Engine Mechanical problem
1997 Pontiac Bonneville 6 cyl Front Wheel Drive Automatic 190000 miles

i changed my motor last week.went very well been driving the car last 5 days .ran good but sluggish .monday nov. 10 2008 i changed the plugs and pcv valve .ran even better old pvc was toast plugs had broken porcillin been arcing for a long time .today i took my wife to work and when she got out of the car it sounded like the belt was sqealing .turns out it wasnt the belt .the sqealing is coming from the engine.if i remove the oil filler cap it creates a major vacumm and kills the engine,unless i hold the accelerater a little.with the cap off the motor stops sqealing .my mechanic suggested changing the plenum.so i braved the cold and changed it tonite .started the car everything was quiete till it warmed up then the sqealing was back again any suggestions? thanks