pearsonbe

The tool read code P0174 System Too Lean (Bank 2).

Here are the values from the Freeze Frame:

Fuel Sys 1 - CL
Fuel Sys 2 - NA
Calc Load (%) - 25.9
ECT (F) - 176
STFT B1 (%) - 21.1
LTFT B1 (%) - 25.0
STFT B2 (%) - 25.0
LTFT B2 (%) - 24.2
Fuel Pres (psi) - 40.5
Eng RPM - 750
Veh Speed (mph) - 0

It looks like the Short Term Fuel Trim for Bank 1 is low. Can someone tell me if I have a fuel delivery problem?

pearsonbe

I just found additional detailed codes in the tool:

P1130 Lack of HO2S11 Switches - Fuel Trim At Limit
P1151 Lack of HO2S21 Switches - Sensor Indicates Lean
P0171 System Tool Lean (Bank 1)

So the lean condition is reported on both banks. I will research the O2 sensor codes.

cliffyk

You need to check the AFR with a wideband O2 system to determine if the engine really is running lean--or if failing narrowband O2 sensors are just reporting the system is lean. As NB sensors die their output voltage falls, which the PCM interprets as a lean mix. Exhauat leaks will cause the sensors to report lean as well.

Your fuel trims, short and long, are all positive meaning the PCM is adding fuel because it is being told by the O2 sensors that the mix is lean, and it is trying its best to enrichen it. The AFR may or may not actually be lean, that is the first thing you need to determine.

The PCM will add or subtract up to 25% more or less fuel as it attempts to keep the mix at 14.7:1, if the AFR is still not correct it will throw a DTC...

pearsonbe

Thanks, Cliff. What is a "wideband O2 system"?

I had a major vaccuum leak caused by a failed PCV hose from the right bank to the intake. The rubber boot collapsed and tore like something cut it. I haven't seen a hose fail like that, especially in a car with 76k on a stock motor. The guy at the parts counter said it is common. Not in my experience. The new part is updated with reinforcements in the rubber. So Ford corrected the design flaw.

I need to replace the PCV valve, clean the MAF, clear the PCM, and see where I am.

cliffyk

A vacuum leak would indeed cause the lean condition.

The regular narrowband O2 sensors used to control the engine can only accurately report air/fuel ratio (AFR) over a narrow range (band) of values, 14.55:1 to 14.85:1--this is why they are called narrowband sensors.



In operation they can only tell the PCM whether the AFR is rich (output > 0.45 V) or lean (output < 0.45 V), but not how rich or lean. So the PCM constantly varies the injector pulse widths to maintain an average of 0.45 V--which represents 14.7:1, the "perfect" air/fuel ratio; called the stoichiometric ratio, the ratio at which all the fuel is consumed.

Wideband O2 systems are an special sort of oxygen sensor that can accurately measure the air/fuel ratio of exhaust gas over a wide range of values--from 10.0:1 to 22.0:1--this is why they are called wideband sensors. It does this by combining a sampling chamber, a narrowband sensor, and a 2-way electro-chemical oxygen "pump" controlled by a microcontroller (MCU).

A sample of exhaust gas is captured in a chamber within the WB sensor assembly and the narrowband sensor reports to the MCU whether that sample is rich or lean. The MCU then tells the oxygen pump to add or remove O2 from the sample chamber (it does this by varying the polarity and voltage of the signal applied to the pump) until the NB sensor reports that the sample is at the stoichiometric ratio (14.7:1).

Because the MCU knows how much voltage of what polarity was needed to do this it can calculate the original AFR of the sample.

The benefit of a wideband system in diagnosing AFR issues is that one can quickly determine if an engine actually is running rich or lean, or if a bad narrowband sensor is just telling the PCM the mix is lean. In performance tuning a wideband system is essential to monitor AFR in open loop operation and make sure it is rich enough to provide maximum power but no so rich as to cause bogging; and also not so lean that detonation might occur and destroy the engine.

pearsonbe

A very thorough explanation. Thanks

Stevo86

Since both banks are reporting a lean condition based on those fuel trims I would look for a vacuum leak. You most likely have some variety of unmetered air getting in.

And then I read your post and saw you had a vacuum leak. Good job on the fix.