My 95 104 engined coupe will start well, and once it comes off it's higher idle[1000rpm] it will stumble and blow black smoke a idle.It also stumbles under acceleration, but will perform as normal under full throttle. The idle still lopes. I took the car to the dealer and they could not get any codes from the computer. They replaced the 02 sensor, no change. I checked the engine wiring harness, and found it to be crumbling, so I replaced it. No change. Getting expensive replacing good parts, can anyone tell me where to look next? Could the computer be a problem? The check engine light is not working, but have not yet had the dash out to see if it is just a bulb... Help!!