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Posted: Mon Aug 17, 2009 9:20pm

Post Subject: Shire engine losing power

For about 12 months we have had an intermittent problem with our (Yanmar) Shire 800 3 cylinder 800cc engine losing power. The engine is around 5 years old and has only done about 400 hours. The problem can occur at any time whilst cruising and appears like a misfire with the revs suddenly dropping from say 1400 to 1000. If the throttle is reduced to tickover for 2 or 3 minutes the problem is usually cleared and normal speed can be resumed. We have replaced the fuel filter (& drained water trap - no water), fuel lift pump, bled fuel system @ lift pump, checked all fuel pipes are tight & add Fuelset at every diesel top up - all with no improvement. Do you have any ideas?


Posted: Tue Aug 18, 2009 9:06am

Post Subject: Shire engine losing power

Dear Andy, I note that you do not make any mention of exhaust smoke during the incidents. If you had said the exhaust smoked (probably white or grey) but it cleared when the mis-fire cleared I would have said it was s a sticky injector but if there is no smoke that only leaves a fuel problem or partial seizure. A partial seizure would probably be accompanied by over heating and as you make no mention of that it makes this less likely, although some very odd internal lubrication problem could still produce it, but it is so long shot as to be ignored. So we come back to fuel. Have you checked the fuel tank breather is not blocked. remove the fuel filler and blow down the breather, it should be very easy to blow. Break a connection on the return pipe to the tank as close to the engine as possible and try blowing back into the tank, again, once you have blown the fuel out of the line this pipe should be easy to blow down. If the engine has any flexible pipes on it both the feed and return pipes from the tank should have one way valves fitted close to the tank, if so make sure they are facing the right way and are not blocked. Now, you do not tell me the age of the boat, although the following is unlikely on a five year old boat it is possible and much more likely on a much older boat. Good practise requires a gauze strainer to be fitted to the fuel pickup pipe in the tank. This can block so (if you are able to bleed the system) remove a supply pipe union and try blowing back into the tank. Personally I would rig up a pipe that fitted onto the car's spare wheel valve so that I could remove the valve and use the stored air as an sort of airline. Do you have a diesel heater with its supply Td off the main fuel line. If so try blanking that feed off just in case the engine is sucking air back through the heater - especially if its a bubble type burner. If all that is OK I am afraid I must refer you to E P Barrus at Bicester for their advice because it could be an odd injector pump/governor fault. Tony Brooks

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