Apologies for the length, but most of it is relevant to the post...
When I bought my 1998 S4, the owner (a friend) gave me what I thought was a generous discount on the price of the car because of an annoying rattle. He had been told by Links Audi that it was a heatshield on the turbo and that, whilst annoying, it was unimportant. In any case, you could barely hear it with the windows up, so I went ahead and bought the car.
After doing some reading, I began to suspect that it was the dreaded manifold problem. I also knew that a Lambda sensor (on the other bank) was dead and that the MAF was suspect because the car was no quicker than my previous 2.5L Mitsubishi Galant (Auto).
The previous owner had booked it into Links Audi to get the cam belt and water pump done before he passed it over to me, so I asked them to look at the rattle and the MAF. They came back with the same "turbo heatshield" excuse and said that the MAF was "within specification". At this time I was unaware that Audi test the MAF sensor at idle
, and they wanted £300 to change the Lambda sensor, so I let it be.
After a week or so I was thinking of selling the car because it was really not up to much considering it was no quicker, no more comfortable and more expensive to insure than my previous car that I sold for less than a third of what I paid for this one.
I read about the VAG-COM test for the MAF and carried it out. Lo and behold, I was getting 160g/s max and it was also throwing Lean codes to boot. I acquired a working MAF (thanks to DavidT), reset the DTCs and suddenly the power was back. I got the Lambda sensor changed at my local garage and it was DTC free and pulling like a train. And yet.... there was still a rattle.
Every time I opened it up on the road, I had visions of pieces of manifold dropping into the turbo, so I decided to take the plunge and have it fixed. AwesomeGTI in South Manchester did the job. They they tried to save me some cash by changing the VVT tensioner first (as well as changing the cam belt again), but I was right with my diagnosis, so they eventually hoisted the engine out and changed the manifolds. Once they had taken the old manifolds off, they were amazed to see just how bad it was.
The top picture shows the loose liner that was causing the annoying noise. Tiny pieces had either corroded away or broken off leaving a jagged edge, but anything that went through the turbo must have been too small to cause damage because the turbos were fine when they checked them.
However, once the liner comes loose, it is free to rattle up against the EGT sensor that sits just in front of the turbo. The jagged edge of the broken liner acts as a mini hacksaw, slowly cutting through the base of the EGT sensor. If you take a look at the lower picture, you will see that the liner had cut about a quarter of the way through. In fact, the sensor had started to lean over at about 15 degrees from perpendicular, reminiscent of a tree being felled - just before it falls over.
The car had been rattling for over a year, so the process is quite slow. Nevertheless, I would have eventually had a lump of metal weighing ~10g heading staight for the tubine blades and Audi were still insisting that the rattle was "just a little annoying".
Now for my theory
We all know that the MAF sensors invariably fail on these cars. When this happens, there are no DTC codes thrown out until it gets REALLY bad, then the engine reports fuel trim = lean. Even then, Audi refuse to accept that there is anything wrong, so they leave the car running with a faulty MAF, causing it to run lean. Running lean causes the exhaust gas to get hotter than normal, causing the manifold to experience higher temperatures than it was designed for. The already weak design simply cannot tolerate the long-term exposure to excessive heat and the liner comes loose.
The morals of the tale:
a) Get a VAG-com lead and make sure your MAF is OK. Running lean will put strain on the poorly designed manifolds, possibly landing you with a bill fo close to £2k
b) If your turbo heatshield is rattling, make sure it really is the turbo and not the manifold especially
if it has been diagnosed by Audi.
c) If your manifolds fail, do not delay getting them replaced. If the fragments from the liner do not mash the turbine, then the EGT sensor will once the liner has sawn through its shaft!