The exhaust system is used to transfer exhaust gases to the rear of the vehicle. If an exhaust leak is present before the oxygen sensors it can cause the sensor to produce a false reading. You might say a leak will not affect the reading because the exhaust is simply leaking out. The problem is that theory is not exactly true. Engine exhaust is produced in pulses as the cylinder's fire. When the exhaust valve opens pressure is created in the exhaust system while the spent mixture exits the combustion chamber. But then a vacuum condition in the exhaust is created after the exhaust valve closes. This vacuum condition can draw raw oxygen from outside of the system and cause a false reading. To inspect for an exhaust leak, start with a cold engine. Then have a helper start the engine and hold the idle at about 1500 rpm. Next, try to listen for any exhaust noises coming from any part of the exhaust system including the exhaust manifold and head pipe. Also look for black soot at any point in the system as this can be the source of an exhaust leak. If an exhaust leak is detected repair leak and recheck system.
The correct fuel delivery is essential for proper engine operation. This means if the fuel delivery is impaired for any reason it can cause a lean mixture code. The main reasons for this condition are plugged fuel filter or weak fuel pump operation.
Your engine is designed to run on specific values, if there are no codes besides a lean mixture code and the engine is running ok the problem is a sensing value problem. A MAF (mass air flow) sensor is famous for causing such a problem. Example: Let's say the engine is running at 2100 RPM at any particular time. When a MAF fails it sends feedback information to the ECM/PCM that the engine is running at 1800 RPM so the computer will lean down the mixture. But the engine is running at 2100 rpm so the mixture is detected lean by the oxygen sensor and the computer will illuminate the CEL (check engine light). There are a few main reasons the MAF reading can be incorrect. First the air intake boot could be cracked or ripped allowing non-metered engine air intake to be consumed. Or the MAF fails because the sensing element inside the sensor becomes contaminated from impurities from the air the engine consumes. Sometimes this element can be cleaned with an aerosol cleaner such as carburetor cleaner. I have had limited success cleaning the MAF sensor, replacement at this point is best. Repair or replace failed components as needed and recheck the system.
Your engine is designed to run on its ability to hold vacuum. If vacuum is allowed to leak it will cause a lean mixture condition. Vacuum hoses are typically connected to the engine intake manifold and will supply engine vacuum to various accessories like power brakes. Some cars are designed with a larger vacuum transfer hose like Ford that connects the intake manifold to the IAC (idle air control) motor. A broken or dilapidated vacuum line can cause the engine to lose vacuum which will cause a lean mixture code. Inspect all engine and accessory vacuum lines to look for missing, torn or dilapidated lines and replace as needed. Also have a helper rest their foot on the gas pedal just enough to keep the engine running. Check the engine when it is running to listen for any whistling noise coming from the engine that is not usually present. Follow the noise and inspect vacuum lines in that area. In addition when the engine is running it will pull inward a broken or weak piece of the hose to create a larger vacuum leak. Check the integrity of all vacuum hoses at each end of the hose. Typically this is where a vacuum hose fails. Replace any vacuums hoses that have failed, clear codes and recheck system.
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Saturday, February 21st, 2009 AT 6:35 PM