Looking to see if you have an suggestions on rough idle that just happened and a gas mileage of 12 miles per gallon. oxy sensors contribute to this problem? my sons first truck and just need some help if anyone can suggest. seems not to not have surging when at higher rpm's but I still have to drive for awhile. Always smells like it is burning too much gas but....

 

Pull out 2 or 3 spark plugs, darker brown or blackish will tell you if engine is actually running rich or if fuel is leaking out somewhere else.

Light brown is good burn, no rich running.

Assuming rich running:
1998 is the first year of Returnless fuel system in Rangers.
On the end of the fuel rail there will be a Pulse Damper needed for the 65psi fuel pressure.
The Pulse Damper will have a vacuum hose attached to it, remove the hose and check it for fuel, if fuel is found replace Damper, it is leaking and extra fuel is being sucked into engine thru vacuum hose.

Clear Flooded engine test
All fuel injected engine computers have this.
At 0 RPMs and if gas pedal is held down all the way(wide open throttle), the computer will shut off fuel injectors allowing a flooded engine to be cleared.

So turn on the key
Press gas pedal down all the way and hold it down
Turn key to START

Engine should just turn over without firing, or starting, there should be no fuel coming in with injectors shut off.

If engine fires then fuel injector could be leaking fuel into engine.

As soon as you release gas pedal injectors will start working.

If engine does fire, then you may be able to find which injector is leaking by repeating the test, but unhook the Coil Pack wire first, so no spark.
Repeat test and crank it a few times, gas pedal held down all the way!
Now pull out each spark plug, wet spark plug tip means that injector is leaking.

Fuel pressure test will confirm a leak in the system as well.
Fuel pressure can be tested with a gauge, using the Schrader valve on the fuel rail.
Fuel pressure should be above 60psi with engine running
Engine off, the pressure should hold above 50psi for a few MONTHS, not minutes, hours, or days.....months


Other end of the system, exhaust
Partially plugged exhaust system can cause your symptoms.
You can test for this with a vacuum gauge hooked to intake manifold.
Or try having your hand over tail pipe while someone revs the engine, you can often tell if flow out is restricted above a certain point.


As far as the O2 sensors, if engine is indeed running rich you should have trouble codes for that, and the CEL(check engine light) should be on.
Does the CEL come on when key is turned on?
It should, and then go out after engine starts or self test is complete.

If it doesn't then bulb may be burned out or...................previous owner disabled it.
Take truck to auto parts store, most will read the codes for free, WRITE THEM DOWN!!!!!!!
Do not buy anything to "fix codes", 90% of the time they mean all the sensors are working correctly and there is a mechanical problem, so don't "shoot the messenger" by replacing a perfectly good sensor because a code mentions it in the description.

O2 sensors tend to last about 100k miles, and tend to make engine run richer as they get older, so changing the front 2 on a V6 can pay for themselves in the next 100k miles.
Rear O2(behind Cat converter) tends to last longer because it "sees" cleaner exhaust, but up to you really.

EGR system, exhaust gas recirculation
EGR valve remains closed when engine is cold or with low engine load, after engine warms up computer will open EGR valve when accelerating, going up hills or other high engine load situations, this cools down the cylinders to reduce NOx emissions which spike at high cylinder temps.
If EGR valve is not closing all the way you will get a rough idle, unplanned exhaust gases are fouling up the air/fuel mix, this will also cause rich running as computer tri4es to balance air/fuel mix.
Build up inside the valve over the years can cause it to not close all the way.
You need to pull it off and inspect it for this build up.