Engine Performance problem
1989 BMW 325 6 cyl Four Wheel Drive Manual 275000 miles

I have a 1989 BMW 325IX with almost 300,000K on it. Recently I have been having very bad MPG. a gallon of petrol can only take me as much as 8-10 miles and I have thick black smoke coming out of my tail pipe and car has problem idling.

The car moves very well and does not do hard start even on cold mornings.

Please help with a list of things that could be responsible for this.

Thanks.

#1 is to check for corrosion at the injector/sensor harness connector under the intake manifold. It is a round black plastic connector that is a twist lock and has 7 pins and is directly underneath the throttle body. This connector is poorly placed and if the throttle body heater is leaky it drips coolant onto the connector and promotes corrosion. Check that one thing and let me know what you find. As to other possibilities for poor fuel economy, are the tire properly inflated, do you keep up on the regular maintenance?(I know it seems like a silly question especially for someone who has 300k miles, but I have to ask!) Is the fuel system in good repair? Have the fuel lines been replaced since new? Have you checked fuel pressure? Is the air flow meter original? Let's start with this and see what happens.