Causes of marine auxiliary engines hunting problems, encountered during operation

CAT ENGINE

The engine has started but there is a fluctuation in the RPM. You think that the tachometer cable might have broken but you hear the sounds of acceleration and de-acceleration. The governor linkages are moving like crazy sometimes increasing the RPM and sometimes decreasing it. The auxiliary engines have an Isochronous governor, which means that the engine has to operate at a constant speed after starting. "Iso" means same and "chronus" means speed and thus it is supposed to be a constant speed governor, unlike the main engine governor, which you can run at various speeds. 

How to troubleshoot or service? 

Firstly,  stop the engine,  if  already on loaded condition ,  Transfer load and stop the engine and check the following reasons ;

1) The engine is cold and leads to uneven combustion. This can be caused by forgetting to put the engine on pre-heating after stopping last time. Check for the engine temperature before starting,  it should maintain 35-40C.  Check the Golden book ( manufacturer manual)

2)  The tachometer or its wire may be out of order. They may be rubbing, or the pin might have turned round and is about to give in. Try lubricating the wire, or changing the tachometer.

3) The fuel line may be having air. Yes!, now you remember you had changed the filter but had forgotten to purge the air and prime the system. This is a good time to do it.

4) Some fuel injectors getting stuck and firing intermittently. This might happen in engines which are run on Heavy fuel oil. These engines are supposed to be changed over to diesel oil for approximately half an hour before stopping. The half an hour figure is only a rough guide line, practically the purpose is to flush the pipe lines.

5) Faulty governor;the governor oil is supposed to be removed and spaces flushed with clean kerosene, before filling up with oil again. Sometimes the pilot valves have some sludge stuck and hence the hunting.

Reference,

Introduction to Marine Engineering: by D.A. Taylor

Maritime power plants and maintenance.