Major Mileage Drop

[Ranxeroxlubna]

Hi, looked through the other threads and didn't find anything that resembled a solution, so here we go.

1998 Merc Mountaineer, AWD 5.0 with 157000 miles. Automatic Trans. 255 70R/16 tires and factory aluminum wheels. I keep them at 35 PSI in the winter and around 38 in the summer.

This truck has only ever had synthetic fluids in it from jump; engine, trans, transfer-case, differentials, even the grease. Regular maintenance and new filters as required.

I was getting around 15-16 mpg, with my style of driving (I like to go fast) for the last year or so, (I keep track with every tank) got around 20 mpg on the highway using cruise.

This summer, as it was idling a bit rough, I decided to do some regular maintenance and I cleaned the carbon off the MAF, the IAC, the intake manifold and the throttle body per instructions I found on the Mustang site. I only used Intake Cleaner specified for new vehicles

Still rough, but not as bad.

I replaced the wires with OEM wires (I had replace the plugs with OEM plugs some months back which ironically put it at a solid 17 mpg + around town) and that seemed to do the trick. Smooth as silk.

As I mentioned in another thread, I was getting an O2 sensor code off and on, so I replaced it with a OEM O2 sensor and it seems to have taken care of the code problem.

I also replaced the thermostat with 195 degree one as per the manual.

The issue is, since the change of wires and O2 sensor, my mileage has dropped to less than 12, if I baby it, and around 9 if I don't around town. Mileage on the highway is just a wee bit better. This was before the change to winter gas here in Minnesota.

Even though it runs smoother than I can remember it running for some time, somethings obviously off, but for all my efforts to hunt the issue(s) down, I can't find anything amiss.

Ideas?

 

[Rhett]

I'm not a big 5.0 guy, but doesn't your truck have more than one O2 sensor? Did you change all of them? I've had bad O2s without any CEL before...

How's the fuel pressure regulator? Usually if its bad you will get poor running condition though. I guess you're not getting any trouble codes.

If you truck has an EGR system, make sure it's not leaking.

 

[Ranxeroxlubna]

I'm not a big 5.0 guy, but doesn't your truck have more than one O2 sensor? Did you change all of them? I've had bad O2s without any CEL before...

How's the fuel pressure regulator? Usually if its bad you will get poor running condition though. I guess you're not getting any trouble codes.

If you truck has an EGR system, make sure it's not leaking.

The other O2 sensors checked out fine on the computer. I have not checked the fuel pressure. Any idea where I can find the information on nominal running specs so I can do a running computer check?

The EGR system seems to be working fine. But I will double check.

 

[Turdle]

That mileage drop indicates to me a bad spark plug or wire. Just because they are recent, do not rule them out. It is very easy to mis route a spark wire where it gets heat damaged. I've also seen brand new out of the box spark plugs Which were not so sparkly-


What I would do is remove the spark plugs starting with bank 1, lay them in order and check the color of the electrodes. They should be exactly the same shade of very light tan. If in doubt take a picture of them, then do the other side.

If there is a problem with the ignition it will be pretty apparent, and the off color will help determine what the issue is.

If this all checks good, then I would suspect the "new" 02 sensor. Your truck has 4 of them, and using a "cheap" version might cause all kinds of problems.

You might have also changed a primary sensor out and replaced it with a secondary type, which will not work as well. You mentioned an OEM sensor, but did you actually buy it from a Ford dealer? OEM means bosch to me, not genuine ford.





Lets us know what you find.

edit,
I referred to your "p0153" thread and read that you replaced the bank 1 sensor 2

p0153 code is for bank 2 sensor 1

There is a huge difference. What was the location on the truck of the sensor you replaced? Was it up above the exhaust, in a tight area to get to?
Bank 2 sensor one is the one right below the transmission, the easiest one to see. It is also very common for it's wiring to fall loose and rub on the front drive shaft-so if you messed with it you might check this out.


edit edit thread moved to the stock 1995-2001 explorer sub forum.

 

[Ranxeroxlubna]

That mileage drop indicates to me a bad spark plug or wire. Just because they are recent, do not rule them out. It is very easy to mis route a spark wire where it gets heat damaged. I've also seen brand new out of the box spark plugs Which were not so sparkly-


What I would do is remove the spark plugs starting with bank 1, lay them in order and check the color of the electrodes. They should be exactly the same shade of very light tan. If in doubt take a picture of them, then do the other side.

If there is a problem with the ignition it will be pretty apparent, and the off color will help determine what the issue is.

If this all checks good, then I would suspect the "new" 02 sensor. Your truck has 4 of them, and using a "cheap" version might cause all kinds of problems.

You might have also changed a primary sensor out and replaced it with a secondary type, which will not work as well. You mentioned an OEM sensor, but did you actually buy it from a Ford dealer? OEM means bosch to me, not genuine ford.





Lets us know what you find.

I will check the wires and plugs.

I replaced the O2 sensor with a factory Bosch unit, and I did check it was the primary one. I bought it at Napa, and I did check the numbers on the unit and the box against the number it is supposed to have in it.

Bosch is the manufacturer of Fords O2 Sensors. The Ford Dealer mechanic told me this is the case and showed me one of theirs, which clearly stated Bosch on it. I know this guy and have known him for many years. It's why he gave me a break and told me what to use and how to get at the plug.

I'll have to wait until the weekend on on the wires.

 

[1997XLTRollover]

I'm very interested to see what you find. My 96 5.0 is getting the same mileage you are. Runs fine but mileage is HORRIBLE and plugs indicate a severe lean condition.

 

[Turdle]

I will check the wires and plugs.

I replaced the O2 sensor with a factory Bosch unit, and I did check it was the primary one. I bought it at Napa, and I did check the numbers on the unit and the box against the number it is supposed to have in it.

Bosch is the manufacturer of Fords O2 Sensors. The Ford Dealer mechanic told me this is the case and showed me one of theirs, which clearly stated Bosch on it. I know this guy and have known him for many years. It's why he gave me a break and told me what to use and how to get at the plug.

I'll have to wait until the weekend on on the wires.

Not trying to cause an argument, however, if you were to have changed the proper sensor, you would not have to be shown how to get at the plug. It's right there. However, if you changed the bank one sensor 2 ( wrong one) the plug would be hard to get to.

And, just because it is a bosch sensor does not mean it is the correct unit. Bosch made both the upstream ( engine control, --faster reaction) and downstream ( cat monitor-slow reaction) sensors.

so, I wonder if , by chance, you installed a downstream sensor in the correct place, or, an upstream sensor in the incorrect place.

Once again, what was the physical location, on your truck, of the sensor you replaced? Can you shoot a picture of it?

 

[Ranxeroxlubna]

Not trying to cause an argument, however, if you were to have changed the proper sensor, you would not have to be shown how to get at the plug. It's right there. However, if you changed the bank one sensor 2 ( wrong one) the plug would be hard to get to.

And, just because it is a bosch sensor does not mean it is the correct unit. Bosch made both the upstream ( engine control, --faster reaction) and downstream ( cat monitor-slow reaction) sensors.

so, I wonder if , by chance, you installed a downstream sensor in the correct place, or, an upstream sensor in the incorrect place.

Once again, what was the physical location, on your truck, of the sensor you replaced? Can you shoot a picture of it?

Hi! Thanks for your input...

After doing a lot of research on my friends directions, I found that on Explorers and Mountaineers of this vintage, all but 1 of the O2 sensors is tucked up in the transmission tunnel. Bank 1 - sensor 2, and Bank 2 - sensors 1 & 2 live here.

I also found that this means you have to remove the center floor console - pull back the carpet - open an access panel at the top of the trans tunnel and VIOLA!, you have access to the remaining 3, O2 sensors. They are still a pain to get to, but after a great deal of research on this, and other sites to confirm my friends directions, I don't think it's possible to reach these O2 sensors any other way.

As I mentioned, I checked out the numbers on the O2 sensor I installed against my (Online) Ford shop manual, the box it came in and the part itself. It is the upstream part, for the correct side. Short of checking it's pedigree to it's origin, I did everything I could to get the correct part into it's place.

Code P0153 - Bank 2 - Sensor 1 was the code that came up, and that I corrected.

Not sure where Bank 1, Sensor 2 is from...?

So, once I got the access panel off, I could clearly see all 3 sensors, the Bank 2 sensors are easy to spot; 1 in front (closest to the exhaust manifold) of the little catalytic converter, and 1 downstream. I replaced the 1 closest to the exhaust manifold.

I wish I could shoot you a picture, but I'm not in the mood to completely disassemble my truck again...It's dang cold here in Minnesooootaah.

 

[Turdle]

Incorrect.

Bank 2 sensor 1 is in the open,

Bank 2 sensor 2, , plus bank 1 sensors 1 and 2 are in the tunnel



Like I said before. Bank 2 sensor 1 is in the pipe directly below the transmission, with the plug right above it. Take a look for yourself. Also check if it's wire is rubbing the front drive shaft.


If you change a sensor in the trans tunnel, you changed the wrong sensor.

 

[Ranxeroxlubna]

Incorrect.

Bank 2 sensor 1 is in the open,

Bank 2 sensor 2, , plus bank 1 sensors 1 and 2 are in the tunnel



Like I said before. Bank 2 sensor 1 is in the pipe directly below the transmission, with the plug right above it. Take a look for yourself. Also check if it's wire is rubbing the front drive shaft.


If you change a sensor in the trans tunnel, you changed the wrong sensor.

You are correct. Once I was told it was the passengers side, I didn't bother to check otherwise. A lot of trouble for nothing it seems.

As they are both upstream sensors, would that sensor diminish performance?

 

[Turdle]

You are correct. Once I was told it was the passengers side, I didn't bother to check otherwise. A lot of trouble for nothing it seems.

As they are both upstream sensors, would that sensor diminish performance?

Yes
Upstream sensors are the input for the pcm to determine proper fuel air mixtures, among other things

Downstream sensors merely monitor if the cats are working or not.

Once again, get a good 02 sensor. the name Bosch doesn't mean anything, They make 99% of the 02 sensors out there. They are then tested and manufacturers get the best of the batch.

you need a good "enthusiastic" or fast acting sensor for sensor 1 on both sides.

 

[koda2000]

When I first bought my '00 AWD 5.0L Mountaineer (during February) the fuel economy was terrible (12-13 MPG highway). I also noticed that the temp gauge was barely moving above "C", but I was getting decent heat. At first I was thinking the temp sender might be bad, but then the CEL came on with a P0125 code (insufficient coolant temp to enter closed loop mode). So I changed the thermostat and the temp gauge started reading normally (approx center of the gauge) and my fuel economy jumped up about 9-10 MPG highway to the low 20's. I suppose a bad O2 sensor could also cause a "failure to enter closed loop mode", by making the PCM think the engine needed more fuel.

Where is your temp gauge reading? Also, stick/rub you figure in the end of your exhaust pipe. Does it come out black and sooty? It should be pretty clean (this is a little easier than removing all your spark plugs right off).

It's pretty obvious you're engine is running rich. The question is why. It might be O2 sensor related, coolant temp or coolant temp sensor related, or perhaps a bad fuel injector. If you pull all your spark plugs and find a sooty one (or if they're all sooty) that will narrow things down. Also check your fuel pressure at the fuel rail. Engine running, it should be around 35-40 PSI. I believe a '98 still used a return line to the fuel tank to bleed off excess fuel pressure, where later models had their FPR in the fuel tank and ran higher fuel pressure at the rail (65-67 PSI at the rail).

BTW, If you have O2 sensor(s) behind the CAT(s) they have no effect on fuel economy, they only measure CAT efficiency.

 

[Ranxeroxlubna]

Yes
Upstream sensors are the input for the pcm to determine proper fuel air mixtures, among other things

Downstream sensors merely monitor if the cats are working or not.

Once again, get a good 02 sensor. the name Bosch doesn't mean anything, They make 99% of the 02 sensors out there. They are then tested and manufacturers get the best of the batch.

you need a good "enthusiastic" or fast acting sensor for sensor 1 on both sides.

Yes, the upstream sensor on the opposite bank would affect performance or no?

As I changed the wrong sensor, I'll change it now. I ask only because I don't want to disassemble my truck again, and yank out that sensor. If I have to, then I will order the opposing O2 sensor and just swap them out.

I'll get it from the Ford Dealer nearby and slap my friend in the head.

 

[Turdle]

Yes, the upstream sensor on the opposite bank would affect performance or no?

As I changed the wrong sensor, I'll change it now. I ask only because I don't want to disassemble my truck again, and yank out that sensor. If I have to, then I will order the opposing O2 sensor and just swap them out.

I'll get it from the Ford Dealer nearby and slap my friend in the head.

To back up
I believe the sensor you changed was actually a downstream sensor.

I know it is easy for me to spend your money, but if the truck has over 125k miles on it, and if an upstream sensor failed, it might be best to replace the upstream pair.

Keep in mind the truck has 2 catalytic converters on each side. A primary catalytic converter and a secondary redundant catalytic converter to catch what the first one would spew if it fails.




The bank 1 primary sensor is right in the bank 1 primary cat tucked up on the down pipe between the frame and engine on passenger side.

as stated bank 2 sensor 1 ( and the bank 2 primary cat) is right below the transmission

 

[Ranxeroxlubna]

To back up
I believe the sensor you changed was actually a downstream sensor.

I know it is easy for me to spend your money, but if the truck has over 125k miles on it, and if an upstream sensor failed, it might be best to replace the upstream pair.

Keep in mind the truck has 2 catalytic converters on each side. A primary catalytic converter and a secondary redundant catalytic converter to catch what the first one would spew if it fails.




The bank 1 primary sensor is right in the bank 1 primary cat tucked up on the down pipe between the frame and engine on passenger side.

as stated bank 2 sensor 1 ( and the bank 2 primary cat) is right below the transmission

I get it. I know there are 4 cats, and I did follow the piping from the exhaust manifold while under the vehicle, and then from inside the car.

I'm a bit **** about details and I marked the tube while underneath so I could see it from above. It's not difficult and it is in plain view. I put the upstream sensor in the wrong upstream side.

I have been working on vehicles since my first Honda 50 back in the 70's. I have done most of the work on my trucks, including 2 transmissions; 1 transfer case; Several front hubs, ball-joints; finding an unmetered vacuum leak in the dead of winter; 1 radiator; 1 turbocharger; a new gas tank, fuel pump and gas lines (rust); a new oil pump and pan (rust got the pan and the oil pump was weak) and literally dozens of other things on the 4 vehicles I currently own - Ford SOHC 4.0 4WD; the current Merc, my wife's 2005 PT Turbo and my dads 93 C2500 HD that I can't part with...plus, it's in fantastic shape.

I didn't verify what side - passenger or drivers, because I was told the sensor was "here, on the passengers side" by the dealers mechanic. I should have, and lesson learned.

Let me break it down.

I changed the sensor 1, bank 1.

I should have changed sensor 1, bank 2.

The question; does it matter if said replacement sensor 1, in position 1, bank 1 affects things negatively, as far as the operation of the motor? Of should I swap them out?

Thank you in advance for your help.

 

[Ranxeroxlubna]

Tip of the cap to Turdle!

Turdle you were correct. Thank you!

Upon close inspection, Sensor 1 on B2 had a bad wire/connector - the pin wiggled more than it should have in the connector. This caused code P0153 to appear, then disappear randomly. It drove me nuts.

I ordered a new Sensor 1 for Bank 1.

I removed and swapped the S1 I had previously & incorrectly installed into B1; installing it into B2, where it was supposed to go, and installed newly purchased S1 into B1 yesterday.

Topped off tank and I am tracking the STFT/LTFT percentages on my scanner while I'm driving, it's being recorded on my laptop. The LTFT for both banks seems to be hovering near 0.7% after it is warmed up and cruising at 65 MPH. The STFT's never go past +/- 2.3% and generally are around 1.5% or less.

This looks positive...however, I will drive until the tank needs refill and let you know what happens.

Thank you again for all your help, and suggestions.

More to follow...

 

[Turdle]

I am confident you have resolved your issue.

Thanks for being patient. You deserve a pat on the back because I was being kinda pushy.

 

[Ranxeroxlubna]

When I first bought my '00 AWD 5.0L Mountaineer (during February) the fuel economy was terrible (12-13 MPG highway). I also noticed that the temp gauge was barely moving above "C", but I was getting decent heat. At first I was thinking the temp sender might be bad, but then the CEL came on with a P0125 code (insufficient coolant temp to enter closed loop mode). So I changed the thermostat and the temp gauge started reading normally (approx center of the gauge) and my fuel economy jumped up about 9-10 MPG highway to the low 20's. I suppose a bad O2 sensor could also cause a "failure to enter closed loop mode", by making the PCM think the engine needed more fuel.

Where is your temp gauge reading? Also, stick/rub you figure in the end of your exhaust pipe. Does it come out black and sooty? It should be pretty clean (this is a little easier than removing all your spark plugs right off).


It's pretty obvious you're engine is running rich. The question is why. It might be O2 sensor related, coolant temp or coolant temp sensor related, or perhaps a bad fuel injector. If you pull all your spark plugs and find a sooty one (or if they're all sooty) that will narrow things down. Also check your fuel pressure at the fuel rail. Engine running, it should be around 35-40 PSI. I believe a '98 still used a return line to the fuel tank to bleed off excess fuel pressure, where later models had their FPR in the fuel tank and ran higher fuel pressure at the rail (65-67 PSI at the rail).

BTW, If you have O2 sensor(s) behind the CAT(s) they have no effect on fuel economy, they only measure CAT efficiency.

Temp is 194-196 degrees when operating during normal operations.
Tires are 35 PSI cold.
New Ford 195 degree thermostat
Plugs are clean, light gray/brownish - dry
exhaust is sooty
Computer says EFI fuel pressure is 43-45 PSI, which is good per this site - http://www.therangerstation.com/tech_library/5_0_swap_in_newer_ranger.html
The O2 sensor was bad...still.

 

[Ranxeroxlubna]

I am confident you have resolved your issue.

Thanks for being patient. You deserve a pat on the back because I was being kinda pushy.

I apologize for the confusion on the S1/B2 - S1/B1 Designations. After a while, it was a bit like "she sells sea shells..." I should have just said drivers side and passenger side.

Plus, the mechanic who steered me in the wrong direction got me the part at his cost and bought me a burrito for lunch.

 

[oharris]

And, just because it is a bosch sensor does not mean it is the correct unit. Bosch made both the upstream ( engine control, --faster reaction) and downstream ( cat monitor-slow reaction) sensors.

I am getting codes for O2 sensors, one upstream one down stream, so I thought I'd go ahead and replace both of the upstream sensors. What is the correct part numbers? I tried to do some research but got lots of conflicting info. Some say "use Motorcraft, Bosch are junk" Others say Bosch is the same thing. Do you have a good part number I can look for? '97 Mountaineer 5.0.