If you want to pursue the vacuum hoses, check at the auto parts stores that rent or borrow tools to see if they have a "smoke machine". That puts a white, non-toxic smoke into the system under two pounds of pressure, then you search for where it's seeping out. You can inject it into any vacuum hose or port.
A more common cause of fuel mixture problems that affects GM cars more than most other brands is mismatched fuel injectors. I only remember two brands from a very high-level school taught by an injector specialist. Chrysler buys their injectors from Bosch in flow-matched sets of four, six, or eight injectors. GM grabs a handful out of a huge bin and stuffs them in an engine with no regard to flow-matching them. By the time they get to around 100,000 and have some wear on them, a few will flow enough less fuel than the others that those cylinders are a little lean. The unburned oxygen is detected by the oxygen sensors and in response, the Engine Computer requests more fuel to all of the cylinders on that side of the engine. No matter how much extra fuel it requests, there will still be that unburned oxygen being detected, hence the lean fault code. Oxygen sensors don't detect unburned fuel, just oxygen. Rebuilt replacement injectors is the fix for that. They can only be purchased in matched sets. About 98 percent of that company's sales are for GM products. Most of the customers are amazed at how smoothly their engines run with those new injectors. That has been the solution to a lot of running and Check Engine light problems that have defied extensive diagnostic attempts.
Low fuel pressure will result in a lean condition too although that is a sign of a pump that may be failing and that won't typically go on for very long. The low pressure would cause a hesitation or stumble too.
Thursday, June 21st, 2012 AT 8:10 PM