What could cause a 94 chevy 1500 to bog down when it first starts?

i have been trying to help a friend of my dads to figure out why his 94 chevy 1500 truck bogs down when it first starts it only last about 4 seconds but uses a lot of gas he has tried to replace every sensor we can find,and the throttle body during the 4 seconds it is bogging down it makes a strong air suction noise but we can not find anything wrong with the vacuum system but there could be something we have missed any suggestions would help a lot. thanks in advance :)