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I'm working from memory here . . .
. . . it's just a piece of Nylon, about an inch in diameter, about a half-inch thick, perforated with dozens of 1/16" diameter holes. On one side of the flame trap is the intake system (under vacuum) and on the other side of the flame trap is the crankcase. So, this passageway is obviously used for crankcase ventilation.
What happens is, over time, these 1/16" diameter holes clog up with burned oil, etc. So the flame trap ends up becoming a plug. Crankcase pressures go up and you blow out various seals. Because this 60-cent part becomes a plug, you end up blowing out your rear main seal, which will probably case $800+ to replace.
My guess is that the purpose of the flame trap is to "breaks up" a flame which may originate in the crankcase and prevents that flame from propogating into the intake system, or possibly, visa-versa.
Regardless, manufacturers don't include parts labeled "flame trap" just for the fun of it. It is likely, that during testing, Volvo determined that there was a certain risk of a fire propogating through various parts of the engine. To prevent this possibility, they included this flame trap.
Unfortunately, this flame trap has become a weak link in maintenance of the 850. And certain dealers probably think that the risk of a fire is so low, and the risk of pissing off customers, because of the failure of a 60-cent part, so high, that they simply remove the flame trap.
I think I would rather keep the flame trap in-place, to protect against the possibility of a fire, but make sure that I replace, or clean, the flame trap on a regular basis.
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