Hi I got a 2001 Chevy blazer Lt with a 4.3 vortec engine. When I first start it, it could start or it could take a few tries. It will idle rough and then choke up and want to quit so I have to keep it at 2000 rpms, anything under it will cause it to quit, when it does quit it will try to start but won't get past 500 rpms. I've replaced map sensor fuel pump sending unit spider injection ignition control module fuel rails. can u please help me