1996 Pontiac Sunfire false map sensor code

Hi, i'm have trouble with my 1996 pontiac sunfire, I keep getting a map sensor code and my check engine lights on because of it. My mechanic and done everything he knows to find the problem and has come up short. He has checked the voltage to the sensor and everything else related with it, he has also put 3 different sensors in the car to make sure it wasn't a faulty new sensor. Also the car is running very rich and has sluggish acceleration. If you plug the map sensor in the car idles very rough and has no throttle response, unplug the sensor the car is drivable and has a little bucking when cruising and a slight lurch when at a stop light, also the brakes are somewhat hard. They did check the car for vacuum leaks abnd found none, i'm at wits end please help. Thank you.