Like s130 I also service and repair the family cars. The first diesel in "the fleet" was my 1999 Seat Cordoba 1.9 tdi bought in early 2001. It had 2 years warranty left on it so the first time I serviced it was about 2 years after purchase. As I was crawling around it and getting used to where everything was I noticed a trace of oil at a joint in the turbo trunking. Yikes! Instant increase in heart rate - thinking to myself; Only just out of warranty and already I've got a blown turbo!" - It was where the pipe joined the inter-cooler. I split the joint and found what to me seemed like a lot of oil inside the ducting - no puddles though. I hadn't given much thought to what it should look like but I thought it should be nearly dry (perhaps with the very slightest trace of oil mist from the breather which feeds in after the air filter but before the turbo. Luckily I am on friendly terms with our local VAG independent workshop so I popped over and let him look at it. Absolutely normal said he! "What, as 'wet' as that said I?" Oh yes, there's quite a bit comes from the breather and some from the turbo seals. I ran that car for many years finally scrapping it a couple of years ago and the only turbo problem I had was with the wastegate actuator diaphragm which ruptured due to old age.
My boy had a 1.9 PD Fabia which, after we'd owned it for a few years, suffered the common problem of it dropping into "limp" when large throttle openings were used. The variable vanes were seizing up with carbon soot, although we didn't know that when it first started trying to fix it. We thought we might have to buy a new/rebuilt turbo for it but actually very successfully managed to "de-gunge" it with oven cleaner. (a filthy job but cheap to do - you do have to drop the exhaust pipe off so you can inject the cleaner though which was awkward and, by golly, the oven cleaner nips!). As we initially thought a new turbo would be needed - before I really understood what had gone wrong with it - I did quite a bit of investigating how we might do it most cheaply and found out quite a bit about turbos. The seals of which you speak, as you seem to know, do look a little like very small piston rings and the main spindle, which runs right through them, has an impeller on each end. There is a nut on one end which lets you take one of the impellers off so you can withdraw the shaft but as soon as you do this the balance on this rotating assembly will be lost. Balance is a critical factor in bearing life, as this shaft - and the impellers - rotate at humongous rpm. Only a turbo specialist has the kit needed to re-balance so I reached the conclusion that it is not a viable DIY task. However a lot of the re-conditioners will supply a "core unit" on to which you can then install your old turbo's inlet and exhaust casings. A reasonable amount of money can be saved but I think you would probably only do it if you actually like doing this sort of thing. Not too sure what the warranty situation would be on a "home" rebuild like this? By the way, although there was nothing wrong with the seals and the car ran fine for another five, or so, years, there was even more oil in it's pipes than my old Cordoba's! Neither car though ever produced visible oil (blue) smoke that I noticed - The Cordoba, with it's old VE (pre PD) engine always produced a small amount of black smoke when on max boost with foot hard to the floor. Always got a "fast pass" on it's MOT though!
I also discovered that most turbos don't just "blow" in isolation. If you have a worn/ruined turbo it's well worth googling for related turbo/engine problems with your model of engine. Some are well known for wrecking turbos and simply fitting a new turbo without sorting the underlying problem will simply right off the new turbo in quick time and invalidate the warranty! A number of the larger recon sites have comprehensive lists of the "problem engines" and what to look for. Also the oil supply to a turbo is critical so most companies will invalidate the warranty if you don't fit a new oil supply pipe! (keep the receipt somewhere safe, they'll want to see it if you claim) Remember too that you need to "flood" your new turbo bearings with oil before starting. (a syringe full of clean oil pumped down the supply pipe, or directly into the casing of the core unit if the pipe is not aligned conveniently, is a must), and don't forget to check the "return to sump" pipe is clear. If it's blocked oil can't circulate and will backup in the core unit which can cause big problems with seals and overheating of the shaft/bearings as the oil carries heat away as well as lubricating. The oil itself needs to be only briefly in contact with the shaft/bearings if it is not to burn and carburise.
Another thing I learned is that if you've been doing anything which is going to get the turbo very hot and then park up - so fast motorway driving, pulling hard up a long hill, etc - It's very helpful to the turbo to let the engine idle for a couple of minutes or so before switching off. Doing this keeps the cool air flowing through the inlet and relatively cool exhaust gasses (the engine isn't "working" at idle) exiting the exhaust so the turbo can shed heat. But more importantly it keeps the oil flowing through the core unit, which will be very hot, so taking more heat away and, even more importantly, preventing the oil lying statically against the blisteringly hot bearings and shaft where it might carburise and leave deposits behind which could block oil supply to the bearings. I'm unsure exactly how long to let this idle continue but what I've done for many years on our multiple Edinburgh/North Devon journeys, when taking service area breaks, is to leave the engine idling whilst I retrieve my trainers from the boot and swop over my very light weight trainers which I like to wear for driving long distances. By the time I've struggled with this and Mrs Jock has got her coat on I think this is long enough and the proof would seem to be in the pudding as my turbos seem to last. Got a petrol turbo now so much higher temps. Potentially more problematic? Lastly, from what I've read, fully synthetic oils are able to tolerate turbo temperature better than mineral oils without forming deposits so, if you've got a turbo you are probably best to always use a synthetic oil. Although those who have read my previous posts will know I believe strongly in using only top quality oils which specifically comply with the manufacturers specifications.
