My 86 Fleetwood gets about 8 MPG and I think it should do better. The engine runs very rich with black smoke exiting the exhaust at idle and when I "step on it". I'm told that the MAP sensor is responsible for this and I will get a new one shortly at $58. My question is --does this sound right and if I replace the MAP sensor is there anything else I should do at the same time?
Next question; The rear end of this car makes a clicking noise and I'm pretty sure it's the ring and pinion. The u-joints are fine. The dealer wants $1200 to $1500 to fix it and my garge wants $450 to just replace the whole rear axel with a used one from a salvage yard.(I will not say "Junk yard" when refering to my Cadillac) What would you do with a car that at best is only worth about $500? Are there any other fixes for this problem?
I really love my old Caddy and already have way more in it than I will ever get back out. Maybe I should cut my losses and dump it. Opinions please. Pete

 

First off I'm assuming you mean 1986, don't see too many Cadillacs from before the company was founded. Second, a bad sensor can definitely make the car run rich. specifically the computer freaks the **** out and makes the computer controlled carburetor run rich and if I recall correctly messes up the distributor advance too (which is also computer controlled). You can also ask the computer what's up. there's a little plug under the dash on the passenger's side near the centre. take a paper clip or solid wire and connect the two top holes closest to the passenger's side. turn the key but don't start it. now watch the check engine light, it'll give you trouble codes. for example, with the engine off, it'll give you 12 by flashing once, pausing, then flashing twice, then pausing even longer, then showing the next code. it'll flash 12 3 times, then give you your codes. I believe it's 33 and 34 that would confirm MAP sensor voltage problems. As for the rear axle, you could take the cover off (over an oil tray) and look inside for yourself if you're not afraid of replacing a gasket and putting in new gear oil, but when gears start making noise that tends to mean they're already broken.

 

The clicking sound from the rear end could also be a bearing going bad rather than a problem with the ring and pinion.
The only way to really diagnose this is to take the car or just the rear end to a pro and let them look it over.
Personally, I'd just do the salvage yard replacement.
As for the black smoke, have you tried to pull the trouble codes yet?