pump, but if there is, get a new wear ring and a new cover and try it out. Some of them have a cover and wear ring that you can just flip over and use the other side. While you're in there, check to make sure that the impeller squeezing cam is in place and OK. I once had a screw holding the cam break and the cam fell out when I changed the impeller. Amazed that it didn't spin when running, but I guess there was enough of a stub on the screw to keep it at the top of the pump.

When I had a similar situation to this, I found pieces Prior Owner's impeller vanes stuck in the little right angle squeeze point of the right angle water exit tube cast into pump.

Doug Shuman

 

[REBrueckner]

I'd take the face plate off the raw water pump and crank the genny....see if the impeller turns...Since a hose, presumably under pressure, provided water flow, likely the outlets and hoses are not clogged....
OR....You can always put a bucket of water by the pump, supply and return hoses from the water pump and out in into the pail...turn over /run the genny briefly and see if water flows....

 

[Bob Bradley]

I did the full rebuild on my pump last year. It worked for a while and then crapped out again. New impellor again this year and still would not hold prime. Every time I started the genny, I had to pull the outlet hose from the pump, which allowed it to prime. Of course it also sprayed salt water everywhere when it did. I'd put the hose on while it was flowing and the pump would work fine until I shut the genny down and tried to start it again. It lost prime every time. I finally bit the bullet and bought a brand new pump recently, to the tune of $758.72. Ouch.

 

[hatterass]

I just put the boat in the water and I check every year for blocked intakes. I pulled the back plate and cranked the motor to see if the new impeller would turn. Put the hose on to make sure tat nothing was clogged and that water would pass through the impeller. I know that there is pressure on the hose but, If the impeller was not turning then water would not go through also, I thought that if I lost the prime that this would prime it. still nothing. The cover looks warn but not to bad. If I could I would take the pump off but the is just about impossible because of its location. I think I will buy a new cover to start, I can get to that.
Dave

 

[bobk]

My guess is you are losing prime. These are self priming pumps. I had the problem with a 1986 8KW Onan. Never did figure out why, but for years I had to prime the pump after changing impellors. Then all of a sudden it began to self prime. If you pull the supply hose, you should see just a little water flow out of the pump. If you get a lot, it is flowing from the discharge hose back through the pump. If you just get a little back flow, pull the discharge hose. If it is dry, it probably lost prime. Now you have to figure out why. Are you using a Jabsco impellor?

Bob

 

[hatterass]

the impeller is OEM from Onan. the funny thing is that this has never happened before. until I didn't use it because my main motor was down for the year.
Dave

 

[bobk]

the impeller is OEM from Onan. the funny thing is that this has never happened before. until I didn't use it because my main motor was down for the year.
Dave

Dave, your reply refreshed my memory. My issues were with Onan OEM impellors and they went away when I went to Jabsco. Costs less too.

Bob

 

[REBrueckner]

Many pump face plates, as well as those on main engines, can be turned outside face inside to get a smooth interior face when the original is worn ...this is a nice trick especially if your are in a remote location without a spare....
When installing new impellers it's always a good idea to :
.....introduce some moisture and or lube into the impeller/ pump housing...perhaps a touch of grease, a little spray soap, a bit of dawn dish soap, a little water from a cup...anything to lubricate before it picks up water....
..... be sure the impeller blades are bent in the correct direction to avoid unnecessary strain....

.....face plate gaskets need to be of the correct thickness to help keep a tight seal against the impeller edge....

 

[Bob Bradley]

Dave, your reply refreshed my memory. My issues were with Onan OEM impellors and they went away when I went to Jabsco. Costs less too.

Bob

Do you have a cross reference between Onan and Jabsco impellors?

 

[hatterass]

unfortunately mine is cast and has the name on the out side of the plate so I can't turn it around. Maybe I should try a jabsco impeller and get a new plate.
Dave

 

[bobk]

Do you have a cross reference between Onan and Jabsco impellors?

Sorry, I sold that boat two years ago and didn't retain the records. But I think I got the impellors from DEPCO. The pump itself was an Oberdorfer.

Bob