I just got18.5 mpg on my 2002 x cab 4x4 on mostly hiway driving, I thought that was awesome

 

So i tried to give it a quick full throttle and check the vacuum, it goes back up to normal as quick as it went down to 0.

 

Exhaust is not blocked and most likely no vacuum leak with that result

 

Im pretty sure my valve cover gasket is leaking, but that wouldnt cause my problem right?

I dont know what to check anymore. Could there be a tranny problem causing this? Oil is all good tho.

 

So ive finnaly recalibrated my ultragauge, and as expected, the mpg is worse than shown, by 10%!

 

So what does that make it?
The MPG that is?

There are only two ways out for the gasoline, burned or leaked.
EVAP system prevents evaporation.

Well, I guess there is a third way out, neighbor with a siphon hose, lol.

If you are burning it then spark plugs will be black instead of nice light brown

 

Yeah MPG, if it used to show 1 liter burned at X miles, then 1.1 liters at same miles after adjusment, from default setting.
I changed spark plug last summer, and they were indeed black, however fuel trim on ultragauge mostly show between -5 to +5 on both bank, and never more than +10 and never under -10, is it possible that the o2 sensors show false result?

 

Yes, if plugs were black and computer is showing "normal" fuel trims then either the computer is at fault or the O2 sensors are.
If MAF sensor was wrong then O2s would show the rich burn.

Spark plugs being discolored are physical evidence of incorrect mix.
Your MPG would also back that up, although never have trusted electronic "best guess" software.
Fill up tank
Reset trip odometer
Drive a bit, say 100 miles on odo
Fill up tank again
5 gallons used.
100/5 = 20mpg

But I am old and have lots of pencils and paper to use up.

Ford dealer can reflash the computer with latest software, which often improves MPG even if it was OK before, $100-$150 usually, but have seen it done for $75 flat fee at some dealers.

O2 sensors work by chemical reaction and fresh air from outside, they used to have vents but now with the 4 wires they get it via the wire insulators.
The chemical reaction uses up the chemicals so O2 sensors do simply wear out, like a battery does, 100k miles is expected, some go longer some shorter.
The black spark plugs would mean shorter O2 life because they would also get a coating of black.

 

I guess its worth trying to change o2 sensors then, should i bother changing the one after the cat?

Do you think i can reflash the firmware myself? I have an usbASP stick and a FTDI stick wich i have used to reflash some electronic with it.

 

No, the 2000 should have PATS so I think you need an active Ford subscription while flashing.
The actual flashing probably only takes a minute or two.
The connection to Ford Database and authentication process probably takes 20-30minutes.
This is done so thieves can't just swap or reprogram computers to get around PATS system, although locksmith programs can add PATS keys in less than 5 minutes, lol, so the delay in flashing seems like a useless "money grab" for dealer shop time.

I would just replace the upstream O2s.
Be sure to reboot/reset computer after changing them.
And pull out and clean the spark plugs so you can tell if you are getting normal burns after this change.
The downstream O2(s) seem to last alot longer since they are in cleaned exhaust stream, but if engine has been running rich for a long time it may need to be changed.
But you will get a CEL and code for that once new upstream O2s get engine back to "normal" air/fuel ratios.
The downstream O2 doesn't effect air/fuel mix directly, it is there to check if Cats are working.