'01 5.4 computer says 'lean' (codes P0171&P0174) but my nose says that it is running rich (smell gas in exhaust)

which do I beleive? (ie what is doing that?)

 

Just guessing

I've been told the up stream 02 sensors can give the exact opposite reading if there Completely fu(/3d up.

Dirty MAF sensor heating Element

 

My bad, should have added the MAF cleaned then replaced and O2s are new Motorcraft units

 

Both, the code means lean condition detected, so the computer fattens up the mixture. You are running rich and can cook the cats.

What motor and year are we talking about here? Most common is an air leak after the MAF. Could also be an exhaust leak, but not likely.

 

2001 5.4 Triton. I used a whole can of propane looking for a vac leak and found nothing.

 

Most common on the 5.4's is the PCV elbow at the back of the throttle body, but it could be anywhere.

The problem with hunting with propane is the computer adjusts the mixture so fast it may be hard to pick up on the rpm change. You should probably be watching live data on a scanner while spraying the propane.

It could be losing vac to the heater system. With the heat blowing to the floor does it switch to the defroster when you accelerate? Just a thought.

Also take a good look at the hard vac lines that run behind the battery. Then check the vac reservoir, the black ball looking thing by the battery.

Plus you didn't say if you have an exhaust leak.

If you type 171/174 for the keyword in the search box you will get a ton of hits, and your solution will probably be there.

Good luck and don't forget to post what the fix is once you nail it down.

 

That's very correct. The computer does correct it to fast for the engine noticeably change RPMs. I watch the short term fuel trims although watching the instantaneous O2 Sensor voltages will also work. Brake or carb cleaner also works.

 

O2 Sensors NEVER cause this code pair, they are only reporting what they are measuring.

MAF sensor - sometimes. It is almost always a vacuum leak problem or a weak fuel supply.

 

Ok had truck put on scanner. O2 sensors reading 0.04 and Short Term fuel trim 28% (cold start). All of a sudden O2 sensors go to 0.8 and stay there. Fuel trim drops quickly to 0 then goes all the way to -16 with no change in O2 sensor reading.

What is up with that?

 

Vacuum leak.

The computer, when it tried to enter closed loop operation, was adjusting the fuel trim to try to get the sensors to start switching. When they reach the limit of enrichment without positive result, the computer essentially gives up trying and declares a fault.

 

I agree it sure looks like it (if it looks like a duck). However all soft hoses have been replaced and hard hoses (and vac tank) checked with a vac pump.

What kind of vacuum leak only shows itself in cold weather?

 

The kind that comes from materials contracting in the cold.

Post # 7 tells how to find it. Your procedure in post # 5 doesn't work as jgger and I explained.

 

I'm not sure a scanner would help me track changes that well, when it does it the idle cycles from 1200 to nearly stalling every 1 to 2 seconds

 

Gee, why didn't you mention something that important..... you're about a week late and you've wasted everyone time...

Take it to someone who knows what they are doing and pay for it to be fixed.

 

While I understand your statement, it was not my intent to waste anyone's time. I asked the question in the way I did because I had already asked the question 'why is the idle rough'. Lots of other people have asked that question and their answers have all been tried and failed. If that had been included (except in response to a post) it would have led to the same answers that as noted, did not help.

When you don't find something on Google, you don't assume it's not there, you rephrase your search. This post was my attempt to understand the subsystems of fuel control to aid me in the larger quest of why the truck idles rough.

You also suggested paying someone to work on it. The top three local shops and the only dealer that is not a 4 hour drive away tried and failed. I have been chasing this cold weather only issue for 5 years now.

So to all of you responding to my post, I am sorry if you feel your time wasted. This far into the project I am trying to be creative in research and diagnostic. My thought is that if I can not see directly what it is, the only course is elimination of what it is not. Thank you all for the posts, every bit does help.