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Discussion Starter #1 (Edited)
Within the past few days I was asked by a family member to inspect a used vehicle for possible purchase. Vehicle is 2010 Chevy Colorado truck with 3.7 liter 5 cylinder engine and automatic transmission. Odometer indicates 149,159 miles.

After inspecting the body and undercarriage and interior I turned attention to looking at scan tool data parameters as engine idled. Engine at full operating temperature of 190 deg F. No current or historical diagnostic codes in the ECU (engine control unit). Upstream oxygen sensor was oscillating between rich and lean condition as expected. Downstream oxygen sensor was oscillating but at a much slower rate than the upstream sensor. So the oxygen sensors were working as expected.

There was no engine misfire and under rapid throttle acceleration no clouds of blue smoke emanated from the rear of the vehicle. So that seemed to be a good indication on health of the engine. The engine starts and runs well.

The short term fuel trim was oscillating between - 0.5% to - 2.0% and + 0.6% to + 2.0% fairly quickly so that was satisfactory. The long term fuel trim was set at + 12.3% and I could not get it to change to a lower setting by accelerating the engine. I am not an expert in understanding fuel trim values and associated implications, but from many auto maintenance videos it seems that any fuel trim value greater than a +- 10% indicates a potential problem?

So the ECU thinks the air - fuel mixture is a little too lean so it must add fuel to correct to bring it into stoichiometric balance at + 12.3%. Is engine wear at 149,159 miles the cause for the long term fuel trim to be excessively high? I do not believe the engine has a intake manifold leak as this would dilute the air - fuel mixture towards a lean condition and the short term fuel trim would tend to stay positive (not oscillate around zero) and add more fuel to compensate.

I wonder if a dirty throttle body or mass airflow sensor (MAF) could under report the amount of air entering the engine. This would cause the ECU to increase the pulse width on the injectors and add more fuel and after some time cause the long term fuel trim to go above + 10% consistently? Just looking for discussion / ideas or if there is something that I may have missed or misinterpreted. At this point I do not believe the excessively high long term fuel trim is a "no go" for purchase but something that needs to be kept in mind.
 

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I don't think that +12.3% is necessarily alarming, it still has some headroom. You are correct that it is adding fuel to keep the stoichiometric happy. The short term trim is close to a net value of zero (which was measured at an idle?).
When diagnosing excessive + or - fuel adaptive values, it is important to find out the engine operating conditions that have to be met in order to reproduce the addition or subtraction of fuel in the short term value.
The fuel management operates in 'cells' or 'blocks' that the PCM uses to store memory of trims as the engine loads and speeds change.
For an example closed loop (warmed up) and climbing a hill at 3000 RPM, a spark plug begins to break down. You may not sense any misfire, but incomplete combustion is sending an increased amount of unburned oxygen past the O2 sensors.
The PCM will see this raw oxygen as a lean misfire and lengthen the fuel injector 'on' time to fatten up the mixture in an attempt to achieve complete combustion again.
The memory cell for this operating parameter (3000 RPM @ high load) will then store this increased fuel requirement in memory as a long-term fuel trim adaptive.
A fresh set of OEM spark plugs and O2 sensors may get LTA back under 10%.
The following article explains how it works:
 

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Discussion Starter #3
. . . I don't think that +12.3% is necessarily alarming, it still has some headroom. You are correct that it is adding fuel to keep the stoichiometric happy. The short term trim is close to a net value of zero (which was measured at an idle?). . . .
Yes, initial viewing of short term and long term fuel trim values was done at idle with engine at proper operating temperature. I then raised rpm quickly to about 1500 rpm and short term fuel trim went to positive values and after a few seconds at a steady, higher rpm it returned to oscillating around zero. Long term fuel trim did not change.

. . . For an example closed loop (warmed up) and climbing a hill at 3000 RPM, a spark plug begins to break down. You may not sense any misfire, but incomplete combustion is sending an increased amount of unburned oxygen past the O2 sensors.
The PCM will see this raw oxygen as a lean misfire and lengthen the fuel injector 'on' time to fatten up the mixture in an attempt to achieve complete combustion again. , , ,
Interesting in that the lean condition / plug break down does not cause a misfire trigger diagnostic code in the range of P0301 to P0305 for a 5 cylinder engine. But the PCM increases the injector pulse width to add a slight amount of additional fuel to bring the air – fuel mixture closer to stoichiometric / chemically balanced. Over time this would increase the long term fuel trim. I can understand how worn spark plugs / excessive gap / eroded electrode could alter the spark pattern and cause improper flame burn pattern in the cylinder. No maintenance records on this truck so the spark plugs could be original.

. . . A fresh set of OEM spark plugs and O2 sensors may get LTA back under 10%. . . .
I can understand that new spark plugs could help in this situation with long term fuel trim. But why would a new upstream oxygen sensor help? To me an oxygen sensor is just a rather raw., crude lean / rich switch. As long as the original oxygen sensor is switching between lean and rich on a rather quick basis it is functioning properly. If the oxygen sensor gets lazy and slows down its response rate of switching between lean and rich conditions, a diagnostic code would be set which is not present in this situation.

Comment about low fuel pressure is informative. Lower, off spec fuel pressure affects injector spray pattern and amount of fuel delivered to cylinder. This could cause a slight lean condition for which the PCM would have to add positive values to base cell fuel calculations over time.

Thanks I C for the link to the document that provides some insight to the workings of fuel trim design in fuel injection.
 

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I don't think that +12.3% is necessarily alarming, it still has some headroom. You are correct that it is adding fuel to keep the stoichiometric happy. The short term trim is close to a net value of zero (which was measured at an idle?).
When diagnosing excessive + or - fuel adaptive values, it is important to find out the engine operating conditions that have to be met in order to reproduce the addition or subtraction of fuel in the short term value.
The fuel management operates in 'cells' or 'blocks' that the PCM uses to store memory of trims as the engine loads and speeds change.
For an example closed loop (warmed up) and climbing a hill at 3000 RPM, a spark plug begins to break down. You may not sense any misfire, but incomplete combustion is sending an increased amount of unburned oxygen past the O2 sensors.
The PCM will see this raw oxygen as a lean misfire and lengthen the fuel injector 'on' time to fatten up the mixture in an attempt to achieve complete combustion again.
The memory cell for this operating parameter (3000 RPM @ high load) will then store this increased fuel requirement in memory as a long-term fuel trim adaptive.
A fresh set of OEM spark plugs and O2 sensors may get LTA back under 10%.
The following article explains how it works:
"Stoichiometric" is one of the greatest engineering terms ever. At Chrysler back in the day we would throw out that term to new hires and watch their eyes go blank.😁😁😁 14.7:1 was the magic number for air/fuel ratio for the gasoline engine back in the day.
 
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