Multiple possibilities, take it to a shop.
When was the last tune up? They are a "Mother".AnswerOK,
Surging possibilities :
Various sensors : esp O2
I am working to resolve this issue with my '92 265,000 mi Aerostar
email me @ AnselRoe@gMail.com & I'll let you know what it wasAnswerTry replacing the coolant temp. sensor at the thermostat housing. Mine caused surging when cold that went away as it warmed up. AnswerI had a mechanic put a short block in my 1987 Aerostar. When I got it back it would rev up and down, over and over. I took it back to him. He cleaned the injectors. He replaced the whole throttle body. He gave up. I got a code scanner, and it told me the voltage from the coolant termperature sensor was low (as I recall). I replaced the coolant temperature sensor, as the previous poster had, but the problem persisted. I then took a look at the connector going to the temperature sensor and the wiring around it. There was another connector identical to the one he had connected to the temperature sensor nearby. I connected this to the temperature sensor and the problem went away. I re-installed the old temperature sensor, and it worked fine, too.
Try the trottle positioner relay, its kindof a square 1"x1"x1/4"thick/ black with a wiring clip going staight into it, 2screws > top front of engine, side mount by air intake, careful, the scews are hard to remove, there on there pretty good, don't strip, if you do, use channel locks to break lose first. My van ran poor with no power up hill and ran crappy. That's what fixed mine
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