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The primary symptoms of a failed turbo (rare in 850s with under 250K miles) is oil smoke on acceleration from standing starts, which then seems to clear up a few seconds later. This is the result of a leaking rear shaft oil seal, allowing pressurized oil to escape the turbo and mist into the exhaust system. (Note: the oil 'seals' are primitive, so some oil leakage from both ends is absolutely normal. On the exhaust side, it carbonizes the impeller vanes and the inside of the upper exhaust tract. On the compressor side, it pools along the duct to the intercooler, and collects in the bottom of the intercooler. This is, I repeat, normal.)
If (a big IF) your turbo DOES need replacing, there are two levels of repair you can do yourself, if you've got the confidence.
First: full rebuild; cost $100 parts, and spin balance if you can get it. I've done this, and it took me a long afternoon. The kit is a few bushings, snap rings, locknut and maybe a couple other assorted parts. You'll need snap ring pliers (a smallish one, and you can probably rent the big one); buy the small one at autozone. A friend of mine owns a machine tool company; he did an acid bath and a 100,000rpm spin balance on the shaft for free. Here's a site that describes the process: http://www.ecanfix.com/~mdhamilton/turborebuild.html)
Second: cartridge install. Remove the cartridge (on my 745T, this was easy - unbolt the oil return line, the oil and water suply lines, and remove a 10mm nut to release the band clamp; the cartridge slides right out). If your supplier will ship you the replacement ahead of time, you can make the exchange in about an hour or two. (At LeMans, they do this in about 3 minutes, while the blown turbo is still hot!) Make sure you have replacement seals for the turbo return line. Should cost about $250-500, depending on your source. This procedure leaves the turbo housing in place - you don't need to mess with the exhaust manifold.
Kits and engineering from http://www.turbocity.com/
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David \\ (98 S70 T5SE Black, misc mods (mostly lighting), red calipers) (92 940GLE)
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