The flame trap is a portion of the crankcase ventilation system, and it normally inculdes a flame arrester. (The flame arrester is omitted from turbo models, but all the rest of the plumbing 'n stuff is present except one small vac line on the 240s.)

The flame trap and flame arrester (which is a disc-like perforated insert, or cartridge, about 1/2" thick and 1" diameter) stops or quenches a flame from reaching and igniting the vapors in the crankcase. A flame "pop" could occur with a backfire, for example. Igniting the vapors in the crankcase could cause a modest explosion in the sump, which could ruin the crankcase gaskets, blow out the seals, and possibly start an engine fire. All of these are $$$$$.

It's essential that the flame trap and all associated plumbing be free and clear. Otherwise your crankcase will develop positive pressure, which can induce oil leaks everywhere, pop seals from their housings, or push out the big plug (rear of the head). Normally the crankcase runs at vacuum at idle (about 1" to 2" of water). If your car is a non-turbo, it's essential that the flame arrester itself be clean and free-flowing. ('85 was the last year for the optional turbo in a 249.)

Oil weeping from the cam seal will dribble over the water pump top "mushroom" seal and eventually swell and ruin it -- requiring that you replace the pump and/or seal.

Read about this in the archives -- it's well documented. Look for "jiggle test."

"...how do I fix my gas gauge..."

Depends on what it's doing (or not doing).

"...find all the little parts I desperately want to replace so the ol' gal will look good again..."

Speaking as a Yankee, go to a Volvo-friendly boneyard or P-'n-P.

"...by "Fosterizing" the relay..."

Need pictures?
--
Don Foster (near Cape Cod, MA)