I've got a 97 f150 w/ 4.2, auto & 3.55 lsd with about 2,000 miles on a ford reman engine, 152,000 total on the truck. At around 80,000 miles (around 2001) I noticed a 1/2 to 1 mpg drop in gas mileage. After doing some research it looked like it might be the O2 sensors, from what I've read they have a tendancy to degrade/clog-up with the miles and need to be replaced to restore gas mileage and driveability every 50k or so. I had been putting this off because the gas mileage loss wasn't that bad, I was not having any drivebility issues, and replacing the O2 sensors looks like a real PITA (so my O2 sensors have 152,000 miles on them).
I've ordered 2 motorcraft front O2 sensors from rockauto.com ($119 total, including shipping). Why did I order motorcraft instead of bosh? I have an xcal 3 with mileage tunes from VMP and the gas mileage is the same as stock with the 87-mileage, worse than stock with the 93-mileage. Justin says that if the O2 sensors have more than 50k on them they probably need to be replaced (which I knew, I was just in denial). Justin also said the bosh o2 sensors do not respond quickly enough to work well with his tunes, and supplied this write-up:

http://www.vmptuning.com/forum/showt...ight=o2+sensor

I just thought I would mention this, as anyone thinking about getting a tuner in the future might want to consider spending an extra $15.00 per sensor to get the motorcraft O2 sensors.

Now the question: how the heck do you get to the front O2 sensors to replace them? from the wheel wells? Looks like I might need to take out the inner fender liners? It is just too long of a reach for me from the top & I can't get to them well enough from the bottom.
The haynes manual does not mention how to get to them.

Thanks.

 

Get to them from the wheel well or from underneath. The fender liners push out of the way easily (at least mine did).

Also, you can buy a special socket that has a slot in it for the O2 sensor wires. This allows you to use a ratchet instead of a box end wrench. The sockets are cheap, like $10+- at AutoZone.

It's really not that bad a job. I replaced mine with Bosch sensors at like 110,000 miles. Unfortunately, it did not make any difference in fuel mileage. FWIW, I think my original sensors were also stamped Bosch.

 

It looked like the O2 sensors were original (with 152,000 miles on them). They were bosh, and had the ford oval stamped on them. For anyone doing this on this old of O2 sensors on a V6, the O2 sensor socket really helped. With the socket and a 1-inch box-end wrench, taking the wheel off and removing the O2 sensors from the wheel well seemed to be the easiest. (the 3/8-inch hole in the top of the O2 sensor socket will require something longer the the 8 to 10-inch handle on my 3/8-inch drive ratchet, at least on this old of O2 sensors). The hardest part was laying on top of the engine and unplugging/replugging the passenger side sensor. The only way I could get two hands on it was laying over the top of the engine (and then I couldn't see it). That plug was deformed just a little from resting against the valve cover making unplugging it and plugging it back in much more difficult than the drivers side (it required quite a bit of force).
This is a picture of the originals:

https://www.f150online.com/forums/me...ms-97-4-2.html

I have only driven about 40 miles so I can't attest to any mpg improvement yet, but the engine does seem to run a little better (and most importantly, no check engine light!)

 

I've put about 3,000 miles on the truck since installing the new O2 sensors and I am seeing maybe just less than 1/2 mpg improvement in-town (none on the highway). During that 3,000 miles I also replaced the fuel filter and air filter.
Here's the $ break-down:
new o2 sensors $120.
new fuel filter $10
new air filter $20
vmp tune $420 (using 87 mileage tune, installed Aug. 08, revised email tune installed Oct. 08)

total expense so far: $570.00- for essentially zero gas mileage increase:
20.5 mpg in town, 18 mpg @65, 17 mpg @70+.

Based on the near zero gas mileage increase, I would have to say the original bosh O2 sensors, with over 150,000 miles on them, were still functioning without any significant degradation in performance, so listen to the people that say to buy the bosh and save a few bucks (and the people that say that O2 sensors last a lot longer than 50-60,000 miles).

The truck does seem to be more responsive and runs a little better, so the expense has not been a complete waste. I do have to wonder how many people that have gone with a mileage or performance tune are really getting significantly better gas mileage.

I've recently done the gotts intake mod, though I don't believe I'll see any significant impact on mpg (I'm hoping for more response at wide-open-throttle).
When it comes time to replace the fan-clutch I'll go electric, as quite a few people have indicated that should net at least 1 mpg & 5 or so hp (I'll believe it when I see it).

 

It mainly helped me out due to my poor gas mileage in the first place. Went from 14 mpg on a tank to 19 mpg with tune and CAI driving mostly highways. If you were already getting descent MPG then its not going to help you out a whole lot if any at all.


With the fans its going to be mainly throttle response gained. some gain in hwy mpg has been seen for some but not many.

 

I wasn't putting money on O2's doing much.

Most don't realize that the matrials used in them allow them to go much farther nowadays.

My truck is at 136,000+ and I'm sure still has factory O2s.

22 MPGs doing 70 on the highway seems good to me.

 

I'm going to have to raise a flag on that. My V6 gets at most 19 mpg at 70 mph. I'd like to see some math of how you got a consistant (not instantaneous) 22 mpg...

 

Could be the gas...

I've noticed with winter mix, it's been like 19ish for me... During the summer I get like 22-23 mpg.

 

I've never gotten much more than 17 mpg at 70 mph, sometimes I'm in the mid-16's.

 

I've seen 21 mpg at 70 with pure gas, no ethanol, and the A/C off. I've also seen 15mpg at 75 on crap gas.

 

aftermarket o2 sensors are junk and probly wont pass a e-test due to it but you should change your plugs oil and monitor you tire pressures for gas milage mine have 210,000km on then and there working fine i average 19mpg with i think in a 1998 2wd is pretty good for 210,000 km

 

While I'd agree I havent seen a 20+ mpg tank in a while with this ethanol, I dont think we have "winter blend" gas down here in florida.

 

my sensors have 113,100 some miles on them (never been changed) and i only get 13-14 mpg. but no diffrence in power at all. just worse millage. is this my problem to? on another thread of mine the guys are telling me that i may need to clean the MAF. this may be your problem also.

 

I would disregard Justins graph, somethings definitely not right. - He needs to explain that a little more in detail. For one "Bosch" are OEM for these models, Motorcraft IS NOT. You won't find a Motorcraft O2 sensor in the 97-03 truck engines. The Cobra engines might have a MC O2's, but not these.

On the 4.2 - Intake,Wires,Plugs,MAF,O2's and the correct oil + level. Clean the TB and elbow/ports once in awhile. You should get at least average mileage.

 

sorry im not big on gas engines. what is the TB and elbow ports? and i check my oil about once a week and i always keep it at the top of the safe zone.