The brass union that connects to the inlet side of my Johnson F4B-9 Impeller pump broke off the other week and consequently needed replacing. I took the opportunity to take a closer inspection
of the impeller whilst my engine
was out of action and I'm glad I did as some of the blades were beginning to crumble. I ordered a service
kit and whilst waiting for it to arrive, I took the pump off and stripped it down to check the bearings etc. Once the kit arrived it was a simple and straight-forward job of swapping old with new and soon the pump was re-assembled and back on my engine
However, despite repeated efforts to 'prime' the system it just would not draw water
up from under the hull
into the strainer box. I was taking the boat out of the water
anyway for it's annual clean and antifoul and thought that maybe some marine
growth had managed to suck itself up and cause some blockage around the inlet under the hull
. Once the boat was on the hardstand
I was able to attach a hosepipe to the seacock inside the engine and give the entire inlet a good flush in the hope that if anything was in there, it would be dislodged. Water poured out easily and thus convinced me that there was nothing blocking the inlet under the hull. I was not able to do any more testing until the boat was back in the water.
The boat went back in yeaterday and although I had primed the raw water
system prior to her being lowered into the marina unfortunately, when I started the engine and motored back to my pen, very little water was coming from the exhaust
. Fortunately,i only had a about 100 metres or so and thus it was never going to be a real risk however, on arriving back I immediately started to strip down the entire raw water
system in the hope of identifying the problem.
I took the pipe leading from the seacock up the strainer off and flushed that out with a water jet. I then removed the pipe from the strainer to the impeller and did the same with that. With the stainers pipework removed I took the opportunity to unbolt the strainer and gave that a good wash in a bucket of fresh water and checked the strainer was serviceable and the seal around the lid was secure and working correctly. I then refitted the strainer and all the associated pipework. Then I removed the impeller cover and made sure that the impeller was indeed going round on the shaft and it was correctly installed. Everything appears to be just fine. I then moved to the section on the outlet side of the impeller. This is just a short piece of hose that bends round and attaches to the heat exchanger
union. I double checked that there was no blockage in this area.
At each seperate point of this process I started the engine to see if I had resolved the problem but to no avail. I even attached a length of clear hose to the outlet side of the water pump hoping that even if the raw water would not pass through the heat exchanger
, I would at least see it exiting the pump. But it wasn't even making it that far.
Why won't the pump do its job? It has all these new parts
in it, it should work better than ever but for some reason, it just won't work. Am I priming it incorrectly? I close the seacock, unscrew the top of the strainer box and pour water in. The water travels down the pipe back to the seacock easily enough and it also travels down the pipe that leads to the pump. I ensure that strainer is full of water before screwing the lid back on. I open seacock just seconds before starting the engine, the initial water that was sitting in the strainer gets sucked through the system and is forced out of the exhaust
as it should but no new water is pulled up from under the hull. Something strange is happening and I am at a loss as to why I cannot get it to work.
Any ideas would be gratefully received.