An interesting point that you brought up was about the "popcorn noise" that you heard on three separate occasions. In all likehood, that "pinging" was detonation. Detonation can occur in an engine that is running too hot. This ties right in with your second mentioned service event -- replacing the thermostat. It also fits with the fact that you had TWO broken cylinder heads. Failed head gaskets are common enough results from over-heated engines, and a broken cylinder head is usually just a more extreme result of the same temperature control problems.

As for the turbo's water-cooling hose bursting: I think this was probably the RESULT of the overheating problems that you had experienced, and not the cause of them all. Clearly, there was a temperature control problem that you tried to get resolved at the beginning of this story, and you wouldn't have been driving around with a split/cracked/leaking/burst coolant line from 16 June to 22 October. Even a pinhole leak would not have resulted in two catastrophic head failures -- not without the coolant level sensor screaming at you quite frequently during this period.

I'm also seeing that, during the overheating event prior to the second head failure, that you were driving in traffic -- translation: hot engine, very little cooling system air flow -- and that you took three minutes to shut the car down after the coolant light went on. If the coolant had started to leak badly prior to the coolant light going on, then you could very well have been driving a hot engine for THREE MINUTES with little or no coolant reaching the head. Your delay in shutting down the engine might very well have cost you that second head.

What I'm still at a loss to explain is the connection, if any, between the INTERMITTENT overheating and detonation that you experienced and the mixture adjustment (MAF sensor) problems that the car apparently had. I'm convinced that there IS a connection between these two issues, and the dealer's replacement of the intake air temperature (IAT) sensor indicates that they were onto something with that, too. I also think that it's possible that a bad signal from the IAT (or a mis-installed replacement IAT) could have contributed to the overheating and detonation. (I said POSSIBLE, not probable.) The fact that the dealer replaced the MAF sensor twice, though, leads me to believe that there WAS, in fact, something going on that was fooling the MAF sensor. Intake or boosted air leaks can cause faulty readings by this sensor, and those may have prompted these two replacements. If there is anything that can cause a faulty MAF sensor reading AND overheating, then that would be worth further investigating.

Unfortunately, I add all of this up, and I come to no conclusive proof that the dealer did anything wrong that could have contributed to the head failures in this car. Sometimes a doctor can spot a heart attack or stroke precursor before an event happens, and sometimes they can't. Proving malpractice will be a difficult case.
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Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html