Lately, when I start up the car when it's cold (like sitting over night) it starts fine, but then when i put it into reverse or drive, it bogs down to like 300rpm and there's a loud hissing sound that sounds like the A.I.R pump that lasts for a few seconds, and then the idle comes back up to normal. Is there a leak somewhere in the system? What else could cause this? I've also been getting terrible gas mileage (like 10-14 mpg on the freeway) and i'm not sure if this is related. Any suggestions?

 

Sounds more like a vacuum leak than anything else. I wonder if the torque of the engine when you put it in gear is moving something enough to create a leak. The only vacuum line I can think of that isn't attatched or affixed to the engine would be the line going to the power brake booster. You might want to check there. It would also account for the lousy mileage.

 

where is this line at? i'd imagine it goes to the brake booster but what does the other end connect to?

 

It's a 1/2" rubber hose coming from the driver's side of the intake manifold. It should be clamped to the nipple that is threaded in to the manifold and clamped to the fitting on the booster. I'm sure the clamps are there. You might want to pull it off and check it over very closely for cracks. If you find even the smallest of cracks it would still be one hell of a vacuum leak since it comes directly off the intake.