I havea 91 Explorer with the 4.0. its very hard to get it to start when it will when you turn the kep the fuel pump whines and it sounds like somethings popping in the intake...if you get it to run when you mash the gas to get the rpms up it sounds like a growl in the intake.....i took it 3 days ago on the road and when you reach high inclines it starts losing power and it crackles and pops out the tailpipe and occasionaly backfires....the check engine light very seldom comes on......also when it will idle and warms up if the heater control is on warm it blows a smoke through all the vents and smells like antifreeze......if anyone knows what might be going on please let me know

 

Sounds like you need to start with a serious TUNE-UP being a 1991, new fuel pump and fuel filter along with a new heater core.