I have a 91 rs that was said to have an older 350 in it. To start things off when I got the car the check engine light would come on after some driving. If I got right on the highway it would not come on till I got off the exit and starting to slow down. It done this for a while say 6 months since I got the car and I haven't done anything about it cause it still seemed to drive well. When I did my first oil change and changed the plugs this happened to me once before but only for one day then went away. Now I recently just added more oil and some lucas oil treatment to it cause it loses oil. ( I am considering the head gaskets are shot) Now the same thing has started up and won't go away this time. could my 02 sensors not be correctly hooked up to the car for the service engine light to come on. because it does go away when I shut the car off. I also believe that there might still be the 305 intake on the car because I think who ever had it did not want to change the computer for the 350 engine. any help would be great and if you need more information I will do my best to help you. thanx Fred

 

check to see if u got some stored trouble codes. if the ses light has come on and off a few times it should have stored some codes.