Ihave a 92 f250 351 efi ex ambo . The vehicle blows black smoke,air is coming out the emission system under the bonnet and the EEC light is on. Tried to do a self diagnostic test but found the fuel pump would not turn off so i couldn't get any codes. Any ideas where to start.

Could be a number of things.
Does the fuel pump run all the time when it's not in self test mode?
If not you may have just picked the wrong pin in the diag connector and grounded the fuel pump.
Otherwise you could have a busted line on one of the thermactor emissions solenoid hoses causing a vacuum leak and because it's speed density it will run rich...explaining your black smoke.

checked that your fuel pressure isnt too high?