Hello.. I'm a noob when it comes to volvos.. i just bought a 91 240 wagon and have been trying to pinpoint a vacuum leak.. the engine revs high at idle and i can hear a hissing noise so i know there is a leak somewhere... anyone know of a good diagram for vacuum wiring? how probable is it to be the upper intake gasket? i have sprayed carb cleaner on top of the intake and it seems to rev up a little. any help would be greatly appreciated.. i'm ready to get this thing running right.. i hear they are very reliable when tuned right.

 

It sounds like you may have found a leak if the idle increases. There are two places that can be suspect. First is the actual intake manifold gasket. Second is the o-rings that seal the fuel injectors. See if you can pinpoint which it is by spraying the carb cleaner around the injectors vs. the intake manifold gasket. Then replace accordingly. I don't believe either is all that expensive or difficult to do yourself.