91 5th ave need help please!

1991 5th ave with 3.3 liter V6, 153,000 miles on it. ok here is what happens when i start it it will run for maybe 4 seconds at idle then shut down, I can keep it running longer if i rev it at start and turn key off and on fast it will stay running but as soon as i leave key on it will die.
we tryed a diffrent map sensor still same thing i was thinking maybe cam-crank sensor could that be it ? any help please this is a young kids ride and money is very tight thanks. oh and how do i read codes on it thanks for any advice.

got it to run it was the computer that the intake air flows past i put a used one on and motor runs great. now i have a new issue it will shift 1st -2nd and third but won't go into 4th it tries three times then stays in third automatic trans. could this be from changing the computer? should i have changed the compter on the passangers side as well? This shifted great before the running issue. thanks for any help.

Your transmission is in limp mode, unplug the battery for about 30 min and try again. You should also change the fliud with atf+3 or atf+4