1999 Wrangler Sport 4.0 oxygen sensor?

one thing to check first is for cracks in the exhaust manifold. very very common problem on that engine. start the engine and let it warm up. climb under the vehicle and see if you can feel any exhaust leaking from the manifold. if there is a crack in the manifold it allows oxygen into the exhaust system causing the o2 sensor to detect a lean condition causing more fuel to be entered into the air fuel mixture.