Car will not start

My car is a 2001 chevy impala 3.4 with 70,000 miles. I have replaced my starter, my ignition switch behind the key and tumbler, checked all my fuses/relays and had my security code reprogramed, The car will crank over and sounds normal, I took it to a mechanic and he cleared all the codes and checked the firing on the plugs and made sure it had 60 psi of fuel pressure. But it still wont start. Do you have any more Ideas PLEASE. Or should I just run it into the river.