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      Until recently on my 2000 B31 I have never had any problems with seawater cooling flow. Water has always started flowing out of the exhaust almost immediately, whenever the engine was started.

      Recently, I have found that there has been a delay (about a minute)between starting the engine and the water starting to flow from the exhaust. However, this has only happened after I have been sailing. When the engine has been started either on our mooring, at anchor, or in a marina, the water flow has started almost immediately. Can it be that the seawater is syphoning out of the system when I’m sailing?

      Coincidentally (maybe) I have noticed a drip from my anti-syphon vacuum valve “overflow pipe” whenever the engine is running. Could there be a link?

      I have now cleaned the water filter (which was quite dirty) changed the impeller (the old one did have some cracked blades) and stripped and cleaned the anti-syphon valve. The anti-syphon vacuum valve did have some salt crystals and when I tried to clean the internal surface of the main body where the top screws into it, the metal around the small hole just fell away due to being really thin. I put the valve back together, and refitted it, but will obviously need a new valve.

      When I started the engine to find out if the clean filter and new impeller had improved things, I found that there was now virtually no water coming out of the exhaust at all! Could this be due to the damaged valve letting in air? I can see water in the pipes to and from the anti-syphon vacuum valve but there is also some air in the pipes, and the water doesn’t look like it is moving.

      I have read elsewhere that the water pump body can wear, reducing the water flow due to poor contact with the impeller blades. How long would a pump be expected to last? Mine is 8 years old now.

      All advice gratefully received.


        Dear Elektra…

        You need to re-prime the cooling water circuit. First, close the seawater intake seacock (the one down by the saildrive unit). Next, remove the filter cover and fill with water right to the brim. Replace the cover, making sure there’s a good airtight seal. Start the engine and as soon as it’s running open the seacock. Should solve the problem….if it doesn’t let me know!!


        Ian Culley
        Another Fantasy



          Thanks for the advice. I’ll give that a go at the weekend and report back.

          Thinking back on it, and looking at the exploded view of the water filter in the engine manual, I don’t remember the rubber seal being there when I reassembled the filter. The seal may have come out with the strainer and may have come off while I was cleaning the strainer in a bucket of water, the contents of which which then got chucked overboard! The water was very dirty so I might not have noticed. I’ve ordered a new seal just in case.



            I think your last thought (missing seal on the filter) could well be the answer. I had problems for quite a while on Playtime with the old Volvo filter lid not sealing properly, despite my best efforts to screw it down tight, grease the seal etc. I eventually replaced the whole assembly with a new one from Vetus. This has a transparent lid so you can actually see the water flowing!
            You can also check the anti-syphon valve by putting your finger over the pipe and seeing if it is ‘sucking’. If it is then the valve is leaking and will reduce the efficiency of the seawater pump.
            Ian’s suggestion re priming the system is also worth trying although the pump should be self priming.
            Good luck



              Without wanting to question your ability, have you re-installed the impeller in the right direction?

              If its installed in the incorrect direction it may not have enough seal to prime or pump water.



                Good news and bad news I’m afraid.

                I checked the filter and the seal was still attached.

                On Friday, I primed the system by closing the intake seacock and filling the filter. I then poured water down the hose from the vacuum valve to the pump (which surprisingly caused the open filter to overflow). I then put the filter cover on tightly.

                I started the engine and opened the seacock as per Ian’s advice. This seemed to do the trick, and water was flowing well from the exhaust. After stopping and starting the engine a few times I thought all was well, and we set off to the Kyles Of Bute for the weekend.

                Everything was fine until I started the engine after sailing for a couple of hours on the way back to Rhu. The symptom of “reduced water flow for a time after sailing” had returned.

                On our return to the mooring I changed the impeller for another new one. When I removed the pump cover plate I found that some of the blades of the impeller were facing the wrong way. I am 100% sure I put it in the correct way.

                Anyway, at least I now have water flow and can use the boat. I can live with the delay in getting full water flow after sailing, as long as it doesn’t take too long. I’ll have to wait a couple of weeks until I sail again, to find out if the new impeller has made any difference to the “after sailing” flow rate.

                I’m thinking of fitting a non-return valve to prevent water syphoning out, but feel that this would be treating the symptom rather than the cause.



                  The only other thing I can suggest is a blockage in the heat exchanger. To clear it requires draining down both direct and indirect cooling circuits and then dismantling the heat exchanger, to remove the cooling matrix (a bundle of little tubes, which can easily become encrusted with scale and the like). Not difficult but fiddly….

                  Good luck!!

                  Ian Culley


                    I have noticed the same water flow on my B37. If the boat is started on it’s mooring water comes out immeadiately but after some sailing it takes a while for water to start flowing. I also suspect that water is syphoning back out of the pipes whilst sailing. Every time the water flow does establish and then remains good.

                    The question is – does it matter.

                    Whilst water is getting to the heat exchanger from the water pump the water flowing round the closed circuit cooling will only just be heating up and the exhaust will not have time to get warm so water will reach the parts that need it before there can be a problem. There would seem to always be water around the impellor so that also does not have a problem.

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