Ranger is a manual trans 4.0L 2003 FX4 with 94K on the odometer, no modifications other than missing the front bumper and skirt since purchase. The last year or so, fuel economy has definitely dropped. I use a ScanGuage II set to show MPG since the last fillup, installed 8 yrs ago when I bought the truck, so I have a long record to go on. Historically, I could average 20-21MPG on the freeway even with Cali gas, 17-18 or so in mixed driving, 14.5 towing a 1000# boat. I typically don't exceed 65MPH. Lately I've seen a lot of tanks with high freeway % at 17-18 MPG, and with high surface street % at 14-15.

A couple weeks back I had just filled the tank and driven 40 miles home on the freeway. Gauge showed 20.5 when I got home. Got up the next day, hopped on the freeway, and watched the tank avg steadily drop down, to below 17. I switched the gauge to instantaneous, and it was 13-14, and stayed less than 17 for the 25 mile drive down the freeway.

I started poking around the internet, and changed the plugs (Autolite Platinum AP5144, same as before), cleaned the TPS and MAF, and aired the tires up to 35-40psi, in prep for a trip from here (SF Bay Area) down to Yuma.

Just got back, and it seems likely that the fuel economy decline is related to external air temperature. I watched the instantaneous MPG reading throughout the trip, as well as intake temp, and when the temp is below 55F or so, I'm getting 16-17 on the freeway, and above that, 19-20. Temps on the trip ranged from around 50-70F, and it's mild where I live as well, so this is not about extreme conditions. The two tanks for the return trip averaged a little over 19MPG each. Morning temps were low 50s and mileage was low, but overall temps were high 60s. On the way down there was a cold front over the Tehachapis, and one tank was only 16.9. These were 99% freeway miles, 68MPH or so, so this seems like solid data.

While driving the freeways around Yuma, where morning temps were often mid-fifties, and I'd at times see the MPG in both the higher and lower bands (not due to short-term throttle changes), it seemed like using cruise control would often result in seeing the lower readings. I'd disengage CC and the mileage would rise. I can't be 100% sure of this, but that was sure my impression. On the trip home, with temps generally 65F or higher, CC didn't obviously change the mileage. As well, the CC doesn't always disengage with a quick light brake tap like it used to. I sometimes have to press hard, and the pedal bucks as the CC disengages. I don't know if that is at all related to anything else, but I thought I'd mention it.

It seems pretty clear now that something needs fixing, so I'm ready to dig in, but wanted to ask for suggestions where to start.

 

Air fuel mix for a gas engine is 14:1, 14 parts air, 1 part fuel, and this is by WEIGHT not volume, which is why "pre-vaproizers" are a scam, lol.
Anyway, most know warm air rises and cold air falls, so warm air is lighter than cold air, so cold air is denser than warm air, heavier.

An engine sucks in air by volume, 4.0l engine sucks in 4 liters of air every 2 rotations of the crank(4 strokes = 2 rotations)
So the volume of air doesn't change but the weight of the air in that volume does change.
For the computer to maintain the 14:1 ratio it must add more fuel for each 4 liters of cold air, the colder the air the more fuel is needed to maintain 14:1 mix.
But you also get more power, unfortunately it isn't an even trade, engine will feel "peppier" but MPG still goes down.

So lower MPG in cold weather is just a fact of life for gas engines and normal.

But overall drop in MPG means there are changes in the vehicles systems.
Dragging brakes, trans, differential, larger tires, ect.....are mechanical reasons for drop in MPG


The engine computer relies on information from sensors to establish best fuel economy, and like all computers there is the old saying, "garbage in, garbage out", i.e. it usually isn't the computer that's the problem, it's the info being fed into the computer that's the problem.

MAF sensor is the main sensor used by the computer to set 14:1 mix
Your 2003 has an IAT(intake air temp) sensor in with the MAF sensor(6 wire MAF)
These two sensors set the "0" in your Fuel Trims
- and + fuel trim readings are the above and below numbers from this "0"

The O2 sensors read Oxygen levels in the exhaust, not fuel levels.
Not enough Oxygen in exhaust means too Rich
Too much Oxygen in exhaust means too Lean
O2 sensor voltage is what computer uses for feed back on its initial 14:1 calculation based on MAF and air temp "0"

- and + is the time the fuel injectors are opened based on the "0" time.
- means injectors are open less time, so O2 was showing low Oxygen in exhaust
+ means injectors are open longer, so O2 was showing high Oxygen in exhaust

At idle computer sets rich mix on purpose, so +5 to +10 is not unusual
While driving computer changes injector timing(pulse width) constantly so -5 to +5 is usual and it should switch very fast, a few times a second.

O2 sensors have a voltage range that tells Computer how much Oxygen they are seeing in the exhaust, Range is .1v to .9v
.1v means lots of Oxygen in exhaust
.9v means very little Oxygen in exhaust

Computer tries to get O2 sensors to give it .45volts the sweet spot for best fuel economy.

The O2 sensor is the only feed back the computer has on the actual Lean/Rich status of the engine

As O2 sensors age they tend to send less voltage, which means computer sees more oxygen in exhaust than there is so adds more fuel than needed, isn't much but will effect MPG.
100k miles is recommended life of O2 sensors.
So if O2 sensors are aging then there is a tendency for lower MPG even when fuel trims are showing normal -5 to +5 when driving, "garbage in, garbage out".

Misfires, no matter how slight, means unburned Oxygen is dumped in to exhaust, O2 sees this Oxygen so computer adds more fuel thinking Lean.
An exhaust leak in the manifold sucks in air, so O2 sensor sees this extra air, computer adds more fuel.


Computer also needs engine to be at correct operating temp to get best fuel economy.
Ranger engine temp should be 195degF to 220degF for best economy
Thermostat needs to be 192 or 195degF for engine to get best MPG.
Some switch to 180degF t-stat, this lowers MPG.

 

Ron - many thanks for the informative reply. Your one post brought me forward about 30 years in understanding of engine management! Sorry for the delay in response, but boat/computer projects came up and have distracted me from this.

I've started out testing what I can, but in rereading your post and digging around a bit, I'm wondering if you think such testing is likely to yield a diagnosis one way or another? I've got no codes, and no issues like lag, low power, or uneven running - just 10-20% worse mileage, that apparently has a low intake temperature feature to it. For what it's worth, the drums and rotors aren't hot either, and the engine temp is typically 188-190 on the mild days we're having now. I recall seeing this closer to 200 in times past so I'll pay attention to where it is once it gets warmer outside.

I'm inclined to just replace the two upstream O2 sensors as a starting point for A-B type swap testing. Do you think a Live Scanner is likely to provide useful information about those sensors, or anything else?

For the record, so far I've got:

IAT: 4.7V feed, and good ground; the intake temps on the ScanGauge II seem believable in both cold and warm ambient conditions, and between sitting and moving, so I'm thinking this works OK. The temp-dependency of the MPG drop keeps me wondering about this though (but of course, what do I know...)
IAC: 10 ohm between the pins; engine stalls when unplugged
TPS: 0.9V closed, smooth transition to ~4.8V and back
MAF: smooth transition between idle and estimated 2500-3000RPM of voltages in the published ranges (doing this from memory, should have written them down); engine stalls when unplugged. I didn't have a way to see a tach while doing the throttle but the expected range seemed to be there.

I've got a long trip this weekend, including plenty of cold and high altitude highway driving, so I'm going to be checking the ScanGauge a lot.

 

'03 Ranger will have the 4.0l SOHC, 207 horse power

MPG Ratings new for 4x4 were: 14 City, 15 Combined, 17 Highway

Couple of things happen in cold weather, winter gas, gasoline gets more methanol/ethanol added, this absorbs water from condensation and prevents fuel line freezing.
And get this, even if a station posts, NO METHANOL, on the pump, in the winter it can have methanol, not up to the station.

SOHC engine also has a Knock sensor, it has 9.7:1 compression ratio that's how they get 207HP.
Anytime compression ratio gets above 9.4:1 engine will ping/knock on regular, methanol helps, so does EGR, but engine will still ping under load, so the knock sensor will cause computer to retard the timing a bit when knock is detected, you won't hear it.
This reduces MPG as well as power, double whammy, lol.

Your scanner should have a spark timing read out, see what advance is and if it suddenly retards a bit when accelerating.
methanol, of course has less power per gallon to MPG will go down

 

I remember seeing the advance readout while scrolling through the options. I'll set that on the main screen and keep an eye on it during this upcoming trip.

Lol I'm pretty confident in the call there's been a mileage drop. For years I got 20MPG freeway over many gallons, to even many tanks of driving. The observations should adequately average, if not control directly, for external factors like gas formulation, vehicle load, driving conditions, speed. I'm pretty consistent, and seven years is a lot of data. I'm mindful and experienced in judging experimental design and analytical datasets, but as well, the recent anecdotal observations using the ScanGauge just don't leave much room for alternative explanation that I can see. Occam's Razor I'd say.

Assuming my assessment is correct, what would you do to fix it? Do you think analytical testing is likely to be fruitful - if so can you recommend what to do? - or should I start replacing key components, and where to start with that?

 

At close to 100k miles replacing the upstream O2 sensors would be OK, 100k is the expected life, although most are good longer.
O2 data is the final say on fuel mix.
When computer calculates 14:1 ratio based on MAF sensor, Air Temp and Coolant temp sensors, it then checks the final results with the O2 sensors.
If O2 data(voltage) shows Lean(too much oxygen) computer adds more fuel, and you would see +5 to +10 on fuel trims, and that gets the O2 voltage back to .45v
So you would see a sign that something is wrong.
Same with a vacuum leak, but on the other side of the equation, MAF sensor data is wrong because of extra air coming in, so computer's 14:1 calculation is wrong, O2 data shows this so computer adds more fuel, same +5 to +10 shows up on fuel trims.

Because this is a Feedback system there would have to be reverse failures of sensors on both sides of the cylinders, i.e. MAF sensor over reports air, O2 sensors over reports oxygen, so computer is running engine rich but you still see -5 to +5
And both upstream O2 sensors would have to fail in the same way at the same time.
Longshot

You can also just check a few spark plugs, if you are running richer then they will show a darker brown color, light brown is normal.

If fuel trims are staying at the normal -5 to +5 while cruising along then either the computer is at fault or you have an overall power loss causing you to use more fuel to cover the same ground/distance, i.e. lower compression as rings age, or less spark advance because of engine knock.
There have been a few reports of failing knock sensors causing loss of power without a CEL reporting failed knock sensor.
This is noticed as an overall loss of power, not extreme, more of a "not as peppy" as before, and it could be gradual.

Partially clogged exhaust system causes engine to work harder, normal exhaust systems create a negative pressure at the exhaust valves, so exhaust is pulled out when valve opens, this lessens the power loss from crank/piston having to push out the exhaust.