When I have taken off the pre-throttle body duct I can start the car but it will immediately turn off. I cleaned out the throttle body with the car turned off but would really like to know why the car won't stay running with the duct off.

 

The reason it wont run without it is because obviously it is something the engine needs to run. Maybe you should put it back on.

 

The computer wants a signal from the mass air sensor that tell it how much air is being drawn through it.
Since the signal is saying no air is going through, the computer shuts everything else down.

 

I was told that to clean the throttle body that the ducting needs to come off and to clean the throttle body while the engine is running,which is what the throttle body cleaner says. Is it just protocol to clean with the engine off?

 

Also I would think that at idle that no air is being drawn through. Am I missing something?