1994-2002 [Subscribe to Daily Digest] |
If the mixture gets lean while on boost, any pump gas octane would not be right. So we all speculate. Latest thinking is MAF errors*. The DI's knock prevention software is not able to handle all situations in any case. So the ECU has to be considered at fault in the end, and that ends up being a software issue. These situations are very rare. So the software would seem to be ok for normal situations, but does not manage knocking properly when something else goes wrong.
* I think that we have a report of cracked pistons, engine rebuild and cracked pistons a second time. The MAF was replaced and the engine was then stable on its third set of pistons.
Speculation so far: lean transients when backing off throttle while still on boost, MAF errors, large blowby volumes introducing oil mist into the intake air stream that lowers efective octane of the AM mix. The first two could be true and coupled.
Oil misting is interesting, we have PVC systems that cannot handle flow volumes, but what causes high flow volumes? If the rings are stuck, that fits into the sludging and lubrication issues. Note that the pistons have been failing on mostly (or exlusively?) sludge free engines. But we have had loose head bolts, and perhaps that is allowing gases to excape past the head gasket and into oil passages. I do not think that the pistons are too weak, no pistons will withstand knocking. But if the knocking is not severe, the highly loaded pistons in the Viggen and 9-5 Aero may be vulnerable where a lower HP variant or tougher pistons could survive and not deform/crack. I hear folks refer to melted pistons, but I don't know is that is a reasonable thing to say. Fire going through a cracked piston could be expected to show some signs of melting. Otherwise, pistons in high output applications operate near their melting points and a relatively small increase in temperature can greatly weaken the pistons, which will then fail from overload.
If a head gasket leaked and gases were able to get into the oil feed passages, the lub oil flows could be interupted. The bearings would not be happy, but if the piston cooling oil jets were then not working, piston temperatures would rise. If Saab had not been incompetent about bolting down heads for most of a decade, we would not have to make this speculation.
Once a piston cracks, compression ratios fail in all or some cylinders. Where does that gas flow go? Once this begins to some extent, there is the possibilty of large PCV flows and again, oil misting might become an octane issue. So perhaps oil misting is the end stage of knocking damage that results after primary knocking damage to one or more pistons. That is again speculation and failure dynamics, but the end point is the same in any case.
Cracked pistons break piston rings, then the sharp ends of the broken and hard piston rings can damage the cylinder walls.
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