YSM 12 overheating-raw water system

[Ian McGain]

I have a Yamar YSM 12 hp diesel on my '79 Hunter 30'.Saturday on the way back into port the warning light came on for overheating.The raw water exhust was blowing air with no water. White steam exhaust. After limping back into the slip, I started to try to find reason.1. Pulled the screen out of raw water filter. Screen was clear and jar full of water. With seacock open, water flowed onto cabin deck.2. Pulled the impeller- this was good, not broken, and the brass bar was not loose from the rubber fitting. When I turned the shaft, the impeller rotated. Bolted water pump back-up and re-tensioned the belt.3. Placed the intake hose into mouth and blew air backwards to hear the air bubbles outside the boat.4. Pulled thermostat. This was open but no water in top of engine.5. Put water pump outake hose in bucket and fired up engine. Water came out with force. Placed thumb at end of hose and it blew strong stream of water.6. Placed hose back on block and re-started engine. Water came back out the back and engine light stayed off.What is going on ??? I did not fix anything but now it works. I purchased new spare impeller and a couple of gaskets. I can pull and break down the water pump in five minutes.Question: If something blocked the raw water intake and the engine pumped itself dry would the water pump re-prime after obstruction washed away or would I have to break the vacum by pulling something apart.I feel like I am playing stump the chump.Many thanks for your thoughts.

 

[Miles]

Intake blocked?

You might have sucked something up against the outside of the hull over the water intake. I've had it happen with nasty little kelp pods that fit the intake thruhull almost perfectly. Something like a plastic bag would do the trick too. After you shutdown the suction stops and it floats off. The pump will reprime and start blowing water out again. It sounds like everything is working ok now but as long as you're at it you might want to take a peek at the mixing elbow just to be sure it's clear too.

 

[Doug T.]

Thermostat

I'd bet on Miles' theory, but you might also have had a stuck thermostat. Your taking it apart might've caused it to open up.(Why was it open when you inspected it, by the way -- was the engine still hot??)

 

[Phil]

YSM-12 Overheating

I just replaced the impeller on my YSM-12 and when I started it I wasn't getting any water through the block. I checked the impeller and it was good. Eventually I found that when I had cleaned the strainer I hadn't made a good tight seal and air was being sucked in and the water pump was pumping air and not water.

 

[Mark]

Had similar problem

I recently had a similar problem. I would get good seawater discharge when starting engine, but would get a high temp alarm at the most inconventient times (in restrictive channels). When the alarm sounded I would note little to no discharge flow. After allowing the engine to cool and restarting it, I would again have good seawater flow. Turned out the problem was the seawater pump impellor. The set screw was still in place to hold the rubber impellor in place, but it had enough play that allowed the impellor to slip occasionally. Replaced the impellor and haven't had a problem since.Mark

 

[John Reid]

Scored pump cover?

I had the same problem with the same model boat and engine. Turned out the cover on the pump had been worn down enough over the years that it was occasionally breaking the seal between it and the impeller enough to lose prime. Replacing the cover (I had first replaced the impeller) fixed it.