Hi all,
Our NB has a Isuzu 38 mated with a PRM150 and there is a dull knocking noise resonating through the hull (in neutral or in gear), best heard standing alongside the boat or in the cabin. It is much harder to make out in the engine bay or standing on the engine covers when in place. Even standing on the bow, you can hear the noise in the water tank below - it is loud.
There was a similar thread by @GrahamSop a few months ago. Graham uploaded a sound file and the noise sounded identical to the one we are hearing, sadly no conclusion on the thread.
We use the boat very little, perhaps 30 to 50 engine hours a year. We keep the fuel tank full during winter in an attempt to avoid condensation (which we have never had a problem with), the sound has been evident for some time. Last time we went out prior to this year was two years ago, but we recently started going out every weekend and the noise seems much louder than I recall from before. We don't know how many hours the engine has done as was previously a hire boat, the tacho has just shy on 13,000 hours on it, and while we suspect a reconditioned engine has been fitted at some point, we have no idea.
The engine runs very well and smoothly, a very small amount of smoke on start up, gone in a second or two, oil stays clean and doesn't seem to burn any.
Things I have checked / tried:
Checked and adjusted tappets.
Changed engine oil filter and oil.
Changed gearbox oil.
Replaced fuel filter.
Added injector cleaner.
Cracked injector pipes with engine running, one injector possibly had less effect on the engine than the other three but was marginal.
Checked water trap (clean), fuel looks clean.
Purchased 20 litres of fuel from a filling station, disconnected the fuel line and ran the engine from the can of fresh fuel for about 45 minutes, during which time the pump sucked about 5 litres from the can. Added the remaining fuel to the tank.
Probed the engine and gearbox for noise with a mooring pin. The noise is very clear and loud where the gearbox meets the adapter plate, the engine, injectors and mounts all sound quiet.
Disconnected the propshaft at the coupling at which point the noise can no longer be heard in the cabin etc., so at least I know how it is being transmitted to the hull. If I then probe the coupling it is very clear, it can be faintly felt through the probe.
Other then removing the gearbox and checking the driveplate etc., I am out of ideas. Several engineers I have sought advice from both say driveplates normally only make noise when a gear is engaged. A fellow boater a few boats up from us has the same engine / gearbox combination and they have the same sound, but it can barely be heard inside the cabin, certainly not to the point I would think it is a problem developing.
The noise is very regular so I don't believe it is an engine mount, as surely disconnecting the propshaft would not make any difference to it? Graham's post went as far as removing the gearbox and driveplate only to find the noise continued - last post he made said he was looking at possible fuel issues - but I have kind of ruled that out with the exception of a bad injector, although they all sound good when listening.
So before I get into removing the gearbox or talking myself into thinking it is an engine problem, any ideas / things to check would be welcome.
Thanks.