Super high fuel trims and P0171/0174

I have read through a few threads about this, but can't seem to find anything related to my exact issue. So here's a go at my first post on here.

2005 OR 74k 6 speed manual.

A bit of background info maybe not necessary but....

I got this truck from my uncle about 7 months ago. It was maintained, well, poorly. I received it still with; og diff fluid, Trans case fluid, and gear box fluid, og coolant, ps fluid and pcv valve. Og sparkplugs, Ac was out, needed tires. Just to put in perceptive it's state.

it also had a strange hesitstion underload, And finally, the famous P0500.

Some TLC, a few parts. Send the dash out to be fixed professionally, and finally I let out a sigh of relief.....just to get kicked in the nuts. Lol

Now to the issue. As mentioned above it had a weird hesitation, I hoped fixing the dash and p0500 would maybe fix this.

It did not. In fact, 30-50 miles after installing the fixed gauge cluster, it did the weird hesitstion, and then i get a check engine light.... I think, F#&$ ME i was wrong it's a wire or the ecm causing the P0500. So I pull out the code scanner and to my surprise I have both the p0171 and 0174 codes bank 1 and 2 lean.

Stupidly I reset them without looking at the freeze data. However I decided to look at my fuel trims and boy oh boy are they acting weird.

IDLE: BOTH BANKS 25% 30%
DRIVING: 25-34%
WOT (BUT ONLY AFTER 3.5K RPM): hooyah!!!! They drop to 4!!!!

Maybe I just need to drive faster and it's a me problem.

What i tried so far is check for vaccumm leaks, inspect and clean maf, replace intake filter.

At this point im leaning towards my fuel pump starting to go, anything else I can look at while I try to get my hands on a fuel pressure gauge? Anything else easy to check?

Anyways I hope I gave good info. All above poor maintenence things were fixed for about a month or 2 before I sent the dash out. To my and my uncle's knowledge the only code it's ever throw until today was the p0500. And it has not come back since the fix although only 50 miles ago.

Thanks fellas.

 

I think I'd want to check fuel pressure.

The reason the fuel trims drop at WOT is because it's going into open loop so it's not using fuel trim, you would need to look at AFR and O2 data to see if your still lean or not.

 

okay good to know. I can pull the individual P.I.D and graph the 02 and the Long term on my way to work tomorrow just to see. As for checking the fuel pressure, do you have a personal preference as to where? i see most people say to just T into the supply side where the fitting is. Adding a Pic. Mine was actually leaking here and I had to repair it. This also didn't fix my hesitsrion issue lol rip.

Is 41 to 46 psi is what I'm looking for at or near the rail?

I don't currently have a fuel pressure gauge but i am always down to add to the collection. if you have a suggestion there

thanks

 

That's the easiest spot to tee in, only other spot is under the truck where the lines from the tank go to the lines on the frame.

Fuel pressure should be pretty steady at around 41psi even under load and not drop below 25psi for at least 5 min after the truck is turned off.

This should get the job done and is relatively inexpensive.

$35 BETOOLL Fuel Pressure Test Kit 0-100PSI with 9.497.896.30 Fuel Line Fittings

https://www.amazon.com/dp/B07LF26ZTJ

 

Good looks man, I am going to order that and give it go. I appreciate the first answer I got is from someone with the tag test don't guess haha

In the mean time, any thoughts it could be a vacuum leak I'm missing? like the intake runner gasket or something? i was thinking smoke test or try the classic spritz starter fluid around hoses Etc..

 

Yea too many people (and may "professionals") fire the parts cannon instead of testing and finding the actual problem. I'm the opposite, I wana prove something is bad before I replace it.

Typically with a vacuum leak you would have high positive trims at idle and they would improve as RPM and load increase, your trims appear to be pretty flat throughout the RPM and load range which would suggest a base fuel/engine issue.

 

okay, so pressure gauge will be here friday. i will test then. if results are not as stated. is there a way to check the regulator VS the fuel pump to see if either or both are bad? or is that not how it works lol.

if the pressures are okay, does that mean injectors, bad things, nightmares? lol

 

Don't see regulators fail very often, usually when they do the failure mode is leaking fuel from the vacuum port.

If pressure is low and you want to rule out the regulator you can disconnect the return line, put it in a container, and see if you have any fuel flowing from it.

If fuel pressure is in spec we'll have to do some further testing, a VE test to verify MAF accuracy would probably be my next step.

Don't generally see all injectors restricted at once but it's possible, now if someone replaced them with some Amazon/ebay 6 for the price of 1 junk yea I can see that being an issue for sure.

 

Welcome to TW…

 

I came across this. Some informative reading.

https://gtc.ca/blog/long-term-fuel-trim-and-short-trim-fuel-trim/

 

i dont know if i fully understand this one. so basically the regulator will not allow back flow when functioning normally? does this need to be done after the system is depressurized, Ie pulling the pump relay and turning over the motor. or?

My uncle only had the truck serviced at the dealer ship. from all the records we got it looks like the only thing done in 20 years aside from oil changes was spark plugs and a throttle body/intake cleaning. they did this after my uncle had brought it there for the hesitation issue. this was about 3 years ago. i cant say if they ever even checked the fuel trims Etc as again, it had never thrown this code or any code at all other than the P0500.

so they ran injector cleaner, cleaned the TB and "upper intake" and sent him on his way.( obviously not the fix but maybe helped) however this is the same dealer ship that sold him a new compressor, but never cleaned the filter too or looked at the expansion valve. It was filled with metal when i checked it, because shocker, the AC wasn't cold when i got it.... blah blah blah i could go on for quite some time about this dealerships tom foolery they did with my uncle.

when i took ownership i did well, everything i mentioned earlier. so they should still be OEM injectors long story short.

 

Thank you! "long time" lurker, finally found something i wasn't able to find in the threads after 7 moths and countless fixes from TW

 

TL;DR:
Your long-term +25–30% fuel trims on both banks mean the ECU is adding a ton of fuel to compensate for a lean condition. Since this happens on both banks, it's not likely to be a specific injector or O2 sensor. More likely culprits:

Most Likely Causes:

1. MAF Sensor Underreporting Airflow (Even After Cleaning)
   
   • MAF issues can cause both banks to run lean if it's underestimating airflow.
     
     
   • Cleaning isn't always enough. If it's original, consider replacing it (Denso only).
     
     
   • Check live airflow readings at idle (should be ~2–3 g/s) and at cruise (~30+ g/s at 60 mph).
     
2. Fuel Delivery Issue (Pump or Regulator)
   
   • Fuel trims dropping under WOT supports this — ECU goes open loop at WOT and ignores trims.
     
     
   • Weak fuel pressure = lean in closed loop, normal at WOT.
     
     
   • A clogged fuel filter (if not in-tank or never changed) or a dying fuel pump can easily cause this.
     
3. Vacuum Leak Post-MAF (Even Small Ones)
   
   • A cracked vacuum hose, brake booster line, intake manifold gasket, or PCV hose will lean both banks.
     
     
   • Spray carb/brake cleaner around the intake while idling to see if trims drop or idle changes.
     
4. Exhaust Leak Pre-O2 Sensor
   
   • This will allow fresh air in and cause false lean readings.
     
     
   • Check for leaks at the manifold-to-head or manifold-to-pipe flanges.
     

✅ What You're Doing Right:

• Checking fuel trims at various RPMs and load.
  
  
• Cleaning the MAF and inspecting for vacuum leaks.
  
  
• Understanding that P0171 + P0174 together = likely system-wide, not bank-specific.
  

Additional Quick Checks:

• Fuel pressure test (you’re already planning this — do it ASAP).
  
  • Spec is usually around 45–55 psi. Check both at idle and under load if you can.
    
• Smoke test for intake/vacuum leaks if carb spray didn’t help.
  
  
• Check MAF readings vs RPM with scanner — may not throw a code even when bad.
  
  
• O2 sensor behavior — are they switching properly at idle and cruise?
  
  
• PCV valve & hose — you replaced it, but double-check hose isn’t cracked.
  

⚠️ Less Likely, But Possible:

• Intake manifold gasket leak (common with age).
  
  
• ECU or sensor ground issues, especially if the cluster/dash was monkeyed with.
  
  
• Clogged injectors — but usually not both banks at once unless bad fuel/filter.
  

My Top 3 Suspects:

1. MAF sensor underreporting.
   
   
2. Weak fuel pump or clogged filter.
   
   
3. Vacuum leak post-MAF.
   

 

The regulator regulates fuel pressure by bypassing fuel and returning it to the tank when pressure goes above it's setpoint so if pressure is low it shouldn't be bypassing anything.
There should be no pressure in the return line but there will be fuel in it.
I would unhook the return line and put it in a container then command the fuel pump on with a scan tool but without one you would either need to start the engine or jump pins 3 & 5 of the COPN Relay to run the pump.

Ah ok, just checking because those Amazon/ebay shit parts get a ton of people in trouble.

 

okay i will check while cruising, i do believe the one picture shows it around 19 g/s which was while going slow, then you'll see the 74 and 110 g/s i was on it in those, idk speed really they were 2nd gear pulls. so ill check at a steady state and report back. side note i have 2 vehicles, should i pause driving the taco a lot until this is resolved. my head says yes, leans bad.

other wise i will check fuel pressure and double check for vacuum leaks at the suggested areas. i have good understanding of anything mechanical, i don't quite know how the O2's should act though...

 

i also want to add a piece of random possibly useful info. sometimes, while at cruise, i feel the truck "come alive" like my foots been in the same spot at ~65mph then all of sudden its accelerating to 75 plus. this sensation quickly fades, and i never thought much of it. as it goes back to normally pretty quick. maybe that points towards fuel pump going out as well? final food for thought before bed. i will report back with new info after work tomorrow

 

MAF accuracy can really only be effectively checked at idle and and on a WOT pull.

At hot idle MAF Grams per Second (g/S) should be about equal to engine displacement in liters so for a 4.0L engine it should be right at 4 g/S.

For the WOT reading we want to do what's called a VE (Volumetric Efficiency) test, do a WOT pull through 1st gear and record peak RPM, peak MAF, IAT, and the barometric pressure then enter them in one of the calculators below.
You want to be above 85%, ideally closer to 100%

https://atgtraining.com/atg-volumetric-efficiency-calculator/
https://otctools.com/ve

Probably wouldn't be a bad idea to get the issue resolved before driving it too much although it does look like we're still in fuel control but not by much (usually around +/-50% total is max fuel trim limit).