Possible causes of a decline in MPG

[ptf18]

Fellows. The wife's '02 S/T fuel mileage has dropped noticeably since July/August. She JUST mentioned it to me the other day when I was riding with her and she had to get fuel.

There's 220k on the meter. I do all the preventative maintenance per the Ford owners manual...and on time.

The S/T runs fine although she did mention that the engine hunts (rpm goes up/down) just abit as she can hear it at a stop light although she doesn't see the tach needle move.

Its not a weather issue as "apparently" this MPG decline had started in July/August. Maybe sooner.

Anyways. Any suggestion on where to start looking fo a cause? The "usual" maintenance items are up to date per the FoMoCo manual ie, plugs, filters, fluids, tire pressures, etc.

Thanks.

What about the O2 sensors? The installed units are the original OEM units

 

[koda2000]

Spark plugs, ignition wires, O2 sensors, coolant temp sensor, leaking fuel injector, air temp sensor, MAF sensor,,, ? Any codes?

Lots of possibilities to consider. You need to narrow it down. When's the last time you did a tune up? How many miles on your O2 sensors.

 

[410Fortune]

start with tire pressure and alignment
Then tune up including fuel filter and air filter

A bad 02 sensor will give you a computer code, so if no codes the 02 sensors are working fine.

 

[ptf18]

Koda and 410. Thanks. No codes. The S/T drives fine. When she gets home I'll look in the log book I keep in the vehicle of when I did a "tune up', etc. As I mentioned, I follow the FoMoCo Owners manual to the T on schedule maintenance. I would say I replaced the plugs and wires at 200K (220k on the meter currently). Same with fluiids and filters ALL done per the FoMoCo schedule.

I use Motorcraft parts.

I'm wondering about a 'weak" O2 sensor as they all have 220k on them. I have ODBwiz and will plug it in and see what it tells me... I have it but have NEVER used it to any degree.

 

[oklagp1200r]

Change out all the O2 sensors even tho no cel from them they are way past due for a change!
How much of a change in fuel mileage are we talking?

 

[410Fortune]

they do recommend changing 02 sensors at 60-100K miles even if they are working properly
Big drops in MPG in my experience are usually tune up items like:
air filter, fuel filter, tire pressures
followed by a bad wheel bearing or dragging brake, bad cv axle, bad U joint,
I like to do the "roll test" how easily will the vehicle roll in Neutral? down a small hill?
Jack up each wheel and tire and spin it, any resistance?

 

[CDW6212R]

Ditto. Change the front two O2 sensors. The other O2's are redundant checking sensors, leave those forever if they don't throw a code.

 

[ptf18]

Fellows. Thank you all for your input. Fuel mileage drop is about 4ish maybe 5 miles per gallon. From what I see in the fuel log book she keeps that has been going on since mid-summer...at least. Should I be "amazed" that a "decline" in mpg was occurring and there was no mention (to me at least)????

As I mentioned I stay atop of ALL the FoMoCo maintenance items listed in the "02 S/T owners manual and have since we purchased the vehicle new in '02. In addition I do change ALL the fluids (not so much the transmission or rear end....sealed for life???) on a recurring basis. Shocks are replaced at 40K as they are life time warranty items and it only involves my labor.

I have replaced "normal" wear and tear items ie brakes, rotors, drums, ball joints, all body bushings and or course tires at which time front end alignments done.

There has been a few "off" repair items over the years... thermostat housing which was replaced with an aluminum unit. Upper and lower radiator hoses were changed when I replaced the radiator and "plastic" fan due to cracks in both items.. The coolant system items were done this summer when the S/T had about 215k on the meter. I have always used MotorCraft parts. Had an oil filter housing oil leak That was somewhat of a head scratcher. to figure out but did get it fixed this summer. There is probably a few other items I'ld don't recall with the log book in my lap.

The S/T "rolls" fine in neutral as I pushed it back and forth in the driveway. Tire wear is even across the treads. I rotate them every 10k using the direction per the owners manual. T.P. is checked every 2-3 weeks, or when we have wide temperature changes.

I am a aircraft mechanic by trade so doing "maintenance" and taking care of stuff (mechanical) is some what in my "blood. Have to admit that at 60+ its not as much... "fun".... as it was when I was....16. .

I have a laptop OBDII diagnostic ap from ScanTool. I get alot of info from using it.... but for the life of me.... I really don't know what I'm looking at. I have had the ap for a year or so and never really had a need to use it. I plugged it in to the S/T and ran it. Found that the post-cat O2 sensor voltage (on a graft) varied more than from what I have read on the "net". As I said I don't really know what I'm looking at so........

I have purchased and received ALL 3 O2 sensors (Motorcraft). Looking at the currently installed units, the 3 look "easy" enough to access. The connector for the #1 (pass side) is what I can not see. Any input on where its at???.

I do not find a definet schedule replacement time for the O2 sensors....from Ford.

From what I have found on the 'net 100k...plus/minus 20k.... for the 4 wire heated O2 sensors is the "recommendation". I suspect its like alot of automotive "stuff" that the manufactures don't want to list to potential new car owners as this "stuff" would only add to the long term maintenance cost. On the other hand most owners today could care less about most (any) maintenance.... until things don't work or break..

Anyways....when it quits raining here I'm going to get the sensors changed. I'll keep you all posted.

Again thanks for all of your interest and suggestions... its what makes ExplorerForum what it is.

ptf18

 

[CDW6212R]

Some of the O2 sensor connectors are very hard to reach, long arms and some scratches are common. The V8 has one that's up top and you basically do it by feel.

The O2 sensors can cause big MPG drops. You've got enough mileage I'd hope for that to be it.

The shocks can go 40-75k easily, they randomly wear out at long ranges of mileage. I'd do the transmission fluid at least every 100k, the gear oil about the same too. It takes synthetic 75-140 in back, the front I like 75-90 using Severe Gear from Amsoil. The Severe Gear is IMO the best and way cheaper than Ford at the dealer($25+ a quart). Amsoil has gone up to about $15 a quart now.

Does that ScanTool device display inlet air temps, IAT? Be sure that sensor is showing a close amount above ambient all of the time. Much older Fords(80's) had poor IAT sensors that could mess up A/F ratios, even a broken wire to the sensor could do that. If your IAT's seem okay, then that's not the issue.

How old are the spark plugs? OEM specs may say 100k is good, but being in that long creates seizing issues. I prefer 50k for plugs, no matter what version or how much they cost. You don't ever want to have to deal with damaged or stripped threads.

 

[410Fortune]

I will say dang nice work thus far! Somebody who actually reads and follows the manual! WOW just WOW, your truck should last you forever if you keep this up

The transmission can be serviced! Drop the pan, let it drain, change the filter and add a drain plug. Then over the next year drain the pan 3-4 more times. Each time you will lose 4-6 quarts of Mercon V then next summer drop the pan again and change the filter again. Cheap insurance that your 5r55e will last a long time! No backflush machines!!

The rear axle also can be serviced and should be, 2.5 quarts of 85-140W full synthetic with friction modifier if you have a limited slip (I like to add the modifier even to open diffs it helps with foaming)

Let us know what you find out!

 

[koda2000]

If you find you can absolutely not get at the O2 connector(s) there should be a large removable access plate under the carpet, on the trans runnel. It requires removing the console and pulling back the carpeting so that you can get to it though. In case you're not aware, it really helps to use an O2 socket on the sensors (buy or borrow).

I don't believe in "sealed for life" maintenance on the rear diff. around 100k seems like a reasonable interval to me for the rear diff. Don't forget the friction modifier if you have LSD. The transmission should be done more frequently. Ford recommends 30k, but that seems excessive to me. I try to change my fluid around 50k.

While changing the rear diff fluid you may want to get into some additional considerations, like axle seals/bearings, pinion seal and LSD friction clutches. Your going to be spending around $55-$60 on fluids + sealant for the cover and you don't really want to do this job more than once if you find any signs of leaks. The LSD clutches just wear out over time which isn't critical, but depends on whether you want to keep your LSD functionality.

 

[CDW6212R]

The LSD clutches only last as long as the rear tires aren't spinning much differently. They can go 200k, or just months if you have one tire off pavement a lot, stopped, and spin the tire leaving. Those cost more now, about $75, or the better CF stuff they make for later Mustangs, $120 or so. If the gear oil is changed regularly, then the axle bearings can last much longer than if the gear oil is left in for 20 years etc. I do the axle bearings and seals when I buy a vehicle, and in normal use they should go 100k or more. The axle seals are weak links, the constant heat will slowly wear them out. Bearings are $15-20, the seals around $5.

 

[swshawaii]

Fellows. Thank you all for your input. Fuel mileage drop is about 4ish maybe 5 miles per gallon. From what I see in the fuel log book she keeps that has been going on since mid-summer...at least. Should I be "amazed" that a "decline" in mpg was occurring and there was no mention (to me at least)????

As I mentioned I stay atop of ALL the FoMoCo maintenance items listed in the "02 S/T owners manual and have since we purchased the vehicle new in '02. In addition I do change ALL the fluids (not so much the transmission or rear end....sealed for life???) on a recurring basis.

I am a aircraft mechanic by trade so doing "maintenance" and taking care of stuff (mechanical) is some what in my "blood. Have to admit that at 60+ its not as much... "fun".... as it was when I was....16.

Being an aircraft tech not questioning you.
Was your wife calculating MPG using the "top off" method in the owners manual or using the fuel gauge?
Owner’s Manual Printing 4 (PDF) (Page 171)

Calculating fuel economy

1. Fill the tank and record the odometer reading.
2. Each time you fill up the tank, record the amount of fuel added.
3. After at least 3 fill-ups, fill the fuel tank and record the current odometer reading.
4. Subtract your initial odometer reading from the current reading. Use one of the following simple calculations to determine fuel economy

• Liters x 100 total kilometers
• Miles traveled gallons used

Ford's scheduled maintenance guide does not recommend changing transmission/transaxle fluid and filter until 150,000 miles for "normal schedule" use.

Rear axle lube also at 150K for RWD only, no 4WD shown. No intervals shown after 150K.
Scheduled Maintenance Guide Printing 3 (PDF) (Page 32)

 

[Turdle]

Check the rear drum brakes are releasing well, not dragging.

Also, make sure the "od off" function works, and, the light for it also works.
check to see if torque converter lock up is functioning correctly-- 2200 or so rpm's at 65mph

 

[ptf18]

Again thanks all. Let me answer some of your questions.

CDW IAT: I just "installed" it into the Scan Tool program. The plugs (MotorCraft SP500) were new at 200k (20k miles ago).

Everyone: Never have opened the rear end. I check the fluid level at engine oil changes. No leaks around the input or left and right bearing seals. How do I know if I have a LSD or the clutches if I do have a LSD? Is it something that I should change out if I have the cover off? For that matter how can I tell what rear end I have anyways? The vehicle VIN sticky tag is nearly wiped clean of its writing. The wife has ALWAYS driven the vehicle like a Grandma...easy...too easy on both the gas pedal and brake pedal. I changed transmission fluid and filter at 150k. I used Motorcraft fluids and filters. After changing the filter and refilling the transmission I pulled the hose going to the AUX cooler and with the engine running, pumped out fluid a quart or 2 at a time. I'ld shut down the engine, top off the transmission and restart the engine again pumping out a couple quarts of "old" fluid'. I did this until I got clean bright red color fluid out and then called it good. I think I used 19 or 20 quarts of fluid...$$$$$. Have had no issues with the transmision but it is something I've always been concerned about. The vehicle has only towed one time and that was a camping trailer for about 3k miles during its 220k life and that was back in 2005.

Koda: Thanks for the "tip" about the access panel....under the center console..... Thanks Ford.... I sure hop I don't have to go that route.


Swshawaii: She uses the "top off" method. I realize that measuring MPG is a wishy washy topic. But I use it more for trend monitoring....such as in my current situation. Just wish the current "drop" would have been brought to my "attention" before the weather turned into the upper 20"s here in Texas.

Turdle: The rear drums are "free". I just had them off (looking at the shoes) when I rotated the tires at 220K (every 10k). I've never fooled with the OD button. What do you recommend to "test" this system. Like wise with the lock up of the torque converter?

Fellows. I really appreciate the input. The wife bought this vehicle new and payed for it. Its one of her "joys' although its starting to need a paint redo. I've looked at new vehicles as a possible replacement but the manufactures are building them with more "stuff" that I personally don't see as needed. BUT the majority of buyers like the "toy" stuff even though I suspect alot of the"toy" stuff never gets used as time goes by.....oh...thats right..... the same buyers get a new vehicle every couple of years so......

I just crawled under the vehicle...again... to look at the #1 bank O2 sensor connector....I think I can access it thru the R/H fenderwell if I pull the splash liner. The others "appear" to be...easier to access.


Now if it will warm up a bit.

Thanks all.

ptf18

 

[410Fortune]

The drivers door sticker there is a code under the AXLE designation, what is that code?
This will tell you the gear ratio you have in your differentials and if the rear axle is limited slip or not

 

[koda2000]

If you can't read your axle code on the door jam sticker, you can look up your VIN number on Ford's ETIS site and look at your truck's build details.

 

[CDW6212R]

The drivers door sticker there is a code under the AXLE designation, what is that code?
This will tell you the gear ratio you have in your differentials and if the rear axle is limited slip or not

Ditto. Find the axle code on the VIN tag. You want to see a letter there, numbers are an open diff, a letter means it's got an LSD. The LSD clutches are kind of easy to change, only the stupid tiny 8mm bolt that locks in the cross shaft is hell. It either loosens, or it breaks, and you cuss a lot, plus the repair.

The V6 trans holds maybe 12 quarts or a hair over, total from dry. Dropping the pan will get maybe 6, the rest is stuck in the TC and the upper passages. Pulling the cooler line is an "okay" method to pump more of the trapped fluid out. I've only done that once, with my 99 V6/5R55E, when the trans broke pulling out of my driveway. I got it back parked, pulled the hose and let it pump until it began to spit, and shut the engine off. Then when I dropped the pan, it had little in it. I'd guess I got maybe 9 quarts total out of it. I didn't measure what went into the big coolant pan. I recall it took close to 9 quarts to fill it, I had bought a case. My trans was broken, I found one small portion of a bearing shell in the pan. The error code was excessive EPC pressure, thus a big internal leak.

The V8 trans has a drain plug in the torque converters, those are easy to get another gallon out of.

 

[ptf18]

The drivers door sticker there is a code under the AXLE designation, what is that code?
This will tell you the gear ratio you have in your differentials and if the rear axle is limited slip or not

Fellows. The numbers/letters on the diff tag are:

S662A
10 88 2D03

Any idea what this tells me?

 

[koda2000]

Fellows. The numbers/letters on the diff tag are:

S662A
10 88 2D03

Any idea what this tells me?

There should be two tags on the rear diff cover. One should look like the image below. This one's for a 3:73 limited slip (3L73) with 8.8" ring gear.

Have you tried looking up your productions specs on ETIS?
FordEtis home page


Ford Axle Codes type/capacity/ratio
(Driver's door jam sticker codes)
44 Regular 2640 3.73
47 Regular 2500 4.10
D2 Limited Slip 2500 3.45
D4 Limited Slip 2640 3.73
D7 Limited Slip 2500 4.10
Explorer 43 Open 3200 3.08
41 Open 3200 3.27
42 Open 4.10
46 Open 3.73
45 Open 3200 3.55
D4 Limited Slip 3200 3.73
D2 Limited Slip 4.10
L73 Limited Slip 3.73