My friend has a '92 F-350 dually IDI with an aftermarket Banks turbo. The truck starts no problem but runs really rough. I had noticed the "water in fuel" light was on, but after we changed the fuel filter, the light went out but it still idles rough. What else could cause a rough idle. We're figuring a wet injector, but he had read somewhere it was something to possibly do with a return line(????). Can anyone help?