Why is your 99 Tahoe running too lean?

What makes you think it's running too lean? Are you getting a code? Try cleaning the MAF sensor with some MAF sensor safe spray cleaner. A lot of times this fixes it. It could also be a bad O2 sensor. If cleaning the MAF doesn't fix it, there is another problem GM has with those engines. I don't know what year they started using the plastic intake manifold, but if yours has this, then I'd bet you have a bad intake manifold gasket. GM used CHEAP gaskets. Felpro makes an excellent replacement. Take some of the MAF sensor cleaner you picked up and spray it where the intake meets the heads. Any change in engine idle would indicate a leak. I just fixed this on my friend's 2004. On the plus side, it's fairly easy to fix. Took me about an hour from start to finish. On my friend's Yukon it showed a lean condition in the codes and this ended up being the problem.