No on the O2 sensors, if they haven't been replaced before then they need to be for best MPG, but they wouldn't cause your problem.
They last for 150k miles, then MPG starts to drop slowly, not much, but new ones will pay for themselves over the next 150k miles.
First thing to check would be the Fuel Pressure Regulator(FPR).
Passenger side top front of engine, it has the Return Fuel line connected AND a vacuum line.
You need to check if this vacuum hose has fuel in it.
FPR diaphragm can leak and fuel is sucked into the intake, causing what you describe.
Highest vacuum at idle so highest amount of fuel sucked in, then when driving lower vacuum so less fuel and you are using more fuel anyway, higher RPMs.
Replace FPR if fuel is found in Vacuum line.
Next is Sticking fuel injector, when you changed the spark plugs did one look much darker that the others?
Sign of injector not closing all the way and leaking fuel
You can do a fuel pressure test, '94 system should have 30-40psi when running and then hold pressure above 25psi after it is shut off, and it will hold that pressure for a few months, not minutes, lol.
And a system that doesn't hold pressure could have a bad FPR or leaking injector, even bad check valve on fuel pump but that wouldn't cause current issue.
Exhaust leak at the Exhaust Manifold can cause Richer running, only because the O2 sensor on that side "sees" extra oxygen which means Lean burn to the computer.
An exhaust leak also sucks IN air it doesn't just blow exhaust out.
So a pipe or muffler leak wouldn't cause that issue.
O2 sensors only "see" oxygen levels not fuel, too little oxygen means Rich, too much oxygen means Lean
If an exhaust leak is after the O2 then it would have no effect on lean/rich
1994 was last year of OBD1 protocol, ODB2 was used in 1995 and up vehicles
You can try reading codes via meter or CEL(MIL) read here: http://www.therangerstation.com/tech...ry/OBD_I.shtml
But from your description the codes will be "engine running rich", lol, and you already know that.
Other sensors or controls(like IAC) can't really cause what you describe, almost has to be a mechanical failure, i.e. leaky FPR or injector