Over time, exhaust byproducts can obstruct an oxygen sensor's ability to read oxygen content in the exhaust stream, in which case, you will notice a drop in fuel economy. However, a malfunctioning exhaust gas recirculation (EGR) valve, may cause several problems: stalling, rough idle, detonation and poor fuel economy. Even so, there are some tests you can apply to the sensor and valve to determine whether they are still operating within manufacturer specifications or should be replaced.
Testing the EGR Valve
To test a vacuum type EGR valve, bring your car engine to operating temperature. As you increase the engine to about rpm's to 2000-3000 rpm by pulling the accelerator linkage with your hand, observe the EGR valve's diaphragm or stem. If necessary, use a mirror. The diaphragm or stem should move as you accelerate. Otherwise, the valve is not functioning.
Another way to test the valve is to apply vacuum to the unit. With the engine at idle, remove the vacuum line from the top of the valve and plug the hose. Connect a hand vacuum pump to the valve and apply about 15 inches-Hg of vacuum. As the vacuum increases, engine should begin to idle rough or stall. Otherwise, the valve is not working. Keep in mind that, if the valve is stuck in the open position, the engine will show a rough idle.
If the engine does not respond to the previous test, remove the EGR valve and clean the exhaust manifold passages and valve from exhaust deposits. If this does not fix the problem, you need to replace the valve.
Many late model vehicles come equipped with an electronic or a digital EGR valve. This type of valve triggers troublecodes when experiencing problems. You can retrieve these troublecodes from the vehicle computer using a scan tool. Then look up the codes in your vehicle service manual to pinpoint system problems.
Testing the Oxygen Sensor
Problems with the oxygen sensor in your vehicle will show up as a slow or not responsive unit. To test a zirconia-type sensor, use a 10-megaohm digital voltmeter. With the engine at idle and within operating temperature, connect the voltmeter power lead to the signal wire of the sensor and the ground wire to the engine block. Record the voltage readings within a period of one minute. The readings should fluctuate between 100 and 900 mV, with an average of 500 mV.
Now unplug a vacuum line and watch the meter readings. The voltage should drop near the 200 mV. However, if you block the air going through the air intake duct, you should notice a voltage increase.
With titania-type sensors, you can measure the unit resistance with the voltmeter and compare the value to those provided by the sensor manufacturer. Also, remember, regardless of the type of sensor your vehicle uses, a faulty sensor will trigger a troublecode, which you can retrieve from the vehicle computer with a scan tool.