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"...the flame trap, but I'm still not 100% sure what that does..."
Long answer to a short question....
The flame trap is one part of the crankcase ventilation system. Vapors from the crankcase are routed back into the intake system so the engine can "consume" and burn them rather than simply venting to the atmosphere. (All cars used to vent, but the EPA put a stop to that years back.)
The danger with routing back to the intake is that if the car backfires, the flames can travel back through the plumbing to the crankcase, resulting in BOOOOOOM! The repairs to replace the gaskets and seals would be staggering -- providing that the car didn't burn up.
Volvo inserts a "flame trap" in the plumbing (of non-turbo cars) that will pass these crankcase vapors but will quench a flame. Hence, it "traps" a flame.
These were common on the lanterns used in coalmines centuries ago, and for just the same reason.
"The 'big plug' on the other hand, I haven't a clue as to what that is..."
Another long answer to a short question...
The B230F engine (first used in '85 models) was used in both the 240 and the 740 models. When used in the 740, the distributor is mounted on the rear of the cylinder head and directly driven by the camshaft.
But when the B230 engine is installed in the 240, the distributor is in the normal position in the block, lower left front side.
When the engine is used in a 240, a big plug blocks the distributor hole in the head. When the engine is used in a 740, the distributor hole in the block is blocked.
When the flame trap becomes plugged, the pressure inside the crankcase can force oil past the seals, sometimes pop the seals out, and sometimes pop the big plug out from the head. That causes a VERY BIG OIL leak. (Happened to my daughter when she was driving back to college in '97, or so.)
The remedy for this is to ALWAYS check, clean, and service the ventilation system, including the flame trap.
Another remedy is to install a safety strap across the big plug. (It exploits the threaded holes used to mount the distributor if the engine were in a 740.)
The picture below shows the plug and illustrates a cheap, easy strap you can make from a scrap of aluminum. The plug is behind the head, next to the firewall. You really can only feel it, not see it.
--
Don Foster (near Cape Cod, MA)
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My problem seems to be solved...somewhat..... [200]