Yes, I know that, but . . . I know that a cooling system is supposed to hold pressure well, but for 11.5 hours? That is quite a long time. Shouldn't it have cooled down enough to depressurize by then?

On the other hand, trying to figure out some kind of head gasket/warped head failure mode that would cause it to hold pressure for 12 hours seems a stretch. To say the least. Many factors would have to fail in exactly the right way to cause that. Further, the one cylinder that, for example, had a *one-way* leak INTO the cooling system (but not from the cooling system) would have to stopped when it was pressurized when the car was shut off. What are the odds of that? Even if that happened, the cooling system still managed to hold pressure for a very long time, so wouldn't that imply that it is able to hold pressure?

I can't wait until it gets home tonight, because this speculation is driving me nuts. This evening I'll see another small piece of the puzzle and go from there.

Meanwhile, anyone who would care to comment on the original post here, or my reasoning, above, or just on experience with Volvo engines that hold pressure for nearly eternity, feel free to do so.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)