My 1994 one ton will only start when I disconnect the optical from the IP. I just bought the truck, and it did not run when I bought it. After pumping some of the diesel out, I see it is red ( as in red dye). I don't think this will harm the optical but, I also think the engine failed after the red diesel was added. I can start in limp home mode, but I should get DTC 17 or 18 if the optics are failing ???

I take it that it was showing a code for the optical before unpluging it? or just by chance you unpluged the optical and it started working at the same time?

Manual says red dye a no-no, but doesn't seem to elaborate as to "why?"

I can clear the codes, with optical pluged in, and I get no codes and no start. Then I unplug the optical, and it starts but of course I get DTC 17, 18, 19.

Try replacing the optical by switching pump covers with one that works or with a new optical.That it will start with the optical unplugged tells me that the opticals signal is keeping it from starting.