Have an 89 Cherokee 4.0. Just bought approx. 2 weeks ago, my 2nd Cherokee.

When sitting still, it's belching black smoke. Upon acceleration, it smokes. Gets 5 mpg and is running rich. Was told that the oxygen sensor would correct that.

Changed oil (& filter), air filter, plugs, plug wires, and oxygen sensor. Put a fuel treatment in.

Still gets 5 mpg and it still is running rich. Any suggestions?

There's an oxygen sensor electrical relay that might be defective it's fairly cheap. I've found them on the web for under $10.00

Thanks. I appreciate this tip. Off to look for one. I'm desperate, as I don't want to get rid of this Jeep, but it's killing me financially (because of the gas mileage). My other Cherokee was totalled and needs front end work, which I hope to undertake next year, so it cannot be driven.