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Which sensor ???
Yes I believe actually the 1986 to 1992 Volvo 240 4 doors are all interchangeable parts but I'm not sure for 1985 and earlier models.
Yes
I have a 240 and with the knock sensor unplugged it will still start so I would have to say no.
1 to 4 , front to rear
Not 100% sure about the '87, but there is no PCV valve on the 1986 Volvo 240 2.3L. Flame trap and Oil trap, but no PCV.
Connected to the intake air duct.
No, they are not user serviceable
Dextron D/M
it's on the radiator, just pull the wires and unscrew it.
on top of bellhousing, behind head, pass-side
O2 sensor/leaky injecters/fuel pressure regulator.