I have an 04 monte carlo, 3.4 that has trouble starting. When I accelerate it starts instantly but won't stay on/idle Thus killing it. It used to happen once in a blue moon now it's very often. I also had trouble with my tank, alot of pressure making it difficult to put gas in. Took it to the shop, they put in a new tank and THAT problem went away but still having issues starting it. I changed the MAF sensor today to no avail, my check engine light is on and pulled codes p0452 and p0455 fuel tank pressure sensor and evap vent leak. I'm at my wits end And really just want to blow it up and walk away. Anyone shed some light on this or had this problem?

 

I'm not a mechanic by any means but just had similar problem. Did you clean the idle air control valve and throttle body when you cleaned the mag sensor? That's what finally cured my hard starting issue. Also check the vacuum lines for leaks. Hope this helps because I know I was going nuts until I figured it out