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BMC 1.8 Random Engine Acceleration

I have 20 year old Calcutt Marine BMC 1.8 engine in my narrow boat. It was re-tuned by them 18 months ago after fitting new injectors and injection pump. It has now started to randomly accelerate and misfire at normal cruising speed, which improves slightly as the engine warms up, I have replaced both fuel filters but this has made no difference. Could this problem be due to air getting into the fuel system, if so how? Any help would be most appreciated.
Many thanks Peter Heath

Asked by: Peter Heath  | 7.26pm, Monday 16 September

WW says:

I think that the problem you describe is indeed air getting into the diesel supply to the engine. Air bubbles will cause the engine to inject a smaller amount of fuel than needed to maintain the speed, causing the engine to slow down; the governor will then increase the fuel supply which will be injected without bubbles so the engine will speed up until the governor compensates. Thus the engine will slow momentarily, and then speed up apparently for no reason.
To find where the air is getting into the diesel system requires systematic inspection of every pipe and joint from the tank to the injector pump. If the air is being sucked in through a bad joint or a cracked pipe there will not necessarily be signs of a leak. It is also worth checking the diaphragm in the fuel pump as this could be a hidden point of leakage.
Good luck!

Rupert Smedley  | 1.01PM, Tuesday 17 September

Readers say:

Many thanks for the quick response and the explanation of what is causing the problem. I will let you know how I get on with the investigation and solution.
Regards Peter Heath

Peter Heath  | 4.58PM, Tuesday 17 September

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