I have and 02 4.2L F150 that keeps giving me the P171 and P174 trouble codes. In other words, the left and right banks are running lean. The truck has 104K miles. Roughly 6 months ago the lower intake manifold was replaced (original cracked and I caught it early) with a used one for budget reasons 95k miles at the time. About a month later, the trouble codes popped up. I did move over to a cold air intake and have checked and sealed any possible leaks. I don't know how how to check the PCV valve because all of the repair manuals describe doing it on the V8's and I just get lost. At this point, my only guess besides the PCV valve is that the intake gaskets have gone bad. Any ideas?

 

You could also try cleaning the MAF with MAF cleaner. That sometimwes causes them. http://www.obd-codes.com/p0171 http://www.obd-codes.com/p0174

 

That has not helped. I've cleaned it several times and the. Reset the computer but it is still not clearing it. Like I said I've sealed from the MAF back on the air intake (air tem sensor, pcv breather). So I'm really at the point of just guessing.

 

There could be an exhaust leak in the O2 sensor area too. How about the EGR tubing? Also spray around the different areas on the intake to see if you can locate a leak. Are all the vacuum hoses okay? sometimes the one to the PCV leaks.

 

I know I'm dealing with this on my truck (4.6L) and it turned out to be the elbow that goes into the intake manifold. I watched a video that said the elbows break down from the constant contact with the oil. In the video he talked about the tubes collapsing from the weakness and that's exactly what was happening on mine even though there was a huge hole in the pipe at the bend. A new tube with both fittings was $30 from Ford. The thing was, the hole didn't make the usual "hissing" sound so I couldn't tell it was there just by listening. It's worth a check...

 

Here is a dumb question, when were the plugs changed?

 

I thought i would share my experience w the p174 code. My 05 5.4l began to stutter severely under mild acceleration and got progressively worse. My pcm never alerted me but the code showed up in my programmer. I tried the suggestions i pulled off forums...began by cleaning MAF sensor w electrical cleaner, checked into hoses or vacuum leaks...no luck. My next plan was pcv valve but i found out it was an entire assembly which cost $180 at the dealership. My plan then was to due cheapest options first...fuel filter than o2 sensor and up. I did the fuel filter today and it did the trick, my truck runs better than it has in months and the avg. mpg has gone up about 3mpg.
I struggled to figure this out and it was frustrating the **** out of me so i hope my experience is helpful to someone else.
*fuel filter requires filter disconnect tool....$3 at parts store.

 

Check out the thread I posted yesterday where I was getting the P0171 plus a P0303 code - https://www.f150forum.com/f6/error-c...1-p0303-62154/

 

Yep clean your MAF and replace your fuel filter. Just had this problem last week and doing this cleared it right up.

 

It's either not enough fuel or too much air... the 4.2s were know for intake manifold leaks... the isolator bolts loosen up and the gaskets go bad... but your lower intake plenum was replaced so I'm sure everything was checked out...

Like a couple said replace the fuel filter... that has helped people before... My problem with lean codes was the connection to the O2 sensor was corroded, I just cleaned it and everything's been fine for a year now... the pcv should be in your valve cover somwhere.