Chevrolet: Multiple misfiring, chevy malibu, automotive store

Question
I have a 2001 Chevy Malibu 3.1 V-6 that I just put a new motor in with only 6,000 miles on it.  I ran the car for a week with the new motor when all of a sudden the car wouldn't start.  It was getting spark and the fuel was getting to the fuel rails but it just wouldn't start.  I worked on it for a week and decided to have it towed.  When they got it to the garage after sitting for the weekend, the mechanic put the key in it and it fired right up.  This time it was misfiring.  Now it didn't misfire on me the entire week I ran it.  The mechanic changes the master sensor, spark plugs, and wires; ran compression checks on it and everything is working fine.  Everything on the car is new.  I brought the car back home because, the mechanic said that he thinks something is wrong with the fuel injection, so I decided to fix it myself.  I have cleaned the injectors, checked to see if I had hooked them up wrong and still can find nothing.  Before I put a new motor in the car, it sat from April 17th to May 31st with a full tank of gas.  My wife put gas in the car on June 3rd and that is the day that car quit starting.  She had driven the car all day and when she came to pick me up at 7:30 in the evening and that is when it quit.  Do you have any suggestions on why the car could be misfiring? The only parts that are not brand new are the fuel injectors and the fuel pump.

Answer
Hi Bill, the fuel pump can be checked with a fuel pressure tester, this will determine if the fuel pump is suppling enough pressure. As for the injectors, you could try some of that injector cleaner you put in with the fuel, you can get it at your local automotive store. There is also a machine that hooks up to your injectors and professionally clean them, this is an expensive piece of equipment and may only be available at higher end shops.(I have seen this machine work wonders with dirty injectors and engine miss) Also you can have your car hooked up to a analizer, this will tell you what cylinder its misfiring on. I hope this helps you, if you need more info just let me know. Regards, Keith.