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Engine changing speed.
Engine speed will automatically increase for a short time.
Air in the fuel system. Leaking diaphragm in feed pump. Fuel pump leak. Dirty filters.
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Engine runs irregularly.
Air in fuel system. With larger engines (OM363, DAF575 for example) the fuel has a separate crankcase where engine oil has to be put in, if this is not there the pump and regulator will work more stiffly and wear out faster.
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Engine runs wild.
Crankcase breather mounted on an air filter should be regularly checked for contamination. With a heavily soiled air filter, oil can be sucked out of the crankcase when the crankcase breather ends in the filter.