I'm having problems starting my 2002 pontiac grand prix se 3.1L. I just bought it from a private owner back in Oct. Of 2012. I plugged in a scanner into the OBD-II and got a PO300 reading for mutiple/random cylinder misfire. Initially the problem began when I cleaned the carbon from the intake behind the throttle body because of a previous reading of inefficient EGR flow. After finishing and I tried to start the car again but couldn't without pressing in the gas halfway to start it. It had a very rough idle and I had to keep pressing the gas to keep the car running. I drove about a mile away to get new spark plugs and on my way home the entire car just stalled out and stopped running on me. Usually happened when I came to a stop. I made it home and shut the car down. I installed new plugs and soon after new wires and tried to start it again but it wouldn't start. Then I bought a OBD-II scanner and got the PO300 code. Since then I've replace the catalytic converter, fuel PSI regulator, battery (because of draining), alternator, and fuel filter. Fuel pump sounds like its working because I can hear it engage, although I know that may not necessarily means it's operating properly, but in replacing the fuel filter I discovered that it had been a long time since its last replacement and the nut was rusted on. I put the new filter on and it was leaking fuel at the outlet side of the filter which also told me the pump was working. I repaired the line with a fuel line repair kit. No leaks and the car still won't fire up. I used an extension and hammer to tap onto the injectors to free up any debris just in case they were clogged. Also, when I went to relieve the fuel PSI from the fuel rail, when changing the fuel filter, there was close to no PSI released when I depressed the schrader valve. When I try to start the car it cranks maybe twice and then goes quiet with the exception that it sounds like something is still spinning. I don't know if its the sound of all the components the serpentine belt is wrapped around, alternator, compressor, etc. Or if it's the starter because of a weird grinding screech I hear on what sounds like the driver's side when I try to start the car, which I know is where the starter is located. I also have an idea it may be the GM TDM security that reads the chip key. I am so lost with this and at the end of my wits. I cannot afford to take my car to a professional mechanic who's more than likely going to try to sell me $1000's in parts before the problem actually gets fixed. Some one please help me.
You have that exactly backward. You are the one who is installing a lot of unneeded parts. That is the most expensive and least effective way to diagnose a problem. You've seen how easily something as simple as a fuel filter replacement can go wrong. Mechanics have learned how to cope with miserable things like that every day. Now look at all of the new variables you've put into the system with all the new parts. Mechanics diagnose and replace just what is needed, but in many cases that means a lot more than a single part. At the very least, have a mechanic diagnose the cause of the no-start, then replace the parts yourself if you think that is the less-expensive way to go.
Replacing the battery won't solve a drain on it. You have to diagnose and fix what is running it down, if anything. The generator isn't even in the picture yet until the engine is running. The good news, without going into all the details right now, is that GM has had a huge problem with their generator design since 1987 and the way to reduce the number of repeat failures is to replace the perfectly good battery unless it is less than about two years old. You have that future problem handled.
Once the battery was run dead or disconnected, all the stored fuel metering data in the Engine Computer was lost. Besides the obvious no-start condition there are going to be other running issues until that data is relearned while driving. That can include rough running, low idle speed and the need to hold the gas pedal down 1/4", and stumbling and hesitation.
When it comes to fuel pumps, there are some generalizations about how they fail. Typically a Chrysler pump will fail to start up but when it does it almost never quits while you're driving. GM pumps are just the opposite. They typically always start up, then die and let you sit on the side of the road. In addition, it's somewhat common for them to run slowly and develop insufficient pressure. First be sure you have spark. If you do, suspect a fuel delivery problem. You can use a mechanical fuel pressure gauge to measure the pressure but many GM cars will not start if the pressure is only a few pounds low. That can be a judgement call for inexperienced mechanics and do-it-yourselfers. If in doubt, spray a little starting fluid into the air filter box to see if it will run for a few seconds. Be aware too that there can't be any leaks in the fresh air tube between the mass air flow sensor and the throttle body. If any air sneaks in that doesn't go through the mass air flow sensor, the computer won't know about it and won't command enough fuel to go with it.
Since the battery was disconnected, you may have to do a 30-minute relearn procedure for the anti-theft system. You'll want to double-check the procedure, but it goes something to the effect of turn the ignition switch on, wait for ten minutes for the security light to start or stop flashing or go out, (I can't remember). Turn the ignition switch off and back on once, wait for the same thing to happen, do that a third time, then start the engine. You may want to connect a small battery charger, and you may need to crank the engine for one second as the first step, then let the ignition switch release to the "on" position.