P0171/174 At my wit's end
 

I've seemingly read every thread about these trouble codes posted on forums to no avail. I thought I'd make a thread specific to me and see if anyone can help as I'm out of ideas as to what to do.

2002 F150 5.4 liter V8 126,000 miles. Seems to drive fine, but idles slightly rough, more pronounced after it's warm but has never acted like it wants to die, just not a real smooth idle. Service Engine Soon light won't go away after a reset. Gas mileage seems ****ty.

I've verified the lean condition with a scanner. Long term fuel trims running at 29.7 at idle on both banks in 0 degree weather when I checked it. Both drop to around 10-17 while driving, with bank 2 number slightly higher than bank one by a few numbers, if that matters. Here's what I've done so far...

-Checked hoses for cracks, see none, tried doing a smoke test, see no leaks.

-Checked fuel pressure, it's good.

-Cleaned MAF sensor.

-Air filter looks good.

-Tested O2 sensors by inducing a rich condition, they jumped to .9 volts.

-Cleaned IAF valve.

-Checked electronic vacuum regulator. Has voltage and ohms at 30.

-Have vacuum at the PCV valve.

-Thought my DPFE sensor may have been the cause. I have 5 volts supply to it, but at idle on a cold start, the output voltage from the DPFE is about 1 volt. Haynes manual calls for .3 to .7 volts on a cold start as there should be no EGR. I replaced the DPFE, and still have the same voltage reading.

Any ideas would be so greatly appreciated. I really don't know what to do anymore, other than take it to a mechanic, but at this point I'm committed to figure it out on my own, with your help, of course. ;)

http://www.aa1car.com/library/ford_lean_codes.htm

The link may or may not help. Good luck with it.

Bob

Vacuum leak. The PCV system is the usual source particularly the elbow at the back of the throttle body. If you have not removed and inspected it do that.

just a shot in the dark, but have you cleaned the upper intake EGR Passages? I have a 2002 4.6 and I pulled the upper intake elbow behind the throttle body at about 115K miles and had a large carbon buildup on the passages for the EGR. It also had a rough idle, and after cleaning these passages, the idle smoothed out.
I did not have a lean code, but a year later, I got a stumbling idle again, and traced it down to the hard plastic line for the PCV had a crack, and one of the rubber elbows was deteriorated enough to have a crack in it as well. Something worth looking at i guess.
Have you looked at the vacuum lines that control your heating? Checked your brake booster vacuum line? those things start to deteriorate, so keep an eye on them.

No but I've inundated it with carb cleaner and wrapped my hand around it with the engine running and no change in the idle. Will remove tomorrow and visually inspect.

Have checked vacuum line and it's good. Haven't checked heating lines or EGR hoses for carbon. Will do so. Thanks.

I'd do that. 90 percent of the time that elbow is the problem. Physically remove and inspect. Also just an FYI carb cleaner is really bad for the rubber and plastic on modern engines.

Cleaning the throttle body should be on every ones to do list of 100k maintenance.

You have a lean condition, which infers a vacuum leak of some type. The best way to find it is to have a smoke test performed. You can do your own smoke test to find your vacuum leak using a cigar (cough, cough) and a piece of hose connected to your vacuum system. Put a rubber glove over the Throttle body so you won't have any smoke escaping and puff on the cigar and blow it into the tube connected to the vacuum system. I did this on my son's Buick GS and we found the injector o-rings were leaking. Once we fixed it, the lean code went away.

Best of Luck!

That rarely works. The PCM is too fast and you'll never notice any change.

Hook up your scanner and monitor the short term fuel trims. Hit suspect areas with your flammable spray, I usually use brake parts cleaner since I usually have it on hand, and watch those trims. When the extra fuel is sucked in, you'll see a distinct change in the STFTs.

You might have leaks at the exhaust manifolds that is letting air in and creating a lean reading at the sensors.

Pulled the PCV elbow and inspected. Looks pristine, no cracks. Will check for exhaust manifold leaks. Wouldn't that be accompanied with a noise though? Is there any reason I couldn't blow a smoke test up the exhaust pipe and watch for manifold leaks??

Does anyone have an idea as to why I'd be reading 1 volt off the DPFE sensor when Haynes calls for .3-.7 volts on cold start up?

Thanks for the replies so far.

The Haynes manual is incorrect. The value should be around 1 volt when the EGR valve is not open.

Follow the test I gave you in post #10.

Have you checked the line itself? Also, check the connections on the other valve cover, where the fresh air enters from the back of the intake plenum down to the valve cover.

The 171/174 codes are for both banks as the problem is common to both.
An air leak sufficient over time is detected by both front Ox sensors.
This detection of excess oxygen causes a shift in the long term fuel trim tables to 29% + the max it can shift them. This generates the codes you have.
The EGR is not the cause of these lean codes. If it were you would hear the motor ping under load and run rough.
There is a big difference between going lean from exhaust gas and lean from excessive air. The reaction in the cylinder is completely different.
Your missing the air leak.
On the rear of the throttle body and out of sight is where the PCV hose is connected as well as another smaller hose.
The rubber Ell often goes soft from crank case fumes and gets sucked open with a hole or it could go hard and crack.
Start there and go over it all again.
Good luck.