It's very likely the feedback pod.located under the dash
pad is either out of range (a voltage setting)or bad.Another
poss. is the evap.temp sensor.Any good M&B REPAIR
garage should be able to to find your problem ,belive
me both of these were very common problem.
MERC.OF BLOOMFIELD HILLS (MI)
20 YRS.A.S.E.,M&B EXP.