my Tahoe 4.8 litre wont start until I press on the throttle

Check you're idle air control valve. The engine uses this to allow air into the motor when your foot is off the throttle (throttle body 100% closed). If the IAC is stuck shut, no air gets in, so it can't start until you press the throttle. This valve can be clogged up with carbon and sometimes a simple cleaning will get it running. Often times the valve needs to be replaced, however.