Yanmar 4LHA engine help please

[Moonshiners]

My dad has a pair of Yanmar 4LHA's on a nimbus 350 (2005) since the boat has been on the hard and been serviced, new prop bearings etc... the port engine (only) has develloped an intermitent problem. The engine will drop from cruising revs (3500) down to 800 for anywhere between 8-20 mins then come back up to normal speed. (also happens at low revs) As it's an intermitent problem, I've never been on board while it's happened. I'm heading up later to check out and change the (newish) primary fuel filters.

The problem seems to occur when it's rough and choppy, idicating movement of something may be the cause. Have had marine engineers on board, but problem did not arise at the time during test and no issues were found.

Service engineers have not made any effort to help sort out the problem.

But does anyone have and suggestions on what it could be, or things worth checking out?

I'm not well up on diesels, so all experienced thoughts welcomed.

Need to get it sorted ASAP and cross channel trip planned shortly...

Thanks.

 

[Tidnock]

Most faults like yours on a diesel engine are either fuel or fuel related have you got adequate and visual water traps, the rough/choppy conditions may be stirring up sediment in the tank are there any other signs black smoke etc. Are the engines fed from one tank or two

 

[Moonshiners]

Hello mate, yea pretty sure it's fuel related, changed out all the filters last night, no gunk, only a couple of small particles on the primary. no water in the traps. So didn't see anything that should cause it. They're only a few years old, used regularly and serviced annually.

Hoping that their was some kind of blockage that has now been flushed through with the new filters, otherwise it looks like it's the pick up pipe... not looking forward to that job!

 

[Tidnock]

Hello mate, yea pretty sure it's fuel related, changed out all the filters last night, no gunk, only a couple of small particles on the primary. no water in the traps. So didn't see anything that should cause it. They're only a few years old, used regularly and serviced annually.

Hoping that their was some kind of blockage that has now been flushed through with the new filters, otherwise it looks like it's the pick up pipe... not looking forward to that job!

do you have one tank or two?

 

[Moonshiners]

Sorry missed that, 1 tank.

 

[Tidnock]

Sorry missed that, 1 tank.

assuming the pickups are close to each other and a similar length, if you have only one tank, the fault may be local to one engine, see how the filter change goes

 

[Moonshiners]

Chance of filters and a thorough bleed out of the system all well for 24 hours then the problem started again, although thsi time I was onboard.

This time rev's were dropping from a cruising 3000rpm to 2500rpm (starboard only) although if more throttled was applied this could be brought back up to 3000rpm so both engines were running at equal revs but quite different throttle positions (startboard as much as 30-50% more in order to align rpm.

reduced both engines down to idle and accelerated again all seemed to be fine again.

Theories are:

Blocked fuel pick up on starboard outlet of fuel tank
Electric throttle system - causing error
Blocked injectors

Anyone got any more ideas on narrowing or identifying the problem please let us know before we end up writing a rather large cheque for fault finding...


Am planning on "trying" access is an issue to lift the fuel pick up and check the gauze/filter next but as always ideas and previous experience always welcome.

 

[Moonshiners]

Just for archive reference, we seem to have found a fault in the black box of the (electronic) throttle, which looks as though it's the source of the problem.