Fuel system problems

My 2001 Cavalier would sometimes not start. It would crank, but not turnover. I took it to the mechanic who could not replicate the problem, but ran diagnosis and determined the fuel pressure regulator was bleeding and replaced that. Almost immediately I saw my gas mileage nosedive and almost immediately the car still wouldn't start. Took it to a second mechanic who kept it several days and finally got it not to start for him and realized it was the fuel pump that was hickey. Replaced that and I haven't had a problem starting since. However the gas mileage is even worse. More than half what it was a month ago when all this started. Just called the first mechanic to ask what might be wrong and all he said was to bring it back in. My faith in him is no long what it once was and I'm not confident that he's not going to sock me with another huge bill. (I'm already over $1,000 in repairs for the month.) Does anyone know what might be going on here?

First thing I would do is take a fuel pressure check. If your gas mileage was OK, before the first mechanic replaced your pressure regulator, than most likely it was not the immediate problem. It's fairly easy to check the fuel pressure as the fuel rail has a screw on, or quick dissconnect on top for this purpose. Secondly, you failed to mention what engine you have in the car. If you are not mechanically inclined & cannot do a pressure test yourself, then find out what the fuel pressure should be for your particular engine. Look in a manual, ie. Autozone, or ask someone. If your pressure is too high, I would have the first mechanic replace your old pressure regulator and see if that solves the problem. Many things can cause higher fuel mileage, bad oxygen sensor, bad tcc solenoid in transmission, dirty fuel filter, spark plugs, dirty throttle body, or a bad sensor sending the wrong info to the computer.

pull the vac hose off the top of the pressure regulator and see if there is gas in the top part, there shouldn't be, if not spray some WD-40 in there. the Diapragm could be stuck up letting too much fuel in

pull the vac hose off the top of the pressure regulator and see if there is gas in the top part, there shouldn't be, if not spray some WD-40 in there. the Diapragm could be stuck up letting too much fuel in
I would also pull a fuel injector out and look at it they might be bad and shooting to much gas in.When ever you get all of these problems solved i would change the plugs because all that gas getting dumped in there will destroy them.