im replacing my valve cover gaskets and its a pretty big job...

i have a whistling/hissing noise when i give my car some gas and i think its coming from the leaking valve covers.

can a leak in the valve cover cause a vaccume leak?

my car runs like crap and gets 10mpg.. this is the last thing i havent done besides the timing chain.

 

Make sure that you dont have a hose that has come unplugged while you were changing the gaskets. Thats the most common. I saw a 3.8 once that had a bad Intake gasket on the bottom side of the Intake, and was sucking air through the interior of the motor. If you took off the oil fill cap while the motor was running, it would almost stall the motor it was so bad.