I thought about posting this under engine tech, but I suppose this isn't engine specific. I've got a '99 3.0 automatic 4x4 and I'm getting awful, awful MPG. I also have 4:10 gears with standard size tires (15" wheels), aired to 32.5 on all four corners.
Now, when I decided I wanted a truck, I went in knowing I'd have a gas drinker to feed. But, I think this is pushing that note a bit.

In the time that i have owned it, I have used seafoam intake spray, two bottles of injector cleaner several months apart, taken it on a 10 hour trip (continuous) twice (to and from home to a few states away), changed the plugs (soon to also change wires) and a new fuel filter was installed. Air filter is clean, throttle plate is clean, and I'm using conventional Pennzoil 5w30.

From full to empty, I inconstantly get 200 miles out of it. By my math, that's 10 miles to the gallon, which is absolutely terrible. Now, granted, I am a bit of a lead foot. I like to get going and get out of other people's way, but that being said I don't burn out every time the light turns green. I take off at about 2,000 - 2,500 RPM if that gives any indication.

I do mostly city driving with some highway, average speed is roughly 40 miles per hour within 'stoplight city'. I burn 87 octane gas only, no additives. No CELs, nothing. I have also disconnected the battery and let the truck relearn it's patterns but to no avail.

I'm going to ease off the pedal next fill-up. But surely 10 MPG isn't right. I don't expect 20, but 10, really?

 

That is terrible millage, I knew a guy with this Phantom 2 Rolls Royce with this massive V12 engine with "normal" low compression.
He was doing well if he got 5 to 7 miles to the gallon.

Obvious question, are your spark plugs clean after your 200 mile trip ?
I know your MAF sensor is clean, what about the sensors that tell the engine that it's cold, maybe the contacts are dirty and the computer is simply squirting too much gas in _ even after it's warmed up.

One is right by the thermostat housing with two wires coming out of it, this tells the computer the water temperature.

The other one is after the MAF sensor, a little ways down (also two wires), this one senses how cold the air is just before it enters the throttle body.

I'm not sure if a dirty connection here would cause the Check Engine Light to come on though.

Also, on the fuel rail there is the fuel regulator (mine has one anyway).
There is a vacuum line that comes off of it.
This is a safety feature in case the regulator fails, if it does the fuel gets sucked into the engine rather then sprayed all over the engine bay.
If the diaphragm has been compromised, maybe that's where the fuel is going.
I simple vacuum pump test to the regulator should be done _ the regulator should hold vacuum.

 

Yeah, they're clean as far as I can tell. I pulled one of 'em out after the trip. Motorcraft platinum plugs are what I used. Here's one of 'em.


Those sensors I have not cleaned, but I'm thinking I may take some time to do so.

As for the vacuum line, I haven't smelled gas in the engine bay or anything like that, but I'll still have a look and see.

The funny thing is, is the engine certainly appears to run just fine. Starts every time, no hesitation, etc.

I'll give those a try after my salvage yard trip.

 

Something else I'll note. I did a little experiment. On the first ten hour trip, I went the speed limit, much to the discontent of everyone around me. Now this was on a highway. Long, flat roads with rarely a stop for hours. Even then my Ranger was hard pressed to get more than 200. I think the most I got was 220.

On the way back, I matched speeds with the other drivers and much to my surprise, it stayed the same. Still was hard pressed to get more than 200.

 

Have you tested speedometer for accurate speed, which is needed for odometer to be correct.

32.5" tires travel farther in one revolution than 29", so odometer will show less miles than actually traveled, you could be traveling 260miles on 18gal of gas, closer to 15MPG

 

Plugs are nice and clean, so if it is burning extra fuel, it's not very much.

You wouldn't smell gas if there was a slight leak in the diaphragm fuel pressure regulator.
It would simply get sucked back into the intake manifold and be burned.
That is my thinking though, that's where the gas may be going.
The regulator is probably OK, but it's worth checking.
If it was leaking gas, there would be a strong smell of it when you pull the vacuum line off.

Something else: I know very little of auto transmissions, but I would think that yours would have a "lock" torque converter.
Is that functioning properly, I don't know how one could test it, but I know there is one on our delivery truck ?
On our delivery truck if I have a full load of furniture and travelling at high way speed up a steep hill, it kicks down when I stomp on the gas.
The converter slips a bit (as it should) when it changes gears and then you can notice it locking up as it stops slipping when the trucks speed increases.
The load on the engine is noticed as well when it locks.

I would think yours should behave the same.

 

@ RonD: Yes, the speedometer is accurate within 1 MPH of what my GPS says. It's a Garmin if you're curious, not just a phone app.

@Jeff R 1: That sounds like how my auto behaves, although sometimes it takes it a moment to respond when the throttle is opened. I haven't checked that line, but I will here after a break. Just came back from the salvage yard.

 

cameron

1st / switch the spark plugs over to SP412 nickel core , that engine was not designed for platinum plugs .

how long since you replaced the oxygen sensors , might be time for new ones

those years of engine computers where not programmed to sniff combusted ethanol in the exhaust flow ( the by product of combusted ethanol is OXYGEN )

dump some seafoam in the fuel tank to neutralize the ethanol and run a few tank fulls of 91 octane fuel
( see if your fuel economy rises ) if not , then your o2 sensors are no longer functioning

a ob2 reader can monitor o2 sensor readings

 

Cameron, my truck stock with just the topper got 14mpg highway. It went up to almost 20mpg when I switched to 31s. I had 4.10 gearing aw well. Now keep in mind you have live axles so your front end is spinning all the time, adding extra drag to your rolling resistance.

 

@David: Aw, man. I spent a pretty penny on these platinums . I'll have a look and see what I can get some nickel cores for, but that'll be a last resort.

I've got some software that can look at the oxygen sensors. Can you tell me about what they're supposed to read? I'll post my results after I test it.

@Jacob: I thought 99 had part time? And also, how did your mileage go up?

 

Actually you may have the pvh setup. I was thinking you were 2000, not 99. It may have been an option.
Mileage went up because the bigger tire helped compensate for the lower gear ratio.

 

you are just looking to see if the oxygen sensors are transmitting data

slow data transfer indicates failing oxygen sensors , no data means failed oxygen sensors

more than likely your computer is running the engine off of a pre-programmed fuel trim

only a deep computer reset will force the computer to start reading from the new oxygen sensors

i was told this by a veteran mazda mechanic , simply dis-connecting the battery for 30 minutes will not work.

only the dealer has the correct computer and software to do the deep reset

 

Well, here's the waveform if you're interested. During the waveform I brought the engine to 2,000 RPMs and also gave it a couple revs after it settling out. It instantly responded to what I was doing with the throttle.



@Jacob: Ah, makes sense. Too bad they're so expensive.


EDIT: Vacuum line does not smell like fuel.

EDIT2: Ran the part number for the plug through autozone, and it came back with a platinum plug? Also claims it doesn't fit, although I'm not sure if that is to be believed or not. Mine are SP500 finewire platinums.

 

Well if speedo is accurate then odo should be too.

And if spark plugs don't show Rich running, which they don't, then you are leaking fuel from somewhere.
10MPG isn't even close to what you should be getting.

 

you need AGSF-32-C or SP-447 copper core spark plugs

do you see the carbon build-up around the base of the platinum tip , that is bad news

i was reading some interesting news from some mechanics several months ago

the ethanol burned in the combustion chamber was preventing the build-up of carbon IF platinum spark plugs were being used .
but the computer was detecting excess oxygen in the exhaust , thus the computer was feeding more fuel into the cylinders than what was required .
he did not state what the total solution was , but he switched the engine over to nickel plugs .
the customer reported increased mileage after that .

you should run several tanks of 91 octane fuel and see if there is any difference in mileage

 

is that data from the throttle position sensors or the oxygen sensors

 

@David: Oxygen sensors, save for the red and green lines which are coolant related.

I'll take a look on the inside of the plugs and take a photo then report back.

@Ron: Makes sense, if only I could find where the fuel is leaking from, as there's no odor of fuel nor any trail of fuel.



EDIT: Looked at the plugs and there is zero carbon on them. Unfortunately my phone refused to focus on the plug, so I was unable to get a picture. But yeah, no carbon.

Searched the part numbers again. Oreillys returns a copper plug for SP447, although claims the center electrode to be made of platinum. No match for AGSF-32-C. I did find a set of autolight plugs #103 that are completely and totally copper, though, for about 2 bucks each. I'll change out my plug wires first, though. They're the originals as far as I can tell. Trying to wait until after the harborfreight sale.

 

This bit of carbon, I don't think that's there on mine.

 

copied from a master tech on the mustang forums

no such part #, if you want "stocK" platinum heat range in a Motorcraft, it's a AGSF-32-PM which is now a SP-493, if you want the same heat range in a copper core, get a AGSF-32-C or SP-447....if you want 1 heat range down from stock in a Motorcraft platinum, you'll want a AGSF-22-FSM which is now a SP-459 or in a copper core AGSF-22-C which is now a SP-505

 

@Jeff: Didn't even notice that small spec of carbon. That couldn't possibly be enough to cause my issue, would it?

@David: Here after the fifth, I'll probably wind up getting a set of coppers. I know someone who can use my platinum if they want. Still blows my mind how it' running like a clock but is guzzling so badly. The exhaust doesn't even smell bad. Doesn't smell like anything, really. On initial cold startup it's gassy as expected, but once it's warm that does away.


After my tires cool down, I'm going to increase their air pressure to 35 and see if that helps.

 

I'm just pointing out what cheese_man said where the carbon build up is, no matter how small.
I'll pull one of my plugs when I get a chance and see if it's the same.

A point to note is that mine doesn't smell "gassy" on cold start up and we both have 99's and 3 litre engines, so pretty much the same.
Might be something going on there.

 

if the engine when running purrs like a kitten ,

the cylinders are receiving too much fuel ,

an engine when running and using the correct fuel to air ratio , there should be a slight vibration from the engine

the vibration is telling you that the engine computer is making constant adjustments to the air to fuel ratio ( this is good )

 

gassy when cold ??

then the platinum spark plugs are not firing properly

you should not smell raw fuel or ROTTEN EGG smells from the exhaust pipe at all

it takes 22,500 volts for a spark to jump a gap

the factory ignition coils only generate 37,000 volts

copper spark plugs only require 14 to 15,000 volts to fire

nickel spark plugs require 22,500 volts to fire

platinum spark plugs require anywhere from 24 to 29,000 volts to fire properly

 

Would you like me to select some data to view and post my results? I can look at just about everything.

 

you need to monitor the oxygen sensor data while driving

you also need to monitor fuel injector firing duration while driving

have some one do the driving while you do the monitoring