Try testing the MAF sensor to see if it's in specs.They usually will set a lean code if they are faulty but you need to start checking somewhere.You may want to pull the hose off the fuel injection pressure regulator to see if its leaking gas.Also check the upsteam O2 sensor maybe its getting sluggish.Test the coolant sensor circuit also maybe the pcm thinks the car is never warmed up.
The Mass Air Flow (MAF) sensor directly measures the amount of air flowing into the engine. The sensor is mounted between the air cleaner assembly and the air cleaner outlet tube.
The sensor utilizes a hot wire sensing element to measure the amount of air entering the engine. The sensor does this by sending a signal, generated by the sensor when the incoming air cools the hot wire, to the ECM. The signal is used by the ECM to calculate the injector pulse width, which controls the air/fuel ratio in the engine.
See Figures 1, 2, 3, 4 and 5
Fig. 1: The MAF sensor is attached to the air cleaner housing
Fig. 2: To backprobe the MAF sensor connector, first remove the rubber boot
Fig. 3: For this vehicle, the MAF voltage should be approximately 0.2 volts with the ignition ON. This sensor is within specifications
Fig. 4: With the engine running at operating temperature, MAF voltage should be 0.85-1.35 volts. This sensor's reading is at the high side of the specification
Fig. 5: Measure voltage by backprobing the MAF connector as illustrated
Using a multimeter, check for voltage by backprobing the MAF sensor connector, as illustrated.
With the ignition switch ON and the engine stopped, voltage should be less than 1.0 volt.
With the engine idling at operating temperature, voltage should be 1.3 -1.7 volts.
With the engine running at approximately 4000 rpm, voltage should be approximately 4.0 volts.
It is important to watch for a linear voltage rise in response to increases in engine rpm, up to about 4000 rpm.
If voltage is not within specifications, check the power and ground circuits.
If the power and ground circuits test okay, the MAF sensor may be faulty.