What's Making This Noise?

**alexaescht** · 03-06-2013, 06:28 PM

Hi all,
The Polo has just started to make this noise recently where below 1,200 rpm under load it will emit a loud knocking noise from the engine. At all other revs, and during idle, it sounds perfectly normal. I was wondering if anyone here could identify exactly what in the engine is making the noise.
Video: VW Volkswagen Polo 6R TDI 2011 Strange Engine Noise - YouTube

Thanks

Alex.

**wai** · 03-06-2013, 06:41 PM

If it is happening at a particular engine speed, then it is resonance. Something is vibrating and this happens to be the natural frequency for whatever is vibrating. It is difficult to trace in a moving vehicle.

With my previous Hiace I would get the buzzing vibration at 2000 rpm. I thought something had failed in the muffler, but it turned out to be an exhaust clamp that had loosened a bit and the end of the U bolt was vibrating against the chassis. The only way the dealer could trace it was to put the car on a rolling road and then be under it while it was run.

Check things like clamps and make sure that there is plenty of clearance around things.

**nomadx38** · 03-06-2013, 06:46 PM

That sounds very similar to a petrol engine when a cylinder is misfiring - had that a couple of weeks ago - my car sounded like a boxer engine!

Now as a diesel doesn't have a spark plug or any form of electronic ignition the only thing I can think of is one of the injectors is playing up and not delivering the fuel properly. If you have VCDS you should be able to check for any fault codes.

**alexaescht** · 03-06-2013, 06:52 PM

Originally Posted by wai

If it is happening at a particular engine speed, then it is resonance. Something is vibrating and this happens to be the natural frequency for whatever is vibrating. It is difficult to trace in a moving vehicle.

With my previous Hiace I would get the buzzing vibration at 2000 rpm. I thought something had failed in the muffler, but it turned out to be an exhaust clamp that had loosened a bit and the end of the U bolt was vibrating against the chassis. The only way the dealer could trace it was to put the car on a rolling road and then be under it while it was run.

Check things like clamps and make sure that there is plenty of clearance around things.

Ok food for thought, but it is really loud and I think it's something more major than just a clamp. Thanks

**alexaescht** · 03-06-2013, 06:59 PM

Originally Posted by nomadx38

That sounds very similar to a petrol engine when a cylinder is misfiring - had that a couple of weeks ago - my car sounded like a boxer engine!

Now as a diesel doesn't have a spark plug or any form of electronic ignition the only thing I can think of is one of the injectors is playing up and not delivering the fuel properly. If you have VCDS you should be able to check for any fault codes.

Yeah I was thinking a misfire or pre-detonation (knocking). Not sure how either works in a diesel though.
Unfortunately I don't have VCDS, so I suppose I'll add it to the growing list of things to be fixed next service...
It's not the dual-mass flywheel is it?

**GTI4ME** · 03-06-2013, 07:36 PM

Sounds like injectors, head to the dealer for confirmation though.

**alexaescht** · 03-06-2013, 07:53 PM

Originally Posted by GTI4ME

Sounds like injectors, head to the dealer for confirmation though.

Ok will do, thanks

**nomadx38** · 03-06-2013, 08:16 PM

And just for a little insight into the workings of a Diesel engine: like a normal petrol engine it uses a piston and valves but no butterfly valve (or throttle plate as people know it as) which allows it to use the same volume on every intake stroke (unlike a petrol which has throttling/pumping losses and lag) which is then highly compressed to 16:1>.

When you compress things they got hot and in this case the air that has just been compressed to more than 16 its normal has gone from an ambient 25 degrees to well over 400. All the injectors have to do is inject fuel into the cylinder and the fuel burns immediately upon hitting the air - and thus the expansion stroke.

The reason diesels have the iconic knocking sound is because the fuel is ignited continuously during the expansion stroke and as the head of the flame is set off more fuel is added - making like a ripple of explosions after each other, where petrol engines are ignited once and all the fuel is burnt up in one stroke

If a injector were to not be injecting right this can unbalance the engine a a lot! Under no load (idle) you can't notice it and up high its not too bad because of the centrifugal force from the high spinning pistons, but at low speeds under load you will feel it most.

**alexaescht** · 03-06-2013, 08:57 PM

Originally Posted by nomadx38

And just for a little insight into the workings of a Diesel engine: like a normal petrol engine it uses a piston and valves but no butterfly valve (or throttle plate as people know it as) which allows it to use the same volume on every intake stroke (unlike a petrol which has throttling/pumping losses and lag) which is then highly compressed to 16:1>.

When you compress things they got hot and in this case the air that has just been compressed to more than 16 its normal has gone from an ambient 25 degrees to well over 400. All the injectors have to do is inject fuel into the cylinder and the fuel burns immediately upon hitting the air - and thus the expansion stroke.

The reason diesels have the iconic knocking sound is because the fuel is ignited continuously during the expansion stroke and as the head of the flame is set off more fuel is added - making like a ripple of explosions after each other, where petrol engines are ignited once and all the fuel is burnt up in one stroke

If a injector were to not be injecting right this can unbalance the engine a a lot! Under no load (idle) you can't notice it and up high its not too bad because of the centrifugal force from the high spinning pistons, but at low speeds under load you will feel it most.

Makes so much sense now! Thanks!
So do you think it's likely the injectors/an injector at fault? I'll certainly ask VW to look at it next service, which should be next week, but if you have any ideas that would be great

**gavs** · 04-06-2013, 01:24 PM

Sounds like a rooted injector ( who would have thought, a vw diesel injector dying!) or a loss of compression. My hilux sounded very similar when I lost compression in cylinder 3.... I hope for your sisters sake, it's just an injector and more likely than not, that's what it will be. Stand begin the car and see if there is more soot than usual upon acceleration. If there is, that's a pretty sure bet its a rooted injector that is just dribbling fuel into the cylinder rather than atomizing it.

Diesels, I love them. Generally if you have a miss, there's only 2 things that cause it. Fuel delivery (work back from the injectors) or timing when an engine hasn't been times correctly