Sudden engine knocking sound / Injector problem?

captainboo

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Just launched yesterday and after about 30-40 minutes under engine going down the Clyde, revs suddenly decayed slightly accompanied by a loud knocking sound. Reduced to idle and sound still there. Stopped the engine and then sailed the rest of the way down the Clyde with a tow by a friend onto my mooring.

Engine is a mid 80's Ford 1.6 with a Bosch VE injection pump and injectors.

Had to change the pump over the winter due to a leak, replaced with a 'as new' reconditioned unit. New pump installed and timed, pipes bled, and it started and ran fine with no adjustments required. Tried to be as clean as possible when reassembling - injector supply unions covered/pipes cleaned and blown through/new filters fitted/bled extensively to flush through before tightening injector pipes etc.

This was the first time the engine has run under load properly since changing the pump.

Did a quick diagnosis yesterday with the engine at idle by cracking each injector union in turn. No.1 - 3 caused idle to drop slightly but no change to the noise however No.4 didn't change the rpm but the noise dissappears and once tightened the the engine briefly idles normally then the noise returns.

Does this suggest that the No.4 injector is faulty and potentially not closing properly causing excess fuel and hence the knocking? (Have I introduced dirt into the system?) I don't think it is a pump problem because of the way the pump works delivering the fuel and there is no individual adjustment for each injector.

Secondly if the injectors need an overhaul any recommendations for the west of Scotland / Glasgow area?

Intending to go to the boat today and check my diagnosis / remove the injectors.

On the plus side it was a great day to sail down the Clyde!
 

RichardS

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I'd be surprised if a faulty injector which was delivering too little or too much fuel or a poor spray pattern could produce a loud knocking sound. The only time I've heard a loud knocking sound from an engine it's been a bearing problem, with a different pitch of knocking depending upon which bearing is on the way out. :(

If it were a bearing I suspect that it might stop knocking if you stopped combustion on the cylinder with the worn bearing ..... however, that is pure conjecture on my part as I've never tried that or read about it and I don't have much experience with diesel engines.

I think that you are right to get the injector checked/swapped as a first step as that is relatively inexpensive and might be the solution.

Richard
 

rotrax

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Diesel "Knock " in old generation engines is pretty much a product of the high pressure pulses in the thick wall steel injector pipes. When an old tech diesel injector pump is in a test rig being calibrated, the sound of it being rotated by the test rig motor is pretty much how an old tech diesel engine sounds.

A neighbour had a Citroen BX19 Diesel estate car, with which he was really pleased. Another neighbour purchased a similar car, brand new.

His car was really noisy, knocking like a good 'un, despite being new.

The only difference was the noisy one had a Bosch rotary injector pump, the quiet one a Spanish built Roto-Diesel.

There was a pressure difference, not in total pressure, but in the way the pressure was produced, the Bosch pump being far more " sudden " in its action. This was the cause of the extra noise. Both cars performed faultlessly and gave good service, but the owner of the Bosch pump one was never happy and changed the car quite quickly.

The fact that the noise disappears when the injector pipe is loosened gives a good guide as to where the problem lies.

IMHO, of course.
 

pcatterall

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I'd be surprised if a faulty injector which was delivering too little or too much fuel or a poor spray pattern could produce a loud knocking sound. The only time I've heard a loud knocking sound from an engine it's been a bearing problem, with a different pitch of knocking depending upon which bearing is on the way out. :(

If it were a bearing I suspect that it might stop knocking if you stopped combustion on the cylinder with the worn bearing ..... however, that is pure conjecture on my part as I've never tried that or read about it and I don't have much experience with diesel engines.

+1 ( Sorry!) that was my original thought!

I think that you are right to get the injector checked/swapped as a first step as that is relatively inexpensive and might be the solution.

Richard
 

vyv_cox

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Check for a broken valve spring on cylinder 4. You could try the simple compression test described on the website, basically just bar the engine over to feel whether each compression is about the same.
 

captainboo

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According to the Haynes manual for the engine if the injector is not working properly and possibly leaking fuel this can result in the knocking caused by detonation of the excess fuel entering the cylinder too early before the correct pressure has built up.

The compression feels good on all the cylinders and when cracking each injector there was a small but obvious drop in rpm on the 'good' cylinders. I think that if I had a valve problem then with one of the good cylinders cracked it would only be running on 2 cylinders and be noticeably lumpier.

Did contemplate swapping the injectors but had to remove the pipes and a number of connections to get the injectors out and now need to clean out the holes due to the rubbish/old paint falling in! Did try brushing the debris away as they came out but had limited access.

The tips all look similar without a lot of carbon but if one is stuck slightly I guess it only has to be miniscule object to allow the fuel to leak out early.

Will get them tested and refurbished now that they are out and then take it from there.

Thanks for the replies and will post the outcome in due course.
 

DownWest

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I had exactly this after fitting a serviced set of injectors. Really loud knock. The blokes looked embarassed when I told them & dissapeared for a bit with the offending one. Refitted it and fine.
 

black mercury

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No an injector can't cause overfueling. I would say you have got a tiny piece of dirt in the supply pipe or the injector itself when you were fitting the pump. The fact that the engine runs normally for a few seconds after tightening the injector union when you were diagnosing the problem points to dirt in the injector. What is happening is when you open the injector union with the engine running the tiny piece of dirt is moved away from the injector nozzle, then when you tighten the union it take a few seconds for the dirt to travel back down to the nozzle, the injector and engine working normally for a few seconds. Marine engines are usually coated in thick paint, and a tiny piece of flaky paint has maybe found its way in. You have done the right thing in removing them, hopefully all will be well. The spray pattern of the injector will be altered badly by a fouled nozzle which will cause a heavy diesel knock. In extreme cases it can melt pistons, although your engine is a pre-combustion chamber engine so that is slightly less likely.
 

captainboo

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No an injector can't cause overfueling.

Interesting and thanks for the information. I was assuming that the injector was not closing properly by a piece of debris and therefore leaking fuel early during pressurisation of the fuel line and causing the knock by the timing of the burn being out. Will hopefully find out in due course.
 

rotrax

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Interesting and thanks for the information. I was assuming that the injector was not closing properly by a piece of debris and therefore leaking fuel early during pressurisation of the fuel line and causing the knock by the timing of the burn being out. Will hopefully find out in due course.

A very perceptive and possibly correct assumption. White smoke often accompanies leaking injectors-but not always.
 

bristoljim

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If an injector is blocked ( fortunately very rare) the noise is pretty drastic & what you described - you have identified the cylinder ok so you are on the right lines getting that injector serviced.
Jim
 

captainboo

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Quick update - injectors now tested. Nos. 1-3 working at about 90% but No.4 is stuck open so problem found.

Going to get them all serviced and should get back in a couple of days and hopefully be as clean as possible during the re-install.

Many thanks to all.
 
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