Sudden Reduction in MGP

Thanks to all who responded to my problem concerning "low mpg". I started with the simpliest of the responses and the problem is now solved. I wrote:

I have a 1983 XJ-6 Series III (4.2 L) automatic trans. that recently developed a problem. Starting and low speeds are no problem. Engine runs smooth. However, at 50-60 mph, it starts cutting out with black exhaust. It is not burning oil. Fuel consumption has dropped from 16-18 mpg to 5-6 mpg when this problem developed. This problem developed overnight, and I did not see a degredation in performance over time. I replaced the spark plugs, but no change. Any suggestions would be greatly appreciated.

I know you got lots of answers allready on this. I had the same symptoms last year. Turned out to be a bad coil. I let it go too long and it cost me a catalitic converter and an oxygen sensor.

...the classic symptoms of a failed fuel pressure regulator. The regulators fail in a mode that allows full pump pressure to be applied to the fuel rail. In other words, 120+ PSI rather than the normal 36 PSI. three times the pressure, three times the fuel, three times the stress on them old rubber lines....

Just for completeness, and to always look for the simplest thing first, I would check for something physically blocking the air intake...

Sounds like a heavily over-rich mixture, check the cold start injector, the airflow sensor and the air-density/temperature doodad (what's that called again?)

Could that over-rich mixture be due to a cold engine? How's the engine temperature? I had a somewhat similar problem two winters ago, where the engine seemed ok under acceleration, but hesitated and missed under deceleration. And my heater didn't work. A new thermostat solved the problem, as the old one was stuck open and the low temp apparently confused the ECU into using thecold start injector all the time, or doing something else goofy to keep the mixture too rich.

The O2 sensor? Mounted in the exhaust header with 1 or 3 wires to the EFI system.

Check that your battery ground is good. Also check that all connectors to your fuel injectors are good and that the chassis to engine ground strap is good. The injectors are normally open and only close when voltage is applied to them. I had a very similar problem due to bad ground. You may also want to check the fuel pressure regulator if the above does not work.

Julio, you solved the problem. The problem was not in the ground, but in the connection of the positive cable to the post mounted on the fire-wall. There was a bit of corrosion on the post. I cleaned and polished all of the battery cable connections (with great care on this post) and reassembled. Power and gas milage is better than it has been in a year!! My trip computer says that I'm getting about 20 mpg -- where it routinely was at ~16. Thanks to all of you for the excellent advice.