As others have mentioned, I'd look for an air leak in every seal from the pump upstream to the hull fitting -including the entire pump. If the pump gets air bound and the impeller vanes are less than absolutely factory perfect, it will have a hard time sucking water, and will just move the air in the pump around.
My water pump was leaking, and I thought it was the cover seal like you do...it wasn't, it was the shaft seals between the pulley and the impeller. They were about $10 each or so and not terribly hard to replace. It's a good idea to replace the bearings if you're going in there to fix the seals. If you cannot get new seals in a timely manner, just put as much thick-tacky grease in the shaft seal as you can...just to slow down the air-water intrusion/leak. The other suggestion about the "cam" in the pump is also a good one. It's held in with a single brass screw. Mine corroded away to nothing. So I replaced it with a stainless cap screw. By the way...it's metric and it cannot be too long or it will stick into the pump and tear the vanes. "Choose wisely" as the saying goes. And another thought, before you go all "acid bath" on the engine...maybe start with a mild acid like vinegar. It's cheap in large volume, and mild enough to the steel parts while working magic on salts. Just a thought and if it doesn't work, you are only out about $15. I'd get the engine running and hot on seawater, shut it down, then take the seawater intake and put it in the vinegar bottle. For a SW cooled engine, I'd guess that 4 gallons would do. Then run the engine for a very short time to suck all 4 gallons into the engine. Now the heat from the block will help the vinegar do it's thing. Leave it for a day maybe. The run it normally. Beware....all those bits that were adhering are going to try to exit the engine...if there are large scales they could block the passages. (like a blood clot or heart attack). -Keith M C&C 35-3 (3GM30F) -----Original Message----- From: Don Jonsson [mailto:dbjons...@shaw.ca] Sent: Thursday, May 23, 2013 21:00 To: cnc-list@cnc-list.com Subject: Stus-List Overheating 3GMD - the plot thickens Hi I had previously sent out a message about an overheating diesel. The culprit, so I thought, was the knuckle going into the mixing elbow, because generally when I cleaned it, it worked again. For a while. So I finally took of the mixing elbow and gave everything a proper cleaning. I checked the water pump. I checked all hoses by blowing through them and the engine. I put it all back together and it all worked and water pumped like it is supposed to, except the water pump leaked a bit - needed a new gasket, but I knew that and just wanted to see if it would work. Got a new gasket and put it on. Started up and no water. I took out the thermostat and checked to see it worked - slightly corroded but it worked, i.e. opened in hot water. Put everything back together and started it up and a bit of water but too much steam. Took the thermostat out and put it in more carefully. Tried again, water running, no steam everything is perfect. Ran it for a while just to be sure. Turned it off. Waited a while (an hour) and started it up again. No water. So the problem is not the mixing elbow (and likely never was as it wasn't that bad). Not the pump as it looks fine. Not the thermostat - I don't think although I will replace it. The problem is sometimes the engine pumps water and sometimes not. And I'm stumped. This is a raw water cooled engine which makes things different. Has anyone had a similar problem and ideas for a solution. Also, it seems hard to get part numbers for a 3GMD and the local mechanic had to go find an old book. Is there any place online to find part numbers for that engine? Thanks to anyone that can help. Don _______________________________________________ This List is provided by the C&C Photo Album http://www.cncphotoalbum.com CnC-List@cnc-list.com
