check engine light

I have a Ford E450 v8 and I have a Banks Power System that was installed around the same time the engine light came on. It comes on about 3-10 minutes after starting the engine & it stays until I stop for the day! I have had the OBD1 checked a number of times...once by a Ford dealership and the last time by a reputable auto repair business. Not one mechanic with a variety of scanners could communicate with the OBD1. Now I need to get an IM to license my RV in a different state! I can't even sell it without the buyer getting an IM....unless of course, the state that a buyer lives in doesn't require an IM!!