I just purchased a 90 S/C 5 speed. It had a hard time starting so my brother and I pulled the upper half of the engine to replace head gaskets. Turns out someone had already done so and did not seal it good. Now it runs pretty good but when we put it back together, we noticed that at the back of the engine, right next to the EGR valve, there some sort of sensor that has a vacuum hose attatched and an electrical plug. Right now it is not even plugged in and it is running okay. Any help to identify this would be appreciated.