So I bought a 2000 b4000 with 260k km on it, was told it ran but barely, said screw it and bought it. Took the heads off and both were cracked along with the headgasket blown, replaced the headgasket, plugs, wires, thermostat, valve cover gaskets, intake gaskets, stem seals and everything along with new heads, it runs pretty good now but it feels like it's got a little miss and throws codes p0171 and p0174, just wondering if I missed an important seal to change or where it's most likely loosing vacuum from, thank you!

 

Welcome to the forum

P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)

When you have BOTH sides/banks of an engine with the same code type then cause would have to be something common to both sides

Just a quick how it works
The computer calculates Fuel Injector OPEN TIME based on size of engine and RPMs, computer knows its running a 4 LITRE engine, so it knows exactly how much air will come in at any RPM, its just math
Computer is also programmed to expect 60psi fuel pressure at each injector

Air/Fuel mix for gasoline is 14.7:1 , this is a WEIGHT RATIO
14.7 pounds of air to 1 pound of gasoline
This is why fuel injectors are rated in pounds/hour, i.e. 4.0l uses injectors rated as 14lb/hr, so they can squirt out 14 pounds of gasoline in 1 hour, thats 2 gallons of gas, lol

Anyway what computer does not know is the WEIGHT of the air, air weight changes a lot, its heavier at sea level and lighter at say 5,000ft Denver location
Air is also lighter on a warm day than on a cold day, "hot air rises", because it lighter than the cooler air around it

This is what the MAF sensor is for, "Mass" Air Flow sensor, Mass is weight
As said, computer already knows volume of air a 4 litre engine will pull in at any RPM, but it relies on the MAF sensor to give it the correct weight of that air
An air leak between MAF sensor and valves in the head, means the WEIGHT will be wrong, it will be light because it never was weighed, so a vacuum leak is not a problem because of "extra air" coming it, as said again, computer already knows the volume of the air, just math, its the missing weight that's the issue

Computer does its OPEN TIME calculation
It opens the injectors for that length of time, say 100ms
It THEN looks at the upstream O2 sensors on each Bank
O2s only see Oxygen, not fuel
O2s are self powered
0.1volt is high oxygen, Lean
0.9volt is low oxygen, Rich

Computer tries to keep these O2s at 0.4volts, by adjusting the OPEN TIME of the injectors
If the calculated injector open time is more than 20% off then computer will set a code
If computer opened injectors for 100ms and saw .2volt it would open injectors longer and longer until it saw 0.4volt, and if that was 120ms then 20% off and it sets a code
Engine was NEVER actually running Lean, that's not what those codes mean, all this adjusting takes maybe 2 seconds


So both banks Lean
Could be upper intake air leak, lower intake would only effect one side
Dirty MAF sensor
Low fuel pressure, there is no fuel pressure sensor
OR
OLD O2s sensors, O2 sensors last 12 years or 100k miles, after that they start to run out of chemicals to generate voltage, so lower voltage, they still kind of work so no "failed O2" codes
And if it was burning coolant that also fouls up O2 sensors
So I would first change the upstream O2 sensors if you don't know their age, may not be the problem but can be, and lean codes are based on O2 sensor feedback

 

Thank you for the very in depth explanation, sounds like I know where to start