2000StreetRod
Moderator Emeritus
- Joined
- May 26, 2009
- Messages
- 10,594
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- City, State
- Greenville, SC
- Year, Model & Trim Level
- 00 Sport FI, 03 Ltd V8
Background
P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)
The above diagnostic trouble codes (DTCs) can be the result of intake leaks, low fuel pressure, faulty MAF sensor, faulty TPS, faulty injectors, faulty ignition or bad O2 sensors.
Many forum members arbitrarily replace components in the hopes of correcting the reported lean condition. Such a method is frequently expensive and often fails to correct the situation. I advocate testing expensive components (if possible) before replacement. Inexpensive (but usually adequate) test tools can be purchased for less than the cost of an unnecessarily replaced component and the test tools can be reused for future diagnostics.
This procedure attempts to resolve an engine lean condition with minimal time and expense by the home mechanic. It assumes that avoidance of cost is a higher priority than expenditure of time. Otherwise, the owner would simply pay a mechanic to solve the problem.
Unmetered intake air is air that enters the combustion chamber without passing thru and being measured by the MAF sensor. Since the PCM adjusts the fuel mixture based on data received from the MAF sensor, unmetered air results in a lean condition. For small leaks the impact on the engine is greatest at idle when the vacuum is high and the metered airflow is small. As the metered airflow increases the unmetered air contribution becomes less significant.
The V6 SOHC engine gaskets between the head and the intake manifold dry and shrink with age. While less common and to a lesser extent, the same thing occurs with the V6 OHV engine. The gaskets are inexpensive but their replacement is labor intensive.
Procedure
Check all your vacuum hoses for cracks or loose connections. Check the main intake tube from MAF sensor to intake manifold for loose connections/leaks. One potential leak source frequently overlooked is the O ring that seals the EGR tube to the upper intake manifold. Another is the EGR valve gasket. Others are a failed diaphragm in the fuel pressure regulator/damper and the power brake booster. Some members spray starter fluid, propane or some other aerosol near connections on a cold engine and listen for engine speed flares to detect leaks. Its hard to use this method for the head to manifold leaks because of difficult access. It's also unreliable because the PCM quickly compensates for changes in idle speed by adjusting the IAC valve. The most reliable method of detecting vacuum leaks is with the use of a simple smoke generator: easy home made smoke generator
Here's a link to a list of vacuum lines: Intake manifold vacuum lines
The PCM relies on the TPS for detecting rapid change in throttle plate opening. When the accelerator is rapidly depressed the PCM momentarily widens the injector pulse width to keep the engine from stumbling under the increased load. The TPS is easily checked by performing the TPS Test Procedure.
The MAF sensor output allows the PCM to determine the load on the engine and establish the proper air/fuel ratio. It can be checked by performing the MAF sensor test procedure
You can test the fuel pressure by performing the
Fuel pressure test procedure.
With lean codes indicated for both banks: It is unlikely that your fuel injectors would fail on both banks at the same time unless clogged by fuel deposits. See Why change fuel filter?
When a spark plug does not fire and ignite the air/fuel mixture in the combustion chamber the unburned oxygen results in a lean condition detected by the O2 sensor in the bank associated with the bad plug.
You can check the O2 sensors by monitoring their voltage output with a scanner but its unlikely that both banks would go bad at the same time.
If you have performed the above and still have lean conditions reported for both banks then you probably have an intake leak. The leak could be in the vacuum hoses or manifold/IAC valve gaskets.
If by the process of elimination you're left with intake manifold gasket leaks as the cause of your lean condition then see Starting my 00M12 Installation . Just ignore the parts about the timing chain tensioner and restrictor.
P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)
The above diagnostic trouble codes (DTCs) can be the result of intake leaks, low fuel pressure, faulty MAF sensor, faulty TPS, faulty injectors, faulty ignition or bad O2 sensors.
Many forum members arbitrarily replace components in the hopes of correcting the reported lean condition. Such a method is frequently expensive and often fails to correct the situation. I advocate testing expensive components (if possible) before replacement. Inexpensive (but usually adequate) test tools can be purchased for less than the cost of an unnecessarily replaced component and the test tools can be reused for future diagnostics.
This procedure attempts to resolve an engine lean condition with minimal time and expense by the home mechanic. It assumes that avoidance of cost is a higher priority than expenditure of time. Otherwise, the owner would simply pay a mechanic to solve the problem.
Unmetered intake air is air that enters the combustion chamber without passing thru and being measured by the MAF sensor. Since the PCM adjusts the fuel mixture based on data received from the MAF sensor, unmetered air results in a lean condition. For small leaks the impact on the engine is greatest at idle when the vacuum is high and the metered airflow is small. As the metered airflow increases the unmetered air contribution becomes less significant.
The V6 SOHC engine gaskets between the head and the intake manifold dry and shrink with age. While less common and to a lesser extent, the same thing occurs with the V6 OHV engine. The gaskets are inexpensive but their replacement is labor intensive.
Procedure
Check all your vacuum hoses for cracks or loose connections. Check the main intake tube from MAF sensor to intake manifold for loose connections/leaks. One potential leak source frequently overlooked is the O ring that seals the EGR tube to the upper intake manifold. Another is the EGR valve gasket. Others are a failed diaphragm in the fuel pressure regulator/damper and the power brake booster. Some members spray starter fluid, propane or some other aerosol near connections on a cold engine and listen for engine speed flares to detect leaks. Its hard to use this method for the head to manifold leaks because of difficult access. It's also unreliable because the PCM quickly compensates for changes in idle speed by adjusting the IAC valve. The most reliable method of detecting vacuum leaks is with the use of a simple smoke generator: easy home made smoke generator
Here's a link to a list of vacuum lines: Intake manifold vacuum lines
The PCM relies on the TPS for detecting rapid change in throttle plate opening. When the accelerator is rapidly depressed the PCM momentarily widens the injector pulse width to keep the engine from stumbling under the increased load. The TPS is easily checked by performing the TPS Test Procedure.
The MAF sensor output allows the PCM to determine the load on the engine and establish the proper air/fuel ratio. It can be checked by performing the MAF sensor test procedure
You can test the fuel pressure by performing the
Fuel pressure test procedure.
With lean codes indicated for both banks: It is unlikely that your fuel injectors would fail on both banks at the same time unless clogged by fuel deposits. See Why change fuel filter?
When a spark plug does not fire and ignite the air/fuel mixture in the combustion chamber the unburned oxygen results in a lean condition detected by the O2 sensor in the bank associated with the bad plug.
You can check the O2 sensors by monitoring their voltage output with a scanner but its unlikely that both banks would go bad at the same time.
If you have performed the above and still have lean conditions reported for both banks then you probably have an intake leak. The leak could be in the vacuum hoses or manifold/IAC valve gaskets.
If by the process of elimination you're left with intake manifold gasket leaks as the cause of your lean condition then see Starting my 00M12 Installation . Just ignore the parts about the timing chain tensioner and restrictor.
