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This is puzzling as a diesel engine does not have a throttle, it is full air flow all of the times and variations in fuel change power output; the engine runs lean most of the time, but can be rich at full power depending on control system.
Perhaps you are referring to an exhaust gas recirculation setup. Having owned a VW TDI, I know that the recirc gas can carbon up the intake manifolds and the EGR valve, that was controlled by a vacuum pot (diesel engines have vacuum pumps for things like that and for the power brakes).
OK: Looked at the manual for this engine for the first time, there are swirl throttles, suggesting that these induce turbulence to improve combustion under some conditions. The exhaust gas recirculation (EGR) flows are introduced to the intake air upstream of these throttles. Perhaps the throttles are to mix the EGR flows into the combustion air. So this seems familiar. With the VW TDIs it is?was a right of passage for DIYers to remove the intake manifold and dig at the hard+gooey deposits that closed off the manifold flows substantially. At a dealer, the labor for that effort was such that a new manifold could be cheaper. Depending on the year, the EGR valve and pot were separate or integrated. So costly either way. Many installed aftermarket or DIY oil separators, as the deposits are a combo of EGR soot + oil in the blow-by gasses. With 22:1 CR and high charge air at all power settings, the blow-by gas flows are quite large compared to a gas-spark engine. Using a full synthetic oil was also considered an improvement. Some created oil traps and then vented the gases to atmosphere to keep oil from combining with soot to create solids.
So I am not really surprised that what you describe has happened. I do not know if the type of oil used would be a factor, or how often changed. Higher levels of blow-by gases from problems with rings or pistons certainly would not help. Fouled injectors with spray patterns that hit the cylinder walls can create high rates of ring wear that would show up as elevated chrome and nickel in an oil analysis.
Ask the dealer how often your problem occurs and what can be done to prevent it. You might lean on Saab to help pay for this if this is a design problem... and to some extent, all diesel EGR systems are vulnerable to some extent to this and that is a design problem as they do not know how to implement this without these deposits happening.
Note that blow-by gasses are introduced into the air intake stream upstream of the turbo compressor. EGR gases are introduced at the intake manifold. If is the combo of oil in the blow-by gases and the soot that creates the mess.
Got to run... getting nagged by the wife. I hope that this has been helpful in your understanding of this process.
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