2.3L ('02-'11) P0171 fuel trims

[tatermatt90]

I have occasionally been getting p0171 pending codes and now it finally made my engine light come on. Here are my fuel trims when the code was stored. Typically my fuel trims are 0 to 5 whenever I test them at idle and 2000 rpm. This spike that sets the check engine light is intermittent. Is it safe to keep driving? And yes i have already checked for vacuum leaks.

 

[Dirtman]

Need more info, mainly what year it is. Was the EGR active when the code set? Was the evap purging? How many miles? When was the MAF cleaned? When were the o2 sensors last changed? When was the PCV last changed?

 

[tatermatt90]

Where would I find egr info? 200k miles. 2003 year

 

[tatermatt90]

I cleaned the maf sensor about 2 weeks ago. They guy before me had just replaced it along with the o2 sensors

 

[RonD]

Yes, its SAFE to drive, the engine is NOT running Lean, the code means the Computers Calculations are Lean so its not getting correct sensor data OR there is a mechanical issue like a vacuum leak or low fuel pressure that's throwing off the calculation
But the computer is adding the extra fuel to balance the upstream O2 sensor, so at no time is the engine running Lean, so totally safe to drive

If engine starts to PING/knock then its actually running Lean

After engine is fully warmed up, and idling, unplug the IAC Valve's 2 wire connector
RPM should drop to 500 or engine may even stall, either is good it means no vacuum leaks
If RPMs do not drop then you have a vacuum leak throwing off computers calculations

Test fuel pressure, 55psi is expected, raise RPMs to 2,500 or so and hold it there, pressure should stay above 50psi, if its slowly dropping then its a fuel delivery issue, could be fuel filter or fuel pump

 

[tatermatt90]

Yes, its SAFE to drive, the engine is NOT running Lean, the code means the Computers Calculations are Lean so its not getting correct sensor data OR there is a mechanical issue like a vacuum leak or low fuel pressure that's throwing off the calculation
But the computer is adding the extra fuel to balance the upstream O2 sensor, so at no time is the engine running Lean, so totally safe to drive

If engine starts to PING/knock then its actually running Lean

After engine is fully warmed up, and idling, unplug the IAC Valve's 2 wire connector
RPM should drop to 500 or engine may even stall, either is good it means no vacuum leaks
If RPMs do not drop then you have a vacuum leak throwing off computers calculations

Test fuel pressure, 55psi is expected, raise RPMs to 2,500 or so and hold it there, pressure should stay above 50psi, if its slowly dropping then its a fuel delivery issue, could be fuel filter or fuel pump

See I had been googling this code trying to fix it and so many people had said it's a lean code and means the engine will be running hotter and could damage it because of the lean condition. Most of them said to park it until its fixed. So I've been afraid to drive it. But I thought like you said, since the computer is compensating for the lean condition the engine isnt actually lean. These arent like carbed engines where they run lean and can be damaged because they dont have a computer to add more fuel. I'll check fuel pressure next

 

[Dirtman]

The computer is reading a lean condition but compensating for it. It's not truly running lean until the computer no longer can compensate. You are running at about 20% and the computer runs out of available compensation around 25% - 30%.

 

[RonD]

Your thinking is correct, internet has alot of very good information, and alot of very bad information as well

STFT(short term fuel trim), this is the computer's calculation in real time based on upstream O2 sensor feed back
STFT 0 is the base calculation, +3% means computer is opening the fuel injectors 3% longer that it calculated to get upstream O2 sensor in correct range, this translates as Lean
-3% would mean computer is closing fuel injectors 3% sooner than it calculated to get upstream O2 sensor in correct range, this translates as Rich

LTFT(long term fuel trim) is used to make the computers life a little easier, over time there will be small air leak, lower compression, worn spark plugs, ect....................just general aging of the engine system
LTFT is an average, overtime, of STFT, so computer doesn't have to relearn fuel trims at every start up

With an LTFT of +17% that means computer's STFT has been getting in to the higher + range for awhile
If STFT or LTFT gets above +15% for very long the Lean code would be set

LTFT is ADDED to STFT during the base calculation, thats what its for
So in this case STFT 0 is actually +17 to start with, so STFT +3 means you are actually at +20

O2 sensors can only "see" Oxygen, not fuel
B1S1 is the upstream O2, close to exhaust manifold, its voltage should change VERY fast, sweet spot is .3-.5volts
.1 = lean, high oxygen
.9 = rich, low oxygen

B1S2 is the downstream O2, after Cat converters, the Cat converter should burn up most of the Oxygen cleaning up the exhaust, so this sensor should be steady at .7-.8volts, it will drop suddenly and then go back up, thats the computer adding extra fuel every few seconds to keep the Cats HOT

Yours is running under .6 but I am not familiar with the 2.3l Duratec specifics, but I think that's low, lean, which can effect LTFT
An exhaust blockage or leaks can also cause Lean readings on the B1S2 O2 sensor


If you find and fix the problem you would see STFT in the -1 to -10 range which would bring down the LTFT as you drive
Some OBD2 readers can clear LTFT, but thats not always a good thing to do unless its already close to 0
LTFT in the -8 to +8 range is OK, as said its there so computer can run the engine with its small aging faults, better than relearning every time
And if you don't clear LTFT and you do see the -1 to -10 STFT after warm up then you will KNOW you fixed at least part of the problem
You would still get the P0171 code until LTFT dropped under +15 from driving

 

[tatermatt90]

I drove around with my scan tool plugged in and watched the fuel trims. The stft are always close to zero. The ltft will range from almost 0 at idle and go up to 18.8 at 2500 rpms and under load. How is it possible for ltft to go so high and the stft be almost 0 or even negative 1 or 2

 

[RonD]

That reads like a Fuel volume issue, the fuel pressure may be dropping as more fuel is needed at higher RPMs/load

What does the B1S2 O2 show at higher RPMs?
As said should be .7-.8volts

Yes the LTFT can change like that, and STFT remains close to 0, and it could also be your reader, using reversed PID for STFT and LTFT
LTFT is not actually changing in computer memory, you are watching real time

 

[Dirtman]

In my experience when LTFT is high at idle but drops under acceleration you have a vacuum leak. When LTFT is low at idle but raises under acceleration you have a clogged fuel filter or bad fuel pump.

 

[tatermatt90]

Fuel pressure is 60 and remains steady with increased throttle. Also holds pressure with engine off

 

[RonD]

Thats good

And what about the B1S2 O2 sensor voltage, does it stay above .7?

 

[tatermatt90]

Thats good

And what about the B1S2 O2 sensor voltage, does it stay above .7?

Jumps all around from 0.0 to .88

 

[Dirtman]

B1S1 should bounce up and down, B1S2 should give a steady number.