Vacuum operated fuel pressure regulator's can leak fuel into the vacuum line if the diaphragm cracks. Pull the vacuum line off and prime the fuel rail to see if fuel leaks out.

 

The pump can produce pressure that will be beyond what the computer can compensate for. The 'fuel trims' will all be low, but the mix can still be a bit rich. A leaky injector would do the same in that the computer may compensate on all injectors, lead to misfire(not that it is in your case) due to lean... or it may be 'within tolerance' even though it is adding excess fuel.
Many auto parts stores will lend tools. A pressure gauge may be borrowable. If you break it you bought it. I bought one for $20-30, I forget it has been so long ago. The pressure should raise when the ignition is turned to ON, and should stay for several minutes before dropping. As the engine is started, vacuum will move a diaphragm and modulate the pressure. As noted, if there is a leak, you'll get excess fuel, but it would likely be a notable amount. The pressure should drop with the engine running, and raise if you go to WOT, or even open the throttle just a bit.
I would check the spec on the temperature as I think a 195F was factory. Just a few degrees can make a difference. Do you remember if the thermostat fit snugly into the housing, held by the rubber gasket? If it had 'bypass' leakage, it would not be as effective.
tom

 

Tech manuals state that the PCM 'adjusts' for mileage. I read that as 'detunes' to keep smog levels down. I've had three Fords that did the same thing as yours at about 150K miles. I can't prove what the adjustments are - that's my conspiracy theory, because nothing else had changed mechanically on my vehicles that I could fix.

 

I would have said that it adjusts for mileage based on two things. The exhaust gas composition and the intake manifold vacuum. Both may change based on a tired engine with valves that don't seat as well, or rings that don't seal as well. Add in timing chain stretch and cam wear and things are not exactly the same on many engines.
The two things I mentioned will affect how aggressively the computer advances the spark timing, and the 'spritz' time of the injectors and the response of the O2 sensor to said adjustments.
If your engine seems to be tired, get a compression check done and see if it actually had a problem. If not, then check the other sensors for their output to be 'within tolerance'. A slow thermostat, or a temp sensor that tells lies can fool the computer and it has nothing to compare for reality. At least for some sensors.
tom

 

Could be due to a gas reformulation due to a CARB type agency ruling, when I drive from Oregon to California my mileage drops 4-5 mpg's with no other change in driving habits or equipment hauled/towed

 

cant see a gas reformulation causing it. If so it would effect everyone in the area and not just me. My gas stations did not change and have since checked different stations with no effect. But thanks for the suggestion!

 

Is this a flex fuel 3.0L????

 

Yes.

Still have not had time to start messing with the fuel pressure tests. Not a lot of free time for the next month. Prepping a race plane for the air races in sept! Will be checking for the tools in the next week.

 

The Flex Fuel 3.0L requires a wide heat range spark plug & you said that's one of the things you said was changed, so did you use the specified Motorcraft plugs????

The flex fuel 3.0L also must use special fuel injectors, so if replacing, we must must use the specified ones.

The flex fuel 3.0L also has a % fuel ethanol detector & if it acts out, can mess with fuel trim, as it must have the fuel injectors adjust their volume delivery with E-0, E-10, E-15, E-85, as the various recipes of ethanol have changing % of O2 in them that must be accurately calculated for, so that the engine doesn't run too lean.

About the time the mpg dropped, did you try, or begin using a different % ethanol fuel, like, E-10, E-15, E-85, or stop using ethanol blended fuel???? For folks wondering about non blended fuel, Yes there are stations that sell non ethanol blended fuel, we just have to search them out. Anyway if you have recently changed your fuel recipe, another reason to put the ethanol sensor on your suspect list.

Seeing as how this ride has over 200K on it & you haven't mentioned ever changing the PCV valve, it & its interconnecting rubber hose & grommet belong on your suspect list, because if they are acting out & the PCV valve is sticking open, or its pintail seat is worn & not sealing well, or its hoses or fittings are cracked/dryrotted, soft & aren't sealing well & is leaking/by-passing air, being down stream of the MAF sensor this amounts to an unmetered/uncalculated for air/vacuum leak & sure can mess with fuel trim, by causing the engine to run lean. Even if the PCV valve rattles, that doesn't mean its sealing well internally. We should change them at least as often as Ford has specified, more often if the engine is using oil.

With the mileage on this puppy, both upstream O2 sensors should have been changed at the same time, because their switching range & speed slows with age, so the cyl bank with the 5 year old O2 sensor isn't likely getting the same fuel trim control as the cyl bank with the new O2 sensor.

Hook up your ELM scantool, enable FORScan software & let it query all of the vehicles controllers for trouble code clues. It might be able to find some code clues, or sensor values that are close to being out of spec, that low end scantools can't, that could shed some light on your sudden mpg drop, like being able to have a look at the ethanol sensors operating frequency range for the type fuel your now using, fuel trim for both cyl banks, fuel injector squirt time, O2 sensor switching speed & range, MAF sensor output, ect.

Good idea to have a look at fuel pressure, as if its low, that could mess with fuel trim.

Lastly, make sure your tires are properly aired up to spec.

More thoughts for consideration, let us know how your mpg trouble shoot goes.

 

Do you have AC? Maybe the circuit to engage the AC compressor clutch is stuck on, try pulling the connector on the clutch for a week or two. Or maybe the clutch is jammed up and won't disengage. I have a switch on my compressor clutch and it's good for 2-3mpg.

 

Computer sees lean and rich conditions and will compensate them out as best that it can. Leak a little air and computer says oops, need more fuel... when it all evens out that's how it runs. Leaky injector makes it look rich and... computer opens up the throttle until things "look" good. Haven't seen anyone suggest a coolant temp sensor yet. If computer "sees" it getting hot it will dump extra fuel to try to cool engine down. 90s chevs had 2 sensors, one for the dash you could watch and one for the computer you couldn't watch.... guess which one always failed.

Biggest problem with EFI is it's so good at hiding problems, it can be hard to figure out what's going on.

 

tomw suggested the engine coolant temp sensor & other things in his #19 post & I agree its resistance range/PID feedback to the computer is worth checking, as a corrupt engine coolant temp PID feedback to the computer can cause the computer to corrupt fuel trim. Here is a link to the ECT resistance/temp profile to test it against Ford Fuel Injection » Engine Coolant Temperature Sensor (ECT), to determine if its PID feedback to the computer is corrupt, or on the edge!!!!

The sites main sensor page, provides details on other sensors that's worth checking.

Lots of possibilities that could cause a sudden 2-3 drop in mpg & with the mileage on this puppy, lots of things belong on his suspect list.

Lots of reasons to have an advanced scantool like the ELM, running a good diagnostic software that's tweaked for Fords, like FORScan, have a look at all of the vehicles controllers for clues.

 

Oh, my bad, so he did. Bacon and eggs won over reading skills.

 

bacon and eggs always win!

 

Yall have seen my posts about chasing problems with this truck. Today I replaced both the temp sensors ($20 and $10 and 20 minute job). Where my temp gauge falls changed the first time I started up the truck and let it warm up to operating temp. Dont know how the 2 interact, but at least one of them changed something.

Will post up the results after a tank of fuel.

also, had a code 0153 come up so going to change the O2 sensors...but thats something new. The fuel issue is over a year old now.