Shortly after taking possession of this boat, I realised that there was a problem with the fuel supply to both engines. The engines would run for approximately 1 hour and then run out of diesel fuel. To get the engines running again required bleeding the fuel lines and then they were both good for another hour.
The engines are 3 cylinder Yanmar model 3GMD. They are original motors and salt water cooled. While salt water cooling is more prone to engine corrosion, it is also much simpler as there no heat exchange with only one water circuit. They also run much cooler between 50-60 degC because salt water is not good at higher temperature and causes a build up of salt depots in the engine. However running at the lower temperature means that the engine is not as efficient and therefore not used these days.
When I first discovered the problem I engaged a diesel mechanic to check over both engines and he told me that there did not appear to be anything wrong and it should not recur. How wrong can you be. We had this problem all the way back to Sydney.
finding the problem
Having a bad experience with Diesel mechanic I decided that it was best to try and sort out the problem myself. Not knowing anything about diesel engines, it was a steep learning curve. Firstly, you need to understand exactly how the engine is set up. Tracing through the cooling system, fuel system, electrical system and exhaust gives you a much better understanding of how everything works.
My first thought was that we were somehow sucking in air at either the fuel tank or the primary fuel filters. However, logic would indicate that this was unlikely to occur exactly the same on both engines. Maybe it was bad fuel? So the first step was to change the primary and secondary fuel filters. There was a lot of dirt in both primary filters and water on the starboard side. Maybe the filters have not been changed for a long time or maybe the fuel tanks have a lot rubbish in the bottom?
Tried running the engines again and they both stopped again after about an hour of running. A check of the filters showed that there was a significant amount of debris coming from the fuel tanks, but not really enough to cause the engines to shut down!
cleaning the fuel
So the next logical step was to ensure that the fuel was good. There is no inspection hatch on the fuel tanks so there is limited access to clean them. Built a small fuel polishing unit that could access the tank via the fuel return line from the top of the tank. In this way it was possible to suck up the rubbish from the bottom of the tank and filter the fuel before returning it to the tanks.
Again tested the engines and sure enough they stopped after about 1 hour of running. But at least the fuel supply now was clean! So the problem was coming from some other source. So when checking out the fuel system for the umpteenth time I noticed that the fuel return lines coming off the injectors was being fed into the the primary filter. It was the same on both engines. This did not seem right as the return line to the fuel tanks had been disconnected? Not really understanding the significance of this I did some research and that it was not normal.
So I contacted Yanmar to get their advice and was told that this is definitely not recommended. When fuel comes out from the injector it has been put under a lot of pressure and this can cause a small amount of gas to come out of the fuel. When fed into the filter it can slowly build up and eventually make its way to the injector and the engine starves of fuel.
Fixing the fuel lines
So this would explain why the both engines were behaving exactly the same. When looking at the plumbing it was a good job and obviously done for some reason. Could not figure out why though? Really did not make any sense. And why would the previous owner not had the same problem?
So I reconfigured the fuel return lines to go directly back the to fuel tanks and took the boat for another run. Voila, both engines ran for 3 hours without stopping. So problem solved.
However I was still not happy as the engines were both emitting a bit of black smoke. This could be because they are old, operating at a low temperature, burning old fuel or potentially lots of other reasons. The fuel looked a bit cloudy, so took samples of the fuel and let it settle overnight. Sure enough there was a fine sediment that settled to the bottom. So decided to get rid of the old fuel that had probably been sitting there for a number of years and replace with new diesel.
This boat has extra fuel tanks located in the bow to enable longer trips under power. When I checked out the installation for these, I was surprised to find that they had electric fuel pumps and primary filters installed back to front! This would never work. So obviously someone has been doing work on the fuel system that just did not understand what they were doing!
Operationa Feedback
No more problems with the fuel system. Have done around 200 hours on each engine and not had any problems with the fuel supply.