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GrahamSop

Engine knocking

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Hi,

My engine has developed a knocking noise slowly over the last few months, and it seems to be getting worse but maybe I'm getting more used to listening to it.

I know a recording is no substitute for someone who knows what they're doing to come and listen, but I've uploaded it here: 

 

 

At first I thought it was my driveplate, which it turns out needed to be replaced anyway, but it wasn't the source of the knocking. Currently both the gearbox and driveplate are removed from the engine so these can be ruled out.

I hope it's something easy to fix like a clogged injector. Tonight I loosened the injector fuel pipes one cylinder at a time to try to see if the knocking changed but the engine struggled when I did this and I wasn't really able to tell.

I really hope that it is not bigger problem like a bearing down below in the engine ?

 

Engine is an Isuzu 35, but unfortunately no idea of the hours because the tacho wasn't connected when we bought the boat. It's at least 800 hours though. I changed oil and filters, and checked valve clearances at the end of last summer. The knocking has been going on since about Jan this year.

 

Any help is very appreciated!

 

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Doesn't sound like a metallic knocking, can't really identify much of a knock at all really. Might be an idea to get the injectors checked though. Before you get the spanners out just check the engine mounts are bolted down well and that one or more is not perished or moving around. Same goes for everything else that's attached to the engine. Does it make the same sound at different revs? if you grab the top of the engine whilst its running and try to stop it vibrating does the noise stop? And if the engine is not running can you rock it and does anything knock. Others will know more.

  • Greenie 2

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It sounded like an injector to me, so if you can get close to the engine whilst it running use a large screwdriver as a stethoscope pointy end on injector plastic bit to ear, do it to all four and if one sounds different it could be a duff one. If its not those listen to the block mooring pin might be long enough if the screwdriver isnt. good luck, and hope its an injector as they are cheapish to repair

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A completely duff one ' nozzle leaking badly' would normally cause a complete missfire on that cylinder as well as knocking. A slightly leaking injector would also slightly knock amd vaguely missfire usually with white smoke moreorless from the exhaust in both cases.

Edited by bizzard

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Having had Listers I found it hard to distinguish anything loud or out of place. ;)

However I was surprised to then read it wasn't a two cylinder. 

Are all four exhaust manifolds equally hot?

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Bee wrote what I would have done. Did it start after refuelling? It sounded like diesel knock to me so just might be caused by poor quality (low cetane rating) fuel. Good idea from p[eterboat. Also try loosening each injector main union in turn and seeing if the knock reduces. If so that injector is suspect.

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1 hour ago, Tony Brooks said:

Also try loosening each injector main union in turn and seeing if the knock reduces. If so that injector is suspect.


I took it from the original posting that they have tried this.....

 

12 hours ago, GrahamSop said:

I hope it's something easy to fix like a clogged injector. Tonight I loosened the injector fuel pipes one cylinder at a time to try to see if the knocking changed but the engine struggled when I did this and I wasn't really able to tell.

 

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If you're going to try loosening the injector pipes again, try raising the engine revs slightly.  This should make the engine less likely to stall and run that bit smoother for the test.  Keep hands clear of spurting fuel.

 

 

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Try levering the engine up at each mounting in turn, sometimes the rubber collapses and lets the threaded pin touch the steelwork under the mounting. Check that the leakoff pipes are not crimped or blocked.

Any water in the fuel?

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Thanks all for the help so far.

I will try loosening off the injector pipes again with some more revs. Is it possible I didn't fully 'kill' each cylinder during the test? I could have loosened the pipes a bit more, I didn't get fuel spurting out but it was starting to trickle, maybe some was still being injected, making the idle very rough and then I tightened up again when this happened.

Engine mounts seem ok, there is no knock when I try to move the engine around, and putting my weight on each corner doesn't change anything while it's running. All other stuff like exhaust is secure as well. I have a stethoscope with which I'll listen to the injectors and let you know what I find.

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If your  Isuzu is one without an air filter , its very easy to get foreign objects dropping in next to the rocker box and into the air inlet.

If ultimately you decide its something stuck in a cylinder I would ignore it . It will eventually get bedded into the piston crown and stop knocking.

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52 minutes ago, Boater Sam said:

If your  Isuzu is one without an air filter , its very easy to get foreign objects dropping in next to the rocker box and into the air inlet.

If ultimately you decide its something stuck in a cylinder I would ignore it . It will eventually get bedded into the piston crown and stop knocking.

Yes it is. And when I last checked the valve clearances I cleaned the air inlet 'mesh' on the rocker cover because it was covered in gunk. So this is definitely possible, except it seems to be getting worse not better.

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2 minutes ago, GrahamSop said:

Yes it is. And when I last checked the valve clearances I cleaned the air inlet 'mesh' on the rocker cover because it was covered in gunk. So this is definitely possible, except it seems to be getting worse not better.

Not a valve adjustment that has gone haywire?

The AA used to advise motorists with a strange noise on the engine/gearbox/axle to "let it develop", Sorted one way or another.

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1 minute ago, Boater Sam said:

Not a valve adjustment that has gone haywire?

The AA used to advise motorists with a strange noise on the engine/gearbox/axle to "let it develop", Sorted one way or another.

I don't think so - it was my first time doing them, but if anything they are all set slightly loose, I made sure none were too tight. Supposed to be 0.4mm so I used a 0.43mm feeler just to be on the safe side. But I will check them now in case.

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If you have an oil pressure warning light you can do a very rough check for bearing and engine wear by running the engine up to normal operating temp so the oil is hot,  stop engine either leave the ignition switched on or switch it back on the instant the engine has stopped and count the seconds it takes for the oil light to come back on. About 3 to 5 seconds is about it for an engine in decent nick.     Or if you have a gauge watch how quickly the needle drops.

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3 minutes ago, bizzard said:

If you have an oil pressure warning light you can do a very rough check for bearing and engine wear by running the engine up to normal operating temp so the oil is hot,  stop engine either leave the ignition switched on or switch it back on the instant the engine has stopped and count the seconds it takes for the oil light to come back on. About 3 to 5 seconds is about it for an engine in decent nick.     Or if you have a gauge watch how quickly the needle drops.

That's a goodun :) 
Something else to try ;)

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1 hour ago, bizzard said:

If you have an oil pressure warning light you can do a very rough check for bearing and engine wear by running the engine up to normal operating temp so the oil is hot,  stop engine either leave the ignition switched on or switch it back on the instant the engine has stopped and count the seconds it takes for the oil light to come back on. About 3 to 5 seconds is about it for an engine in decent nick.     Or if you have a gauge watch how quickly the needle drops.

Thanks bizzard, I have a warning light. Not sure if I understand you correctly: 

Run engine up to temp

Stop engine

time how long it takes for the oil light to come back on

If it's faster than 3 seconds, is this good or bad?

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If its less than 2 seconds, it BAD.

4 minutes ago, Glynn said:

It's good.

Sorry, wrong way round!

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1 hour ago, Glynn said:

It's good.

No, it’s bad. 3 seconds ok, 5 seconds good. Anything above 5 seconds excellent. 

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2 hours ago, GrahamSop said:

Thanks bizzard, I have a warning light. Not sure if I understand you correctly: 

Run engine up to temp

Stop engine

time how long it takes for the oil light to come back on

If it's faster than 3 seconds, is this good or bad?

The longer the light stays on the better. The oil pressure ebbs away and drip back into the sump when the engine is stopped.  It leaks away past bearings and whatnot. So obviously the more worn the engine is the faster the pressure and oil ebbs away. The light comes back on at around 4-8 psi before the pressure finally  to drops zero.

  • Greenie 1

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That is good advice for some engines but some more modern engines don't seem to follow that rule, the light on my Beta goes out the instant it fires and comes on pretty much the instant it stops, its done it from new. Same on my 1.5 Renault diesel car, I think it may be because of modern, thin oils.

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22 minutes ago, Bee said:

That is good advice for some engines but some more modern engines don't seem to follow that rule, the light on my Beta goes out the instant it fires and comes on pretty much the instant it stops, its done it from new. Same on my 1.5 Renault diesel car, I think it may be because of modern, thin oils.

Quite true, noticed it happening on the run up to retirement, an old mechanics trick that no longer seemed to work.

 

 

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Forgive me but having listened to the OP's recording several times over, it sounds to me like a perfectly healthy modern diesel engine. 

 

I'm really not getting what the problem is. 

 

 

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For what it's worth, the oil light goes out about 2.5 seconds after pressing the stop button.

Maybe the recording doesn't capture it very well above the normal running noise, but 100% it is there and it never used to be. I can try to get a better recording over the weekend.

 

Is it worth trying a bit of injector cleaning additive in the diesel, if only to see if it changes the knock?

Or would I be better going straight to taking them out and having them overhauled?

My engine is currently my only source of hot water (other than the kettle!) so I don't really want to have to send the injectors off for a few days, at least not if I don't have to.

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