Perkins M30 sudden knocking

[38mess]

This morning my Perkins tick over started to drop then it picked up to normal. Then after a long run when we were back at tick over a knocking noise started. I stopped the engine and checked it over but all seemed good. Then on restart the knocking had disappeared. I did notice what I took to be some fuel on the nut under one of the injectors. Could this be one of the injectors playing up?. It's just run for an hour like a sewing machine.
Any ideas please.

 

[sarabande]

Could there have been a ingestion of air under load into an injector ? Then on restart, not yet enough induction to suck the air in again ?

 

[38mess]

Could there have been a ingestion of air under load into an injector ? Then on restart, not yet enough induction to suck the air in again ?

I don't know...I just tried it again at 2600 rpm up a river and it sounds rough, but at 2000 rpm it's like a sewing machine.

 

[38mess]

.

 

[Refueler]

Just a thought .... maybe a tiny drop of water got injected - if one cylinder got a shot of water / diesel - it could create a knock ...

I'm no engineer - so maybe not ??

 

[Refueler]

Just a thought .... maybe a tiny drop of water got injected - if one cylinder got a shot of water / diesel - it could create a knock ...

I'm no engineer - so maybe not ??

I would still be extremely cautious about it .. until sure engine is OK ....

My old 4-99 Perkins had a knocking sound .... which was very hard to trace where from .... finally I spent about an hour while heavily tied up to secure location - running engine FWD - ASTERN ... idling ... throttling with engine box / cockpit sole removed so I could watch and listen to as much of engine as possible.
It turned out to be one engine mount had failed .... now that I could watch gearbox / shaft and engine ... I could see the engine move and the knocking was connected with that movement.
Boat in its berth - called in yards diesel guy - he ran engine and agreed - changed engine bearers and renewed all mounts ... sweet as anything after.

 

[38mess]

Thanks for the reply.
Yes I had the same experience with a failed engine mount in a previous boat, took us ages to finally diagnose it. The mounts on this boat seem fine, they are less than a year old. But I will check later on when I get down to the boat.
I'm pretty sure it was a spec of something getting into the injector. About an hour before the noise appeared I was taking pictures from my boat of my friend zooming around in his cruiser, he kicked up quite a wake and bounced me around a lot, maybe stirred something up in the tank which somehow got passed the filter?
I don't know, but the noise hasn't appeared since
I have a spare set of refurbished injectors which I am going to replace next week and see what happens.
Maybe I'm overthinking this

 

[peteK]

Water in your fuel can cause knocking,I have had it with a Perkins M20.

 

[Graham376]

Usual way to check for a bad injector is to crack open the fuel line to each in turn with engine running. If they're good, revs will drop each time one's cracked.

 

[38mess]

Usual way to check for a bad injector is to crack open the fuel line to each in turn with engine running. If they're good, revs will drop each time one's cracked.

The knocking had stopped before I had a chance to do this.