I have a 1998 ranger 4x4 4.0 V6.
Recently My gas mileage has taken a big loss. in the mornings I detect a strong fuel smell when I start it up.
I think its most likely the O2 sensors that need cleaned or replaced.
I looked up the new sensors on Rockauto, but I am not sure which ones to get. Its not necessarily the brands, but upstream and downstream. Is there a difference? I know there are two, one coming off the manifold right before the exhaust pipe hooks up.
Can anyone tell me if I am on the right track or if the fuel issue could be something else?

On another subject, I am having a ridiculous time trying to replace the EGR valve. both the compression nut and the two 10mm bolt are not budging. I have tried liquid wrench.

 

On a V6 engine there are two upstream O2 sensors, one on each "bank"/side of the engine
On Rangers from 1995 and up there is one O2 sensor downstream, after Cat converters, it is there to test if Cat Converters are working, it often doesn't need to be replaced until you get a Check Engine Light and a code for B1S2 O2 sensor problem
Upstream O2s should be changed every 100k miles, 150k for sure

Some later model Rangers did have Dual Exhausts, they would have two downstream O2s, one after each Cat converter on each side

Smell of fuel on cold start should be looked into closer.
Before starting open the hood and pull off the Air Plenum(large air tube), that runs from the air filter box to the upper intake
Pull off upper intake end
Open throttle manually and smell intake for fuel
Now cycle key on, count to 3, and off, 3 times, do NOT try to start engine

Now smell intake again, if you have a strong gas smell then you have a leaking injector, OR Pulse Damper
Pulse Damper is on the Fuel Rail passenger side near firewall end, it will have a Vacuum hose attached.
Pull off this vacuum hose and check it for gasoline, if found then Damper will need to be replaced, if clean and dry then one of the injectors is leaking.
But in either case you should have a Trouble code for RICH running, so I would check for computer codes

No magic trick to getting exhaust parts off, best method I know of is to take it to an exhaust shop and let them deal with it.
They spend all day, every day getting rusted and stuck exhaust parts off, they know the best and fastest way to get the parts off.
Sometimes parts can be saved sometimes not, they must be cut out and replaced, but if they can't save it then you sure couldn't, lol



Upstream and downstream are the same type/kind of O2 sensors, the different part numbers are for the length of the lead wire that they come with, i.e .6" lead, 12" lead, 18" lead, to plug into vehicles wiring harness
So there would be a different part number for upstream passenger side and upstream drivers side IF they needed different length wires to reach vehicles wiring harness plug in


O2 sensors use a Chemical reaction to detect Oxygen in the exhaust, the chemicals get used up so they wear out, just like the chemicals in a car battery get used up after 5 or 6 years.
So cleaning O2 sensors doesn't help much if chemicals are used up.

 

Yes there is a difference. The upstream sensors are the ones before the catalytic converter. Downstream is the one after the cat. There should be 3 in total, an upstream on each manifold or directly after the manifold and one downstream directly behind the primary cat.

before you go replacing the 02 sensors you should check some other things. With the fuel smell you describe on startup you may have a leaking fuel injector.