Rich condition, no CEL, poor mpg

[infamousjax]

2001 mountaineer v8 AWD long term fuel trims showing -10 and -8... It runs decent but I'm getting only like 12mpg in the city with the air on.

O2 upstreams oscillating like normal, but one of those downstream stays at like .3 and the other bank downstream shows at like .8

I know downstream doesn't effect performance but kind of interesting to see. I may have a super small exhaust leak that stops when it gets warm

I have replaced:
IAC
TPS
PCV
Spark plugs
Cleaned MAF
Cleaned throttle body

I'm looking towards the EGR system now... Can a clogged tube or bad DPFE cause this rich condition?

Trying to squeeze out the best mileage I can get out of the old girl

Any advice is welcome! Thanks

 

[V8]

I had a MAF sensor go bad and overfuel my '01 5.0. The condition was apparently caused by the IAT (Intake Air Temperature) in the MAF going bad and telling the engine it was running too lean for temp. A Snap-On scanner failed to catch this condition, but a an el cheapo $25.00 code reader finally gave me the IAT code. That MAF was just over a year old at the time. Don't hesitate to try different scanners.I did occasionally get lean codes for either or both banks that went away after I cleared the codes, only to return erratically. On the dark side of this, the overfuel condition cost me the #1 CAT on bank 2. I also have been finding a number of small vacuum leaks and I removed the intake plenum to facilitate my search and destroy mission for vacuum leaks. It's just time for me to put this issue away, and I'm replacing all the heater hoses as well. Might as well make the best of the situation.

As an aside. my 'sploder averages 13 mpg year 'round in town.

 

[infamousjax]

I had a MAF sensor go bad and overfuel my '01 5.0. The condition was apparently caused by the IAT (Intake Air Temperature) in the MAF going bad and telling the engine it was running too lean for temp. A Snap-On scanner failed to catch this condition, but a an el cheapo $25.00 code reader finally gave me the IAT code. That MAF was just over a year old at the time. Don't hesitate to try different scanners.I did occasionally get lean codes for either or both banks that went away after I cleared the codes, only to return erratically. On the dark side of this, the overfuel condition cost me the #1 CAT on bank 2. I also have been finding a number of small vacuum leaks and I removed the intake plenum to facilitate my search and destroy mission for vacuum leaks. It's just time for me to put this issue away, and I'm replacing all the heater hoses as well. Might as well make the best of the situation.

As an aside. my 'sploder averages 13 mpg year 'round in town.

Thanks for the reply
scanner says IAT looks good... About 2x the ambient temp.

I might buy MAF and throw it in there if I cant fix this issue though.

She runs pretty good as it is, I guess my MPG is about normal. These things sure do guzzle fuel

 

[J_C]

Yes, I also get around 12 MPG in town with my '98.

 

[V8]

Thanks for the reply
scanner says IAT looks good... About 2x the ambient temp.

I might buy MAF and throw it in there if I cant fix this issue though.

She runs pretty good as it is, I guess my MPG is about normal. These things sure do guzzle fuel

I mentioned this because it was so off the wall as to seem silly. You might try another scanner if the problem persists. My bank one O2 sensor #1 showed good, I've never had an O2 sensor code. However, when we put the "big" scanner on it it showed very low voltage output. It appears we didn't get a code because the O2 sensor was putting out at least something.