The flame trap is the volvo "PCV" valve on non turbo engines. It is a quarter sized plastic device resembling a tiny spahgetti strainer that is mounted just before the entrance to the throttle body in the air intake hose. It redirects blowby gases back into the incoming air stream to the engine to be reburned and prevents a crankcase explosion should the engine backfire. It is a PITbutt to get to so is often just ignored during service. It is prone to blockage from oil vapor residue, condensation etc and once plugged up raises the crankcase pressure and can blow main oil seals leading to $$$$$$$$$$ repairs especially on FWD car.

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