The FSI Rattle: Explained?

[PlasticMac]

I thought I'd try and get to the root cause of the tappet like noise that is common to the FSI engine at startup.
It has long been put down to the hydraulic lifters, but I only very vaguely understood how these worked, let alone how they account for the noise that was being attributed to them.
In my mind, I had an image of a camshaft operating at one end of a rocker arm, pivot in the middle, and the other end opening the valve.
Wrong.

I discovered that the AUA/BBY engines have a very similar arrangement, which gave a very clear description of how the cam shaft operates the valves. The FSI engine is the same.
In the FSI, (and 1.4 petrol), engines, the valve is operated by a cantilever arm, supported at one end by an hydraulic piston/pivot point, ("Hydraulic Support Element" in Audi speak), and bearing on the valve stem at the other. In the centre, the arm has a roller, by which the camshaft operates it.
This arrangement allows the actual camshaft to be smaller, and lighter, using the cantilever arm to amplify a small cam movement to a larger movement at the valve stem.
The self adjusting function is provided by the hydraulically supported pivot, which, by varying the volume of oil, and so pivot height, provides the correct valve clearance.
Here's a pic from SSP247, which shows it clearly:

The pivot height is set by oil pressure, via a spring and ball valve. Once set, the volume of oil is fixed, (ball valve closed), and becomes a fixed rigid element.
The valve clearance setting process takes place at startup, but requires the oil pressure to reach operating pressure, before it can happen. This will take a few seconds, hence the noise. It sounds like badly adjusted tappets, because that's what is is!
Another clip from SSP247 explains how it works.

I've attached an extract from SSP247, that covers the whole mechanism and it's operation in detail.
Mac.

 

[Indi]

I

I thought I'd try and get to the root cause of the tappet like noise that is common to the FSI engine at startup.
It has long been put down to the hydraulic lifters, but I only very vaguely understood how these worked, let alone how they account for the noise that was being attributed to them.
In my mind, I had an image of a camshaft operating at one end of a rocker arm, pivot in the middle, and the other end opening the valve.
Wrong.

I discovered that the AUA/BBY engines have a very similar arrangement, which gave a very clear description of how the cam shaft operates the valves. The FSI engine is the same.
In the FSI, (and 1.4 petrol), engines, the valve is operated by a cantilever arm, supported at one end by an hydraulic piston/pivot point, ("Hydraulic Support Element" in Audi speak), and bearing on the valve stem at the other. In the centre, the arm has a roller, by which the camshaft operates it.
This arrangement allows the actual camshaft to be smaller, and lighter, using the cantilever arm to amplify a small cam movement to a larger movement at the valve stem.
The self adjusting function is provided by the hydraulically supported pivot, which, by varying the volume of oil, and so pivot height, provides the correct valve clearance.
Here's a pic from SSP247, which shows it clearly:

View attachment 124278
The pivot height is set by oil pressure, via a spring and ball valve. Once set, the volume of oil is fixed, (ball valve closed), and becomes a fixed rigid element.
The valve clearance setting process takes place at startup, but requires the oil pressure to reach operating pressure, before it can happen. This will take a few seconds, hence the noise. It sounds like badly adjusted tappets, because that's what is is!
Another clip from SSP247 explains how it works.

View attachment 124279

I've attached an extract from SSP247, that covers the whole mechanism and it's operation in detail.
Mac.

I don’t think this is the whole story as the 1.4s don’t suffer the same thing, at least not to nearly the same extent…

I was poking around online a while back and saw some reference to the original VAG oil filter on the FSI (the post was in relation to the slightly later chain driven 1.6fsi iirc) having a one way valve, which stops the oil draining back down. The theory was that the aftermarket filters don’t have this, so once warranties ran out and VAG filters were no longer used, the startup rattle became apparent.

No idea if this is true, but I plan to try and find one for my next oil change to see if it makes a difference.

Still doesn’t explain why the 1.4s are different though, possibly a different oil filter orientation or something?

 

[PlasticMac]

I

I don’t think this is the whole story as the 1.4s don’t suffer the same thing, at least not to nearly the same extent…

I was poking around online a while back and saw some reference to the original VAG oil filter on the FSI (the post was in relation to the slightly later chain driven 1.6fsi iirc) having a one way valve, which stops the oil draining back down. The theory was that the aftermarket filters don’t have this, so once warranties ran out and VAG filters were no longer used, the startup rattle became apparent.

No idea if this is true, but I plan to try and find one for my next oil change to see if it makes a difference.

Still doesn’t explain why the 1.4s are different though, possibly a different oil filter orientation or something?

Are the 1.4s that quiet? Virtually silent valve operation, (Edit: at idle)?
I think the noise is noticeable on the FSI because it is there, and then it's not. That's what you notice, a noise then it's gone.
I think we're looking at the oil pressure dropping, not draining, as in a missing check valve.
The difference is that when the engine is off, and the pressure drops, oil remains where it's should be, and circulation is virtually instantaneous on startup.
If the oil actually drains back to the sump, then there will be no oil circulation for a short time at startup. Not good.
Big difference between pressure drop and volume loss.
Mac

 

[Indi]

Are the 1.4s that quiet? Virtually silent valve operation, (Edit: at idle)?

Yeah, from what I remember it’s about the same as the FSI, bar the startup rattle, though it’s been a few months since I’ve driven mine.

When I started driving the FSI I did note it was a more refined and slightly quieter drive, but I put this down mostly to my 1.4 having an undiagnosed metallic rattle at curtain rpms (sounds like a loose heat shield or something but I’ve been unable to find the cause)

 

[james.mckenzie]

Nice write-up Mac. Out of interest what do the German forums say about this phenomenon?

 

[PlasticMac]

Nice write-up Mac. Out of interest what do the German forums say about this phenomenon?

I haven't looked. I'm convinced, so I'm sticking to my theory! I wish I could understand why the AUA is different. Maybe the details of the design of the piston assembly are different.
Mac.

 

[VETRA]

Thanks for studying and for the write-up Mac!
No doubt this is the reason indeed I can only quote you - It sounds like badly adjusted tappets, because that's what is is!

This does explain why the FSIs sound like diesels at startup and then in a sec or two go silent.
What it does not explain is - who do ALL the other engines with 'hydraulic support elements' don't do that.
Probably, due to absence of that on-way-valve in our FSIs, I guess

 

[PlasticMac]

Thanks for studying and for the write-up Mac!
No doubt this is the reason indeed I can only quote you - It sounds like badly adjusted tappets, because that's what is is!

This does explain why the FSIs sound like diesels at startup and then in a sec or two go silent.
What it does not explain is - who do ALL the other engines with 'hydraulic support elements' don't do that.
Probably, due to absence of that on-way-valve in our FSIs, I guess

I've also wondered why the FSI does it, but not others.
Something about the FSI seems to cause the oil pressure to drop, when the engine is stopped.
One possibility I've considered is that the oil pressure driven vane pump mechanism, that operates the variable inlet camshaft "relaxes" on engine shutdown, and relieves the pressure.
Other theories are available.
Mac.

 

[A2Z]

I've also wondered why the FSI does it, but not others.
Something about the FSI seems to cause the oil pressure to drop, when the engine is stopped.
One possibility I've considered is that the oil pressure driven vane pump mechanism, that operates the variable inlet camshaft "relaxes" on engine shutdown, and relieves the pressure.
Other theories are available.
Mac.

It would be nice to get the definitive answer from the VAG Engineer/s who designed the FSI. He/She/they would certainly know and possibly have a fix/solution, as the obvious concern is whether it could be doing the engine harm long term? Might be a bit of a mission to get though Mac, if you're up to it?

 

[PlasticMac]

It would be nice to get the definitive answer from the VAG Engineer/s who designed the FSI. He/She/they would certainly know and possibly have a fix/solution, as the obvious concern is whether it could be doing the engine harm long term? Might be a bit of a mission to get though Mac, if you're up to it?

Might be "Mission Impossible"
Mac.

 

[PlasticMac]

I just look for mechanical differences between the FSI and other engines. For the FSI oil circuit, the oil powered inlet camshaft adjuster stands out.
Mac.