The people at auto parts stores usually never were mechanics so they won't be able to tell you anything other than what they're heard from their regular customers, or what some parts suppliers provide in training classes.
As for those gaskets, they often package multiple gaskets in a set to cover the most common or the most involved repairs, then you just use the ones you need for your project. We always have gaskets left over. We hate to throw them away, but before long we forget what they fit.
You're right to suspect the intake gasket that was taken apart, but the leak would have to be pretty significant to set that code. First look at the fresh air tube between the mass air flow sensor and the throttle body. If any air sneaks in there from a crack or a loose hose clamp, it won't be measured by that sensor and the Engine Computer won't command the right amount of fuel to go with it. That will result in a lean code.
You might also ask at an auto parts store that rents or borrows tools if they have a smoke machine. That will allow you to inject a white, non-toxic smoke into the intake manifold at 2 psi, then you can watch for where it sneaks out.
Monday, October 20th, 2014 AT 4:46 PM