What's Making This Noise?

[alexaescht]

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]

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]

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]

What's Making This Noise?

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]

What's Making This Noise?

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]

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

[alexaescht]

What's Making This Noise?

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

Ok will do, thanks

[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.

[alexaescht]

What's Making This Noise?

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]

What's Making This Noise?

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

[PoloDave]

Diesels, I love them.

+1, I'm hooked since I drove a chipped 330D with 550nm. Don't care for hp, buckets of torque ftw.

[pontiff]

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.

alex... im in officer and have a vag-com cable so if you wanted to bring the car out we could plug it in and see what there is to see.

it does sound like engine knocking to me also.. does it do it if you hold that rpm at which it occurs while driving but while stationary?

i had the AC / fuel hose rattle on me yesterday only at idle. as soon as i moved it to the other clip it was fine, but it does wobble around the place....

[alexaescht]

What's Making This Noise?

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

It has a DPF Gav, so any soot is filtered out of the exhaust stream before it reaches the tip. It's still driving fine above those rpm's and there are no warning lights so me thinks we'll wait until next service. Thanks though

alex... im in officer and have a vag-com cable so if you wanted to bring the car out we could plug it in and see what there is to see.

it does sound like engine knocking to me also.. does it do it if you hold that rpm at which it occurs while driving but while stationary?

i had the AC / fuel hose rattle on me yesterday only at idle. as soon as i moved it to the other clip it was fine, but it does wobble around the place....

It only makes the noise under load (I.e accelerating hard or going up a hill) not when stationary. Once again, I highly doubt it's a cable or clip thing because it's just too loud to be.
Thanks for the VCDS offer, I'll think about it but my 'position' in my family makes things difficult

[pontiff]

It has a DPF Gav, so any soot is filtered out of the exhaust stream before it reaches the tip. It's still driving fine above those rpm's and there are no warning lights so me thinks we'll wait until next service. Thanks though



It only makes the noise under load (I.e accelerating hard or going up a hill) not when stationary. Once again, I highly doubt it's a cable or clip thing because it's just too loud to be.
Thanks for the VCDS offer, I'll think about it but my 'position' in my family makes things difficult

your position? the women tell you what to do eh

Car is under warranty and you live in harkers, just drive it into berwick VW and get one of them to go for a drive with you. Think of it this way, you are doing them a favour as by leaving it, may be causing more damage that will just be a warranty job for them anyway, where as if you get it looked at sooner it may be able to be headed off at the pass.

[gavs]

What's Making This Noise?

It has a DPF Gav, so any soot is filtered out of the exhaust stream before it reaches the tip. It's still driving fine above those rpm's and there are no warning lights so me thinks we'll wait until next service. Thanks though

So do half the diesels in our fleet and they still blow soot/smoke. All diesels do regardless of the marketing spin. I was following a brand spankers ML class last night and that was puffing out smoke like puffing billy, but you can only see it at night with headlights on. If the injector is performing badly enough no dpf will stop it smoking. I spent a lot of time stripping down and putting hilux diesels back together. Granted modern injection is more complex (read, dangerous. I saw the resultant photos of a toyota tech crossing a common rail injection system the wrong way. He is now known as 'lefty') but it is fundamentally the same. High pressure fuel delivered by a spring loaded injector into a cylinder that due to the high level of compression ignites the fuel. In fact, I'd hazard a guess to say that with the software now loaded into these modern diesels they are easier than ever to work on because there are less things to line up and time correctly and adjust. It's all done with the click of a mouse instead.
I digress!

Just try it and see. And don't let your parents and sister wait, get it looked at immediately, especially if its an easily reproducible problem. There are known faults with vw diesel injectors and the dealership won't tell you that until you ask what the sticker is on the inside of the drivers door.