Perkins M30 sudden knocking

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.

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 ?

sarabande said:

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 ?

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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.

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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 said:

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 ??

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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.

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

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

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.

Graham376 said:

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.

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