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Smoke for exhaust when trunk is heavily loaded

I own a 1994 Mercury Grand Marquis LS with a 4.7 L V8 engine and 162,000 miles. I am the second owner of this vehicle and i have kept it in pretty good condition. I recently started to notice that when my trunk was loaded with a lot of weight (200-400 Lbs) and I would stop at a red light and then begin accelerating slowly that a lot of black smoke would come out of the exhaust. This happened a few times and once it happened after I had removed the weighty object from the trunk and started to drive off. What could cause this condition when there is a heavy load in the trunk? I plan on towing a small light boat and don’t want to have the same smoking problem especially since I was pulled over for the smoke once before.

P.s. Beside the occasional smoke the car runs like a champ !! Excellent acceleration and no engine hesitation. I guess the only thing I can mention is that the fuel ecconomy is slightly less then what I would expect. I think i am getting about 17 MPG.

Engine under strain…mixture rich/oil = black exhaust? That’s my guess.

Thanks for the quick response. What can I do to fix the problem? (Other then not towing heavy items in the trunk). Change plugs, fuel injectors, fuel pressure regulator, fuel pump, MAF? Or Do i have to adjust the idle air some how? To achieve a better fuel/air mixture?