John Brookshire

Long and short - 03 4.2 with the P0171 and P0174 codes.

I changed the isolator bolts two years back and the IMRC bushings. DPFE valve, IAC valve, EGR valve, PCV valve, all 02 sensors, and MAF have all been replaced in the last two years (this truck was not very well maintained before my purchase, but was low mileage @ 103k)

Evap valve has been tested and is good. Fuel pressure at the rails tests good. Smoke tested multiple times from multiple inlets and I cannot locate any vacuum leaks, which was my first suspect given the lower - I also paid particular attention to the PCV boot and tube)

All fuel trims under load look good (-1.6 to +6), but as soon as I let off the gas LTFT goes to 28.1 in both banks.

I havent been able to test for exhaust leaks before the rear 02, but under visual inspection they look good (truck has never been outside of GA, so nearly no undercarriage rust)

What am I missing? Maybe plugged cats? I changed out both valve cover gaskets a year back and both are now leaking pretty good, which is what made me think I've got some excessive backpressure.

I'm at my wits end with this thing and I have to get the CEL off for required emissions.

Any more thoughts are appreciated.

Dee Veloper

Since the lean condition affects both sides it must be caused by something common to both sides. With no vacuum leaks and good fuel pressure, I would be suspect of the MAF. Even though it was replaced only 2 years ago, it's worth double checking, especially if it isn't a Motorcraft product.

Do a smoke test through the tail pipe and watch for any exhaust leaks. Plugged cats seem unlikely.

Otherwise, you may need to get a better scanner to monitor real time events when going from no load to load. Or take it into shop for a thorough diagnostic. They usually have better diagnostic equipment.

John Brookshire

Voltage on the MAF tests good on the driveway, but that thought crossed my mind as well.

I hadn't made it that far yet, but it was on my weekend list

My scanner does live data. Everything looks good until I let off the throttle. Even at 3000 RPMs the second I let off the gas the fuel trims hit 28.1.

At the moment I'm fairly broke - we have a dog that had unexpected compilations from a surgery and cost a large chunk of cash.

Thank you for the insights and suggestions. You reaffirmed a few thoughts i had as well.

Dee Veloper

I know you checked for a vacuum leak but that data is indicative of a vacuum leak. See attached video.

John Brookshire

I read up and watch videos such as this thoroughly right after i bought the truck 3 years ago, which led me to the upper intake gasket and isolator bolt repair.

This time I'm stumped. I bought a smoke machine because I was certain I was missing a vacuum leak with propane. I've checked high, low, side to side -- Waited until dark and used a flashlight and the only area that smokes is at the air filter, which is obviously pre-MAF.

BareBonesXL

What happens at idle? After things settle down. Not really clear what you're describing. Do the trims stay high at idle? Or do they shoot up when the throttle is closed then drop down to normal?

And what does freeze frame show? High trims for a moment might be what you see on live data but might not be what sets the codes. Freeze frame shows conditions that caused the code to set. Engine temperature is important since if they set cold you need to test cold.

Dee Veloper

One other thought is that voltage tests are not always conclusive evidence of a working MAF. A check is required for the barometric readings as well. Faulty baro readings can produce a lean condition as well.

John Brookshire

LTFT is 28.1 when the engine is cold and warm, unless rhe throttle is pressed, at which point it normalizes.

At idle, 28.1. At 3000 RPMs/no throttle 28.1.

Long and short is under all engine conditions with no throttle applied, LTFT is 28.1. I might add it was 26.4 until I cleaned the MAF, at which point it increased, leading me to think the MAF is faulty, as was suggested.

I tested under freeze frame conditions, which were set cold. I wiped the codes and it set under warm conditions as well.

ProjectSHO89

Dee and BBxl both offered excellent observations.

An additional thing to consider is that the PCM utilizes multiple internal trim tables, generally based on load, speed, atmospheric pressure and the like. It is quite likely that the behavior you observed at different throttle positions is due to the PCM shifting between different tables, each of which may have had previous adaptions. That's why you have to reset KAM after doing anything before testing the next thing.

You indicated you'd tested with propane but you didn't mention the HOW. If you were spraying and listening, well, that usually doesn't work as the PCM is way too fast in adjusting so you'd never hear any change. You have to watch the live data of your four fuel trims while spraying to see where any intake tract vacuum leak might be.

FWIW, I've never heard of a restricted exhaust causing "lean" codes. Neither did the engineers who wrote the service literature.

I've attached a TSB that Ford issued a long time ago to help techs diagnose various concerns including yours. Section F and the related "Tips" section are to be your primary focus.

Coleroad

100% you have a vacuum leak. Those symptoms could also be a exhaust manifold leak but highly unlikely since it would have to on bothers sides. If lean fuel trims improve with rpm that is proof that it is a vacuum leak and or exhaust leak before the upstream O2, but as stated for the exhaust it would have to be doing it bilaterally. Some harder to find leak possibilities, brake booster, cracked intake manifold, throttle body shaft bushings. Keep looking because it's leaking somewhere.
also just because a part is new or newer doesn't mean it's a good part.