Ive got an 88 GT. I replaced my radiator (and stupidly not my thermostat) and my car got stolen that night. It was recovered one day later and when I picked it up the radiator cap was off, wedged next to the battery and the car sounds like crap. I was able to drive it home 9 miles but didn't have the normal power. I don't think the head gasket is blown because the oil is very clean still... It kinda sounds like it has a really bad exhaust leak but more coming from the engine?? Cracked intake manifold maybe??