2004 Pontiac Montana will crank but will not start at times.. It has new spark plugs, new spark plug wires and coils. yesterday morning I tried to crack her up and she will crank but not start . unless I step on the gas pedal and then she will start. once I let go of the gas pedal she will not run. when the RPMs reaches somewhere around 5 to 600 RPMs she will stop running. this morning I went to crank her up again to check and see. she started right up. this is not the first time this happened. it happened about a month-and-a-half ago maybe two months ago. so I decided to replace the coil packs. she did okay for a little bit. then I got a code of 0303. so I decided to take one coil back and get another one as a return refund. because I thought it was for coil itself but it was not. so when she is running you can hear there'* some Miss somewhere. and this has been going on for maybe a month or so. so what I do for now is just reset the code back to clear. the color has not come back on in a few days but she still runs a little rough you can hear the miss. so I know it'* not the spark plug , the spark plug wire or the coils. I only ran her a couple of minutes and let her idle for 2 minutes just to see. and she might crank up for me all day maybe for a week or so and maybe she'll just stopped again.

You have a mis on #3 and probably more issues.
Swap the fuel injector with companion cylinder and see if the mis moves.
That mis will ruin your cat. Probably already has.

I went back to the van and I ran it for about 10 minutes. she'* doing okay but with a little miss. yes she started right up. so when I will do the Nexus switch injectors. oh I did replace the upper intake gasket almost a year ago. go just in case maybe I'll just go ahead if I need to replace a fuel injector she go ahead and get to upper. let me ask you one more thing. when I am driving her on the road doing 35 45 50 miles an hour 55 miles an hour she she does okay. do you think again I may have ruined the engine? I do thank you for your information for your expert advice. hopefully I can get back with you and let you know that everything went well in the vehicle is running great

carfixer007 didn't say anything like[ruin the engine]:

. . . he said [ruin your cat] as in catalytic converter. Usually less expensive, but will still give you a bad time.

In short: If you let a bunch of unburned gasoline get to the catalytic converter, it will often heat it up so much that the core melts and plugs the outlet end of the catalytic converter.