Yanmar 3YM30 / SD20 raw water problem

[Beach View]

Hello CF, I hope you can help me with a cooling problem in a Yanmar 3YM30/SD20 combination on a Lagoon 400.

When I started the port engine this weekend I noticed no raw water was coming out of the exhaust. I checked the strainer, the RW impeller, alternator and RW pump belts, and everything was fine. I disconnected the RW intake hose from the pump and blew into it, and while I know the water intake on the SD20 is a couple of feet below the waterline, it seemed like it took too much pressure make bubbles outside. Then I swam under the boat and checked for fouling on the intakes, but didn’t see anything major. When I run the port engine at low RPMs little, if any, water comes out. When I run it at 2000-2300 RPMs water comes out, but not as much as on the starboard side. So far the starboard engine temperature alarm has not gone off.

I welcome any suggestions, and thanks in advance for your help.

Bill

[Rapanui]

We have the same configuration on our FP Athena. The possible reasons are many but from inlet to exhaust its could be:

1. Blockage in SD20 leg. The inlet is quite a way below the surface 2-3ft so pressure required to blow through will exceed 1 psi which is a lot for lungs to achieve. Try an inflatable dinghy pump to blow back through the leg, obviously raising the hose end above the waterline! This will also test 2 below.
2. The horrible brass/bronze sw inlet valve on the SD20 is notorious for seizing and only giving partial opening or closing. I work waterproof or lithium grease onto threads when in open condition and working it shut and open until it gives full travel. Doing this twice a year just about keeps it operational. Some people replace with 1/4 turn ball valves.
3. If the strainer is above the waterline, they can suck in air which stops the raw water pump working.
4. There are reports of the raw water impeller rubber impeller spider to shaft spline bond separating so the impeller does not spin effectively.
5. If you or a previous owner has ever had an impeller break up, the vanes will probably end up blocking the heat exchanger.
6. The heat exchanger could be blocked by scale.
7. The exhaust elbow could be blocked.

All these will take a while to check in sequence. I would start in the following order:

1. Take off raw water pump outlet hose at the heat exchanger and direct into a bucket. Start the engine, run for a few seconds and it should have a good flow similar to starboard engine out of exhaust. If so, the problem is almost certainly in the delivery side heat exchanger or exhaust elbow.
2. If no or poor flow then the problem is in the intake or pump. If the strainer is below water line, take off top and water should flood out. If the strainer is above water line, take off hose inlet and lower hose below water line and again should flood out. If not, its likely the s/w inlet valve or leg. If water flow ok, then its the strainer or pump, in which case short circuit the strainer by joining inlet to outlet. Run engine again for few seconds and if ok its the strainer, else its the pump.

Good luck

[Rapanui]

I should have also added that the raw water pumps can lose suction if the impeller cover or the body is scored by the impeller. If there is no strainer or the strainer is below water line, this does not matter so much as the water will naturally flood into the pump so it is always primed. If the strainer is above the water line, then there is a a siphon break which the pump has to overcome, and any lack of suction due to leakage can make it hard to prime and deliver enough water.
If you are new to the boat and engine, the Johnson raw water pumps are prone to leaking from the shaft seals and seem to wear fast which consequently destroys the bearings as well as spraying salt water all over the engines, not good. They are very expensive over £300 each in UK so a little TLC is well worth it. The bearings and seals are cheap and readily available from bearing suppliers and are reasonably easy to refurbish. I would keep 2 sets of bearings and a handful of seals onboard. Details are in https://www.cruisersforum.com/forums...ld-175009.html see posts by 'bobnlesley'. I do the following 2 things to prolong the life of the pumps:

• Change the impeller and shaft seal each year. I take the pump off to do this and after removing the old impeller, I then remove the shaft seal (use a dental pik or small screwdriver) and push a new seal in using an appropriate sized nut socket. The seals cost about 2$!
• Keep the water pump belt tension as low as possible other wise it will knock the bearings out. There is so much wrap by the belt on the crank and pump pulleys, you need very little tension to stop the belt slipping.

Thats kept my original pumps going for 9 years so far!

[Beach View]

Rapanui,

Wow, thanks for your great reply! I will try your trouble shooting methods. I agree, the SD20 valve is terrible, I may end up replacing it. I’ve also rebuilt the RW pumps a couple of times, and I found a great source for ceramic/SS bearings. My last rebuilds are going on 4 years. I’ll post my findings on the cooling water. Thanks again Bill

[Beach View]

Rapanui— Yesterday I went through my cooling system from top to bottom, and everything checked out OK. Ultimately I backflushed the SD20 with a garden hose via the RW inlet hose. At first it felt like there was a little back pressure, then it flowed freely. Port side now pumps plenty of water at idle, so I’ll call it a win! Again, thanks for your logical trouble shooting guide, I’m still looking for a way to clean the inside of the SD20 though...Bill

[Rapanui]

Originally Posted by Beach View

Rapanui— Yesterday I went through my cooling system from top to bottom, and everything checked out OK. Ultimately I backflushed the SD20 with a garden hose via the RW inlet hose. At first it felt like there was a little back pressure, then it flowed freely. Port side now pumps plenty of water at idle, so I’ll call it a win! Again, thanks for your logical trouble shooting guide, I’m still looking for a way to clean the inside of the SD20 though...Bill

Great result, thinking of adding your backflush routine when I go back to Croatia in a few weeks as the SD20s will have been in the water for 15 months, the last 9 months sitting afloat.

I do not know if it is true, but someone on the forum reported that there is another vertical water entry hole facing downwards on the front face of the leg and you can get a small diameter rod in to clear debris. Is that true?

[Rapanui]

Originally Posted by Beach View

Rapanui— Yesterday I went through my cooling system from top to bottom, and everything checked out OK. Ultimately I backflushed the SD20 with a garden hose via the RW inlet hose. At first it felt like there was a little back pressure, then it flowed freely. Port side now pumps plenty of water at idle, so I’ll call it a win! Again, thanks for your logical trouble shooting guide, I’m still looking for a way to clean the inside of the SD20 though...Bill

Great result, thinking of adding your backflush routine when I go back to Croatia in a few weeks as the SD20s will have been in the water for 15 months, the last 9 months sitting afloat.

I do not know if it is true, but someone on the forum reported that there is another vertical water entry hole facing downwards on the front face of the leg and you can get a small diameter rod in to clear debris. Is that true?