'02 F150 V6 - Lean... or not?

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2002 Ford F150, 4.2L/auto 2WD. Approx. 213K miles.

Background: did intake gaskets on this truck about 2 years ago and has run well since. The downstream O2 sensors were hacked off during an engine swap before it came under my care but that hasn't affected its operation since, that I could tell.

Recently it began to "cough" or have a slight stumble when under load, at speed, and a lean code showed up on both banks. No visible vacuum leaks, no response to carb. cleaner sprayed around the intake, etc. Upon finding fuel pressure at the minimum allowable value with quick leak-down but 80 PSI and no leakdown with the return plugged, I replaced the fuel pressure regulator - no improvement to the lean/stumble.

Upon watching the sensor data, the upstream O2's aren't acting at all like they should, and I'm seeing momentary, random voltage gaps in TPS and MAF without touching anything. After observing this I'm hesitant to start spending money looking for air leaks or buying sensors if it could be a larger electronic problem. Any advice?
 
Just a sample of hot/idle oxygen sensor data.

F4CFA691-9FCE-4FD3-9C17-D86DCF373E8A.jpeg
 
I thought I got a shot of the MAF dropping to zero too, but I guess not - the TPS and MAF would independently, maybe once every minute or two, drop to 0V for a split second.

0CCECA1D-BC29-4790-8018-246A06DD324F.jpeg
 
Take a look at the EGR system. I only say that because I had a Taurus that would act like it was running real lean all of the time (of course this was way before shops used scanners to diag stuff). Turns out the DFPE sensor was bad, but not bad enough and it caused the EGR valve to be working slightly wrong all the time.
 
Originally Posted by Alex_V
2002 Ford F150, 4.2L/auto 2WD. Approx. 213K miles.

Background: did intake gaskets on this truck about 2 years ago and has run well since. The downstream O2 sensors were hacked off during an engine swap before it came under my care but that hasn't affected its operation since, that I could tell.

Recently it began to "cough" or have a slight stumble when under load, at speed, and a lean code showed up on both banks. No visible vacuum leaks, no response to carb. cleaner sprayed around the intake, etc. Upon finding fuel pressure at the minimum allowable value with quick leak-down but 80 PSI and no leakdown with the return plugged, I replaced the fuel pressure regulator - no improvement to the lean/stumble.

Upon watching the sensor data, the upstream O2's aren't acting at all like they should, and I'm seeing momentary, random voltage gaps in TPS and MAF without touching anything. After observing this I'm hesitant to start spending money looking for air leaks or buying sensors if it could be a larger electronic problem. Any advice?



Did you ONLY do the intake gaskets or did you also put in new isolator bolts? The problem with these engines is that the factory isolator bolts on the lower intake plenum weren't built to withstand oil exposure, and well the PCV system causes oil mist to coat the entire plenum internals.

Miller88 has a good point about the DPFE as well. The revised DPFE part should have a small dome on top so that it can 'breathe'. The faulty original version was flat on top, and eventual moisture exposure would cause it to fail throwing an OBD code.

But I can tell you this - I had the exact same truck and before I traded it in with 130K miles, it was really moody about engine performance. My gas mileage dropped considerably with no obvious signs of trouble and I never figured out why.
 
Did you clean the MAF and throttle body, making sure the throttle plate is closing fully?

What is your actual LTFT? I know at some point it exceed 25% to throw a DTC but when you take it out for a run, does the LTFT drop to 8 or below and then go back high at idle? The effects of a vacuum leak are pronounced at idle but when you open the throttle up, the effects are negligible if at all. If your LTFT drops to a more normal % at WOT, then you more than likely have a vacuum leak somewhere (intake hose, pcv hose, brake booster hose, evap). If it doesn't drop, then your lean condition is likely fuel pressure/fuel delivery related (injectors, filter, pressure related).

(the odds are pretty low that multiple O2's went bad on BOTH banks simultaneously)
 
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Originally Posted by Miller88
Take a look at the EGR system. I only say that because I had a Taurus that would act like it was running real lean all of the time (of course this was way before shops used scanners to diag stuff). Turns out the DFPE sensor was bad, but not bad enough and it caused the EGR valve to be working slightly wrong all the time.


This. If if has the aluminum depe sensor replace it, the exhaust gasses would corrode the sensor internally. The updated one is plastic.
 
What do the fuel trims look like? If they correct under load.....I usually suspect a vacuum leak. If the get worse under load.....I suspect a fueling issue.
Hook up the fuel pressure gauge & drive it while observing the gauge.
These engines are notorious for the variable intake runner arm retaining clip/s to break & the arm fall off the control rod.

The TPS voltage drop-off is of interest.....If you trust the scan data/logging. I'd definitely suspect the TPS! Low TPS voltage at higher actual throttle angles will make it run extremely lean.

Rich....High O2 voltage following heavy throttle input is completely normal.
 
Clinebarger: I didn't monitor fuel pressure while driving - if this data suggests that could be the cause I can go back and do that. The variable intake runner linkages are hooked up.

What I'm seeing is short term fuel trims staying within reasonable (I think) proximity to zero.

Long-term trims are staying consistently high, seemingly a bit worse on one bank than the other.

The last two frames show driving, at sustained speed, in OD (frame 2) and 3rd gear (frame 3). I was attempting to get conditions of lower airflow/higher load vs. higher airflow/lower load - if an air leak is the problem, fuel trims would possibly be further out of whack at lower engine speed with higher load, yes?
 
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Your B1 LTFT is 6.2 and B2 is sitting at 4.7, I personally wouldn't be concerned. For older vehicles like yours, as long as your LTFT is within 10pts of 0 either way, this is considered acceptable. If the LTFT is greater than 10% positive or negative, the PCM is compensating for more than just normal wear-and-tear.

(none of your LTFT values at the different rpm's exceed 10% and fwiw my LTFT goes between 2 and +8%, just depends..I get lower LTFT's on long highway runs than stop/go in town)
 
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Originally Posted by Alex_V
Thanks for the input Mad Hatter. Any info I can gain in this area is more than I currently possess!

No worries brother..from your LTFT values you can get a sense of the condition of your cylinders and valves, whether or not they're sealing properly allowing air in (intake) or unspent fuel out (exhaust). With those LTFT numbers I'd say you're doing alright for a 17yr old engine.

Here's an interesting read ...

Internal Engine Condition and FT's
 
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