I fill up my '04 T4 gas tank every 150 miles and during the next 30 miles when I stop the car engine rpm's drop to 400. I press gas pedal inmediately to rev it up then I have no idea if it would stall. This happens randomly and just one or two times; after that, engine runs OK until the next fill up. I've tried different fuel brands and filling up at different times of the day but the problem persists.

 

im thinking its a coincedence that it happens when you fill up. try checking the vacuum hose for the fuel pressure regulator below the cover for the throttle cable. sometimes the fuel pressure regulator breaks and fills the vacuum line with gas causing hose to rot away. when that happens the car runs rough. its a long shot but you can start there.

 

Do you have any check engine light on with perhaps any evap emission codes or purge valve codes??
When you fill your fuel tank, the liquid fuel volume is displacing all of the fuel vapors in the empty tank. All of these vapors are forced in one big push into supersaturating the charcoal canister. Typically, your canister purge valve should keep these vapors contained in the canister untill you are at a optimum operating condition for the computer to start purging them into your intake air system. If this valve is sticking open even just a little bit then you can get not only the conditions that you describe but also commonly a hard start immediately after refueling.

 

Thank you very much for your replies. Actually I do have CEL related to evap system but mechanic insisted it was due to a vacuum leak caused by a worn fitting connecting purge valve and air intake hose, which was replaced. I will do checkings as per your suggestions and will revert with the results.