If they're bad, replace. It's not just the black soot you see on the outside that makes them go bad. There's electronics inside them also. Scanner will tell you if they're bad. It's usually hard to return electronic parts to the parts store if you installed them once, unless you have a buddy working there, so its best to make sure before forking out the cash for new ones.

 

As mentioned above, ODBII scanner and an app on your phone (I use Torque) will tell you whether your O2s are switching properly.
Have you checked your coolant temp sensor yet? What does the gauge on the dash say for water temp? If the sensor is broken it'll tell your ECU that the engine is at -40C and will run it super rich to compensate.
You can also use Torq to tell if you're in closed loop or not. If you're stuck in open loop even when the engine is warm then there will be other problems to look at.

Edit: WOAH - do you really have an 88? If so it must be a one-off import and it won't have OBDII. Did you mean 1998?

 

Yeah, I was wondering about that myself. Very first post, it's a 98. Automatic, I'm assuming, so 3.55 gearing?

02 sensors are actually a very delicate solid state electronic semiconductor. CAUTION! SCIENCE CONTENT! There is a zirconia element inside that has a thin coating of platinum on it. When carbon monoxide changes to C02, it creates a tiny electrical charge. This charge is read by the computer to change the fuel mixture so all of the monoxide becomes dioxide before leaving the converter. This is why they are expensive and have a limited life. They can be damaged by things like unburned oil and coolant or extreme temperatures. Also because of the microvoltages involved, the wiring is critical.

The normal life expectancy is 80,000 miles, for a quality one. Any longer than that is a blessing. Having said that, 3 of the original 4 factory sensors in my WJ have 255K miles on them. SO...it's crap shoot. They can go bad at any time.