0 STFT = computers calculated fuel needed based on MAF air flow and engine load in real time, on the fly
+1 means it is adding more fuel than calculated
-1 means it is adding less fuel than calculated
-10 to +10 areas are fairly normal, when you get above or below +/-15 for any length of time there is a problem, at -20 or +20 computer will set codes for it
At idle computer ignores O2 sensors and STFT should always be +, +3 to +7 would be normal, this keeps engine from overheating.
O2 sensors need to be above 650degF to work, they generate their own voltage, 0.1 to 0.9 volts
O2 sensors read Oxygen levels not fuel
Too much oxygen is Lean exhaust, .10 to .45 volts
Too little oxygen is Rich exhaust, .45 to .90 volts
When driving computer uses upstream O2 sensors to see if 0 fuel trim is correct, it then opens injectors longer and shorter constantly to keep O2 sensor voltage between .35 and .55 volts, so STFT numbers should jump around like crazy, but as said between a range, like -1 and +5, or +1 and +6
Downstream O2 sensor, after cat, is used to check if Cat is working, it should always have lower voltage, high oxygen lean, it means cleaner exhaust so Cat is working
STFT tends to always be in the + numbers, this is because of smaller vacuum leaks and less than the highest fuel pressure at the injectors, and nothing wrong with that.
System is setup for high miles, things get loose and things leak with age.
Long term fuel trim(LTFT) is the average over the long term, like it says.
This is only used when starting engine cold, cold engine needs a Rich mix(choked) and O2 sensors can't be used, so computer needs a way to know fuel system condition as the years pass by, LTFT serves that purpose.
Now "average" probably isn't the right word, but it is the right example, it will always tend to be a higher + number than you might see and think is your "average".
The Downstream O2 sensors is also part of this "average" which is why it tends towards the higher + numbers
Your lower MPG could be a few things, one of which is the Knock sensor.
Since you have a real time scanner have a look at Knock sensor and spark timing.
4.0l SOHC has 9.7:1 compression ratio, which means it will Knock/ping on 87 octane if things are not perfect, and they are not perfect long, lol.
When computer advances spark you lose power so lose MPG, and the knock sensor detects pinging before you hear it, and they have been known to detect timing chain noise as a ping so you are running with advanced spark all the time.
You could put some 91 octane in and see if MPG improves, it shouldn't, 87, 89, 91, and 93 octane all have exactly the same power when burned in a gasoline engine, higher octane number just means less of a chance of pinging/knocking under load
Last edited by RonD; 07-13-2016 at 12:12 PM.