I just bought a 1990 Aerostar from a private party, and the guy says that the EGR valve is going bad. The symptoms that I can definitely notice with out having any prior experience with Aerostars is that it is real sluggish at times, but at other times it runs like a top. When you push on the gas while it is being sluggish it almost feels like it is trying to come out of gear in very quick bursts (automatic trany), especially on hills. Starting it is a major pain. It won’t start unless you peg the throttle to the floor, and even then it takes 1 - 2 minutes of trying at times, and I think it has a easier time starting when cold. If you peg the gas while it is idling it will try to bog down or even die, bug at higher RPM it has great throttle response.