Help? 92 Marguis Engine Light
 

Here is a little back story first. I was given a 1992 Mercury Grand Marquis from my girlfriend's father. It was his parents car. Back when the grandparents were allowed to drive, the grandmother confused the gas for the brake and drove it through a concrete wall into a Rite Aid. They never had it repaired, but left it in their garage for the next 3 years. I come along, in need of a car and hook up a battery and it turns over. I am also a DIY kind of guy. I have replaced the front brake flex lines, rotors, calipers, pads and have replaced the vacuum line on the PCV valve. I get an engine light that comes on about 15 minutes of driving. If I shut the engine off and turn it back on, it goes away for another 15 minutes. I was told its the ports, but I have ran every fuel additive through that engine and I have also heard the EGR could be clogged. Any help toward me understanding what it is and how to fix it would be greatly appreciated.