Vaccum leak and very poor gas mileage among other problems.

Hello, I have a 1987 Jeep Cherokee 4.0 (automatic with part time 4x4). I am getting very terrible gas mileage, 9 miles per gallon or less, engine is running very rich and I can hear a vacuum leak under the hood. The leak seems to be coming from the drivers side of the engine, near the intake manifold but all the lines I have checked are fine (not cracked and nothing makes the leak stop. There is a lot of black smoke coming from the exhaust because of the rich mixture and the engine is running very rough. Also I am getting a very loud noise from the front of the engine, almost like the power steering pump, but that's not it (cranking the wheel makes no difference) could it be from the timing chain or the water pump? One last thing, I am getting some noise from the rear end and have noticed that it is leaking at the front seal. It only makes a noise on deceleration. The input shaft to the differential has play. Should I just replace the bearing or am I going to have to replace the whole ring and pinion gears? Also there are no metal debris in the differential case. Any help would be appreciated.

If you can hear the vacuum leak, you should be able to isolate it. Have you tried the "propane torch" or "spray bottle" technique, spraying around everything vacuum related until you see a change in idle quality?

You must resolve that vacuum leak before moving onto other possibilities for a rich mixture (like map sensor, coolant sensor, fuel pressure regulator, etc).

Noises can be tricky. For the noise in the front end, I generally use a "mechanics stethoscope". Very helpful in isolating tough to find noises. They aren't very expensive. Be careful poking around a runnig engine.

Is your exhuast manifold cracked or leaking? If so the 02 sensor is getting an incorrect reading causing it to make the truck run rich