P0171 and P0174, where to start?

[GrumpierGrunt]

Hi, 1999 Ford Ranger 2WD Auto single cab 130k miles.

My truck has been throwing 0171and 0174 codes for running lean. My initial research suggested this was a vacuum leak, but I think it might be a fuel delivery issue, as the long term fuel trims really get lean during a period of acceleration.

My STFT seem like they’re in good shape, but at about 2300 RPM while accelerating my LTFM shoot and max out at about +25. STFT look good at

At about 2400 RPM at hard acceleration both the STFT and LTFT tend to sync up, to about + 10. I don’t think is lean enough to throw a code

I had this issue come up earlier in October. I just reset the computer by disconnecting the battery and dumped in a can of octane booster. About 100 miles before the codes came up that time, I had filled my truck up with a tank of flex fuel. Made it back and drove about 100 miles before the light came on. In this most recent case, the code came on about 40 miles after filling the tank up with regular gas. Not sure if a coincidence or what, but I normally drop in twenty to thirty bucks at a time.

I replaced the fuel pump, fuel filter, and injectors over the summer. Also a MAF and Valve Cover Gaskets. But have put many miles on those new parts before having this issue.

Where to start? Should I reset the computer and test the fuel pump pressure?

 

[RonD]

Lean and Rich codes are solely based on O2 sensor feedback

O2 sensors are the only sensors that wear out
12 years or 100k miles and they are done

Yes, the most common reason for both banks to show Lean codes would be upper intake vacuum leak

But if O2 sensors are old........................then you are basing diagnostics on false data

How old are your O2 sensors?
1999 should just be getting its 3rd set of O2s at 23 years old

 

[GrumpierGrunt]

Hi again Ron! I changed out the 02 sensors probably around 20k miles ago, or about two years ago . It was probably the second set of 02 sensors, but I regret putting Bosch sensors in.

I’ll have to my OBD2 sensor with the fuel trims, but the Bank 1 Sensor 1 voltage seems suspicious. Should it be more or less the same as Bank 1 Sensor 2?

 

[RonD]

No

O2s only see Oxygen, and only when they are heated above 600degF

High oxygen is lower voltage, 0.1v
Low oxygen is higher voltage, 0.9v
O2s generate their own voltage, but are Heated by a 12v and ground wire

O2s closest to the engine are "sensor 1's", upstream
O2s after Cat converters are "sensor 2's", downstream
All the same sensors, just different locations

Computer tries to keep Upstream sensor 1s between 0.2v and 0.6v by changing the open times for fuel injectors on that bank of the engine, so Sensor1's voltage will jump around very fast

Cat converters use unburned fuel to heat themselves up, computer will throw extra fuel into the engine every few seconds to keep the Cats hot
Cats use up most of the oxygen in the exhaust to stay hot and to burn up pollutants , so sensor 2 O2s will have a more steady voltage of 0.75v to 0.85v, low oxygen

 

[Sharky]

try installing a new fuel filter ? ......this cured the issue on my 04

 

[GrumpierGrunt]

I changed the fuel filter less than 3,000 miles ago. I used a motor craft filter.

can an exhaust leak cause this issue? I have an exhaust leak at the manifold, it seems to be worse on one side than the other. Just thinking that if oxygen from combustion is escaping, could it cause a false lean?

EDIT: this is unlikely. I will reset the computer and do some more diagnosing with my scan tool

 

[RonD]

Exhaust leaks SUCK IN oxygen(air) so yes manifold leak would cause lean codes on that one bank of the engine

After the upstream O2s exhaust leaks don't matter as much

 

[GrumpierGrunt]

I’m getting lean on both banks, but could a massive exhaust leak trigger both? I did. notice that one bank tends to run slightly leaner.

 

[RonD]

No, it would only effect the one side

Lean and Rich codes don't mean engine is actually running lean or rich
If engine is pinging/knocking its actually running lean
If engine is blowing grey smoke and MPG is in the toilet its actually running rich

Computer knows(is programmed for) running a 3 liter engine, in this case
So it knows how much air it will suck in and how much fuel it should use at any RPM, it's just math
So it calculates how long to open each injector based on RPM and engine load
If O2 shows higher oxygen level(lean) computer will open injectors 5% longer to lower the oxygen level at O2 sensor
If still high oxygen it opens injectors 10% longer
ect..............
These adjustments happen in milliseconds

If computer gets to 20% longer(lean) or 20% less(rich) it will set codes, letting driver know there is an issue
Engine is never running lean or rich, the codes are because computers base calculations were leaner or richer which indicates a problem
Its a computer and the golden rule for all computers is "garbage in = garbage out"
It could be a computer issue, but golden rule usually means computer is OK but computer is basing its calculations on BAD DATA(garbage)
Air leaks on intake side means MAF sensor data is off
Exhaust side, O2 sensor data is off
Fuel pressure low or high, fuel flow thru open injector is off
Any sensor can throw off data

 

[GrumpierGrunt]

I disconnected the battery long enough to reset the computer, zeroed out the LTFT. No code yet and LTFT are not getting nearly as high as they were before resetting the computer. Gonna see what it does, it’s too cold to really mess with anything else.

is it normal for high STFT when accelerating? Any info on where these two trims should be normally is appreciated.
Also, is it possible that bad gas caused this light?

 

[RonD]

Yes, STFT is short term so will go very high when you are accelerating
STFT 0 is calculated for best fuel trim for best MPG at any RPM
When you press down on the gas pedal the computer says "your the boss, screw economy", lol, but it still holds best MPG trim to have something to go back to
At WOT(wide open throttle) computer IGNORES all sensors, and just dumps fuel in as fast as it will go

If you actually cleared LTFT, didn't think you could, then it will go up and down for a few weeks until a Long Term average can be established