The oil cap should be located on the passenger side of the engine, way in the back of the engine compartment, to the left of the air intake filter area, and to the left of the dipstick (right next to dipstick).

this picture is my source, look for the yellow cap and dipstick in the photo. (Look up from the right corner of the battery, and you'll find the cap.)

http://www.justmitsubishi.com/~midmi...42954-23sm.jpg

i have a 1984 E150 van. cannot locate oil cap. i found all the other fluids. i feel stupid. can you help? thanks.