Ch. 26

More documents

Recommendations

Info

6IO CHAPTER 26 What is the Difference Between a "False Leantt and a "Real Lean" Oxygen Sensor Readingl A false lean signal is a result of oxygen flowing past the oxygen sensor that did not result from combustion inside the engine. Two examples of a false lean oxygen sensor indication include: l. A cracked exhaust manifold or an exhaust leak upstream from the oxygen sensor (between the exhaust valve and the oxygen sensor) can cause a false lean. As an exhaust pulse occurs, an area of lower pressure develops behind the pulse of exhaust. This lower pressure areadraws in outside air into the exhaust stream and flows past the oxygen sensor. The oxygen sensor voltage drops as a result ofthis extra oxygen brought into the exhaust at the leak. The drop in oxygen sensor voltage is interpreted by the engine computer as a message that the mixture supplied to the engine is too lean, and it increases the amount of fuel supplied. As a result, the mixture now being supplied to the cylinder is too rich because the oxygen sensor was fooled and provided a false lean signal to the computer, 2. An ignition misfire as a result of a defective spark plug wire or fouled spark plug can cause a false lean. When a spark plug does not fire, the unburned gas and air inside the cylinder is pushed into the exhaust manifold by the piston(s) on the exhaust stroke. The unburned gas and air contain oxygen that is detected by the oxygen sensor as too lean a mixture. i F##T."H? Remember, the oxygen sensor is a sensor ; to detect orygen, not unburned fuel (hydrocarbons I or HC)! As a result of this oxygen being detected, the voltage produced by the oxygen sensor is lower. This lowervoltage signal is interpreted by the computer as a sign that the mixture being supplied is too lean. The computer then increases the amount of fuel delivered. This extra fuel can often cause more spark plug fouling and even more unburned oxygen passing the oxygen sensor. Because a lean condition can be false,the wise service technician checks the exhaust system and the ignition system before trying to correct a lean indication. The O" Sensor is Lying to You A technician was trying ro solve a driveability problem with a V-6 passenger car. The car idled roughly, hesitated, and accelerated poorly. A thorough visual inspection did not indicate any possible problems, and there were no diagnostic trouble codes stored. A check was made 6n the oxygen sensor activity using a DMM. The voltage stayed above 500 mV most of the time. lf a large vacuum hose was removed, the oxygen sensor voltage would temporarily drop to below 450 mV and then return to a reading of over 600 mV. Remember: * High O2S readings : rich exhaust (low 02 content in the exhaust) o Low O2S readings : lean exhaust (high 02 content in the exhaust) As part of a thorough visual inspection, the technician removed and inspected the spark plugs. All the spark plugs were white, indicating a lean mixture, not a rich mixture that the oxygen sensor was indicating. The high O2S reading signaled the computer to reduce the amount of fuel resulting in an excessively lean operation. After replacing the orygen sensor, the engine ran great. But what killed the oxygen sensor? The technician finally learned from the owner that the head gasket had been replaced over a year ago. The silicone-silicate additives in the antifreeze coolant had coated the oxygen sensor. Because the oxygen sensor was coated, the oxygen content of the exhaust could not be detected-the result, a false rich signal from the oxygen sensor. Post-C atalytic Converter Oxygen Sensor Testing The oxygen sensor located behind the catalytic converter is used on OBD II vehicles to monitor converter efficiency. A changing air-fuel mixture is required for the most efficient operation of the converter. If the converter is working correctly, the oxygen content after the converter should be fairly constant. See Figures 26-20 and26-21. ffi SPEED DENSITY Fuel-injection computer systems require a method for measuring the amount of air the engine is breathing in, to be able to match the correct fuel delivery. There are two basic methods used: :: L. Speed density method 2. Airflow method
fomputer Sensor Operation, Diagnosis, and Service 6l I MAP sensor. The value of the intake (inlet) manifold pressure (vacuum) is a direct indication of engine load. TP senson The position of the throttle plate and its rate ofchange are used as part ofthe equation to calculate the proper amount of fuel to inject. Temperature sensors. Both engine coolant temperature (ECT) and intake air temperature (IAT) are used to calculate the density of the air and the need of the engine for fuel. A cold engine (lolv coolant temperature) requires a richer air-fuel mixture than a warm ensine. Figure 26-20 1996 and newer vehicles use an oxygen sensor behind the catalytic converter. The purpose of the oxygen sensor is to sense the percentage of oxygen in the exhaust to check the efficiency of the catalytic converter. The airflow method actually measures the amount of air as part of the computer input information necessary for accurate fuel delivery control. There are three basic types of airflow sensors used on port-injected engines: the air vane sensor, the hot film sensor, and the hot wire sensor. The speed density method does not require an air quantity sensor, but rather calculates the amount of fuel required by the engine. The computer uses information from sensors such as the MAP and TP to calculate the needed amount of fuel. AIR VANE SENSOR This air vane sensor uses a movable vane that translates the amount of movement of the vane into the amount of air being drawn into the engine. An air OXYGEN SENSOR BEFORE THE CONVERTER VOLT OXYGEN SENSOR AFTER THE CONVERTER VOLT GOOD (EFFICI END CONVERTER 1.25 OXYGEN SENSOR AFTER THE VOLT 0.00 5.00 5EC/DlV CONVERTER BAD (INEFFICIEND CONVERTER Figure 26*? I The post-catalytic converter oxygen sensor should display very little activity if the catalytic converter is efficient.
Page 1 and 2: Computer Sensor Operation, Diagnosi
Page 9 and 10: Canrputer Sensor Operation, Diagnos
Page 11 and 12: i,cllipr-:;,*r' :i*ri:c,r iJ{:**-:1
Page 15: Computer Sensor Operation, Diagnosi
Page 23 and 24: ?24-7 Select DC volts and turn the
Page 25 and 26: P25-l Oxygen sensors can be tested
Page 27 and 28: P25-l3 Push the MIN/MAX button to v

Ch. 26

Create successful ePaper yourself

Delete template?

Save as template?