White smoke from engine & hard to start after fill up

[toberek]

We're stumped...
We have a 2004 LeSabre.

Check engine light keeps coming on with a loose gas cap/emissions code.

Starting yesterday, the dash also displays a "gas cap loose" code at every start up.

We have replaced the gas cap - same code & display.

Here are the strange symptoms:
Every time we fill up, the car does not want to start. Turn the key and it tries to start. We have to push on the accelerator, then turn the key - it takes a second or two, then it starts and white smoke comes out from under the hood. We have to keep the pressure on the accelerator for a few seconds until we get it into gear - once we start down the road we have no problems. Car always starts fine unless we have just put in gas.

Over thanksgiving the transmission was replaced, the thermostat was replaced, the ignition module & some ignition coils.
This all started after these repairs.

We're stumped and out of cash after Thanksgiving... We would appreciate any ideas - thanks!

[HotZ28]

Welcome to AF! Could it be possible that during the transmission exchange, one or more of the EVAP lines were disturbed? Check the EVAP vapor lines and hoses, the EVAP vent lines and hoses, the fuel fill pipe and hose, the EVAP canister, the EVAP canister vent valve and the EVAP canister purge valve. If there is not an obvious line/hose, or wiring connector loose, you may have to take it to a dealer for a “nitrogen smoke test”! When conducting this test, they use the Tech II scan tool to command the purge solenoid closed and then introduce the nitrogen smoke into the system while using a high intensity white-light in order to inspect for smoke leaks in the aforementioned areas.