1990 dodge dokota 3.9L
My truck keeps spitting nd sputtering when I drive it but I can put it to the floor and it goes. I can hear a vacuum leak on the top back side of the motor at an idle. Really has me wondering.
Quit wondering, chase that vacuum leak down and fix it 
At the rear of the motor - that could be the vacuum tree that feeds a chunk of the vacuum lines. Most likely, due to age, the rubber and hard plastic have deteriorated.
What's also back there is the line to the HVAC system inside, to run the blend doors. There's also a line back on my 1988 that runs the heater control valve, not sure if the 1990 has one also.
But yah, chase that vacuum leak down and seal it up and see what happens.
RwP

At the rear of the motor - that could be the vacuum tree that feeds a chunk of the vacuum lines. Most likely, due to age, the rubber and hard plastic have deteriorated.
What's also back there is the line to the HVAC system inside, to run the blend doors. There's also a line back on my 1988 that runs the heater control valve, not sure if the 1990 has one also.
But yah, chase that vacuum leak down and seal it up and see what happens.
RwP


