I have been experiencing a P0174 (System too lean condition bank 2) and P0171 (System too lean condition bank 1) on my 5.3l Vortec powered Silverado. The only symptom that I noticed was a slight stumble at idle. After reading forums I suspected that either the fuel filter, fuel pump, MAF sensor, or my intake gaskets were bad. I sprayed sensor safe carburetor cleaner around the base of the intake and didn’t notice an increase in idle, so I proceeded to clean the MAF and changed the fuel filter. I reset the check engine light and drove the truck for a few more days until the CEL came on again. Since I didn’t have any reason to suspect an intake leak, because it passed the carb cleaner test, I ordered an OEM fuel pump and installed it. Once again I reset the CEL, and in a few days the light came back on. Now I was suspect of the intake gaskets and decided to go high-tech on the issue. I purchased two wireless OBDII readers. The first one I purchased was a cheap unit that didn’t work. Then I ordered an iCarsoft OBDII Wifi unit to use with my android smart phone.
After setting up the iCarsoft unit to work with the OBD Car Doctor Free app it was time for a test drive. Below you will see two screenshots of my short term and long term fuel trim showing the values under acceleration and at idle.
Under steady acceleration, my short term fuel trim was between -7 and +3, when I let the engine idle the short term fuel trim went to over 25% indicating a lean condition during idle only. The same was true for long term fuel trim, under acceleration I was reading 15%, and at idle it went to 25%. This helped rule out an issue with fuel delivery, and pointed toward the direction of the dreaded 5.3 intake gaskets.
Now for the true test. I let the truck idle in park, and got out the trusty carb cleaner again.
I sprayed all around the base of the intake, paying more attention to the back of the engine. As I held the trigger on the spray can I noticed the fuel trim % slowly declining, but the engine idle sound never changed.
The ECU was adjusting the fuel trim at a faster rate then my ears could pick up, making it impossible to determine that the intake gaskets were bad using the carb cleaner method without some a diagnostic scanner. This is one instance where high-tech and old school can work together to more efficiently solve a problem.
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