Volvo Repair: 240 with rough idle, intake manifold gasket, lean mixture

Question
I have a '90 240 with the b230f engine.  The engine runs poorly at idle, often stalling when I put the car into reverse.  My mechanic cleaned the throttle body but this seems to have made the problem worse.   He thinks the engine is running lean.  The built in code reader gives out a 2-2-1 code, "engine compensating for rich or lean mixture at cruise".  Any ideas?

Answer
Hello,
     I'm not sure what the code is...for lean.
    Many reasons why a engine can run lean..
    Air mass meter problem
    Fuel press.reg.
    retarded timing
    vacuum leak
    intake manifold gasket leak
    E C U...I think

    Good luck,

    Bilbo