Un Burnt fuel can also be a result of poor secondary ignition (from the coil to the plugs), have you checked your cap, rotor, plug wires and plugs yet?

Start with the inexpensive stuff first, before moving on to cats etc. That reminds me, did you take your car for a 20-30 minute pretest run and heat the cat(s) up?... It can help quite a bit

Be sure you have the correct non- resistor plugs and a fresh oil change.

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K