I have a 2000 GMC Sierra 2500, 160,000 miles. 2 wheel drive. Just had tune up with new plugs, wires, etc, and runs and idles great, but have difficulty starting. Sometimes we can hear the fuel pump whine, sometimes not, starts the best when it is a loud whine. Seems to start easier after being off for just a short while. Also has a strong smell of gas. Could this be the fuel pump or something else? We checked the engine codes and nothing showed up. There is no engine light on. Also, the air conditioning goes in and out. It is cold, but will start to get warm, and after we turn off for a few minutes and turn back on it will come back on cold for awhile, then keeps repeating. Do I need a charge?