How -to If You See the “Check Engine” Light, Pt. 4

A Computer-Controlled Vehicle Needs a Good Oxygen sensor. These Tips Will Help You To Check & Replace One.

EDITOR’S NOTE: IN the first three segments of this series we searched for the reason why the “Check Engine” light had been coming on sporadically in our project car. After a tune-up and the use of some scanning tools, we found our problem to be a “dead” oxygen sensor. In the final installment in our series, we’ll take a closer look at the operation of the oxygen sensor, review some sensor tests and also discuss “closed loop” operations.

Computer Functions

Generically, the computer oversees five functions: fuel, ignition, emissions, performance and system diagnostics.

The Major Input Sensors Include MAP (manifold air pressure)orMAF(mass air flow), engine speed (RPM), and coolant temperature. The “fine adjustment” sensors include air temperature, knock, throttle position, and the oxygen sensor.

The computer outputs controls the fuel injectors (time on, fuel squirts), ignition advance (total advance), and the emission controllers (exhaust gas recirculation and air injection).

The computer includes “look-up tables” to control the right amount of fuel to maintain a 14.7:1 average air/fuel ratio, the correct ignition advance, and the fine adjustments to the fuel injectors.

All this, with correct tune-up maintenance, provides good performance (startup and drivability), optimum fuel mileage and low exhaust emissions.

In this final section of our four-part series, we’ll discuss the oxygen sensor as a “fine adjustment” component. We’ll look at how it functions, discuss “closedloop” and catalytic converter operations and offer tips on how to test an oxygen sensor, and how to install a new one if that’s deemed necessary.

Oxygen Sensor Benefits

Oxygen sensor feedback systems provide three main benefits: 1. Optimum gas mileage.2. Good drivability. 3. Minimum HC (hydrocarbons), CO (carbon monoxide) and NOx (nitrogen oxides) emissions out the exhaust pipe.

Feedback System Definition

A feedback system is defined in Webster’s various dictionaries as: “The Return of a portion of the output of a process or system to the input, especially to maintain performance or to control a system or process. One definition says feedback is useful for producing changes in an electronic circuit that improves performance or in an automatic control device that provides self-corrective action.”

The self-corrective action in an oxygen sensor feedback system is based on oxygen sensor voltage input to the computer. The computer controls the carburetor mixture control mechanism or the injector time-on. In a computer-controlled engine this is “closed loop.”

The Oxygen sensor as a Battery

The oxygen sensor is a “battery” that is designed to produce up to 1.0 volt depending on the exhaust gas oxygen content during engine operation. The O2 sensor temperature must be at least 600° F to produce this voltage.

If an engine is running with a rich gasoline-air mixture, exhaust gases are lower than would be the case with a 14.7:1 air/fuel mixture. Rich mixtures contain very low oxygen content.

Lean mixture exhaust gases on the other hand, that is, leaner than would be found with a 14.7:1 air/fuel mixture, contain high oxygen content.

A“zirconium”oxygen sensor switches from a high to low voltage around 14.7:1 air/fuel mixture which is the “ideal” mixture for combustion of gasoline and air.

The oxygen sensor “senses” the exhaust gas oxygen content and sends the computer a “signal voltage” depending on the exhaust gas oxygen content.

Feedback Operation

As described in the definition, the “closed” loop provides a “self-corrective action.” As the exhaust gas flows past the oxygen sensor, the oxygen sensor responds to the exhaust gas oxygen content producing a signal voltage as an input to the computer.

The computer responds by sending a “control” signal to the mixture control system (metering rods in a Rochester Quadrajet carburetor, or “trimming” the injector pulse on a fuel-injected engine).

The computer maintains the 14.7:1 ideal air/fuel mixture by “trimming”the mixture in response to oxygen sensor voltage input. This Is The “self corrective action” defined above and the “closed loop” operation.

When the exhaust gas oxygen content is low (a rich mixture), oxygen sensor voltage is high (0.8-1.0 volts).As The computer “sees” this input, it quickly decides the mixture is too rich. After calculations and decisions are made within the computer, it (the computer) leans out the mixture by keeping the metering rods in the jets for a longer period of time (higher dwell time) or shortening (trimming) the injector pulse width (less time on).

This, in turn, causes the engine to run leaner, resulting in excess exhaust gas oxygen content (lean) and a low oxygen sensor voltage (0.1 or 0.2 volts).

As The computer sees this low voltage input, it quickly decides the mixture is too lean. It (the computer) then decides to richen the mixture by keeping the metering rods out of the jets for a longer period of time (lower dwell time) or lengthening the injector pulse width time.

As the computer sees rich- and lean mixture oxygen sensor voltage input, it quickly decides to order the opposite.

The computer, therefore, becomes an “averaging” device.

In this application, the computer controls ratio changes from rich to lean and lean to rich, maintaining the vehicle air/fuel mixture around 14.7:1 on the average.

Catalytic Converter Operation

The oxidation/reduction catalyst wants to see correct exhaust “feed gas” flowing into the converter. At this condition, the catalyst efficiency should be at maximum to oxidize HC (hydrocarbons) and CO (carbon monoxide) to CO2 (carbon dioxide) and water.

The catalyst “reduces,” chemically speaking, nitrogen oxides (NOx) down to nitrogen and carbon dioxide. This “reduction” is accomplished with the small amount of carbon monoxide (CO) in the “feed gas” from the engine exhaust.

If the exhaust gas mixture is incorrect coming out of the engine exhaust as monitored by the O2 feedback system (there are tune-up problems), the catalyst efficiency decreases.

A Catalyst Working Overtime: Imagine what an ignition misfire or a rich mixture from the engine does to the exhaust emissionswhen the feed gas enters the catalytic converter system. High HC and CO adds extra feed gas for the catalyst to burn (oxidize). The catalyst is not able to handle this excess feed gas.

In this case, the catalyst may self destruct by overheating, resulting in rapid meltdown, and plugging up the exhaust system. In addition, the reduction of NOx does not occur with incorrect feed gas mixtures.

Benefits of Correct Feedback Operation: So, again it behooves us to make sure the engine tune-up and emission controls are operating in “closed loop” for optimum mileage, good drivability, and low emissions. That all starts with the tune-up steps discussed in Part 1 (November) of this series.

Rich Mixtures: Rich A/F mixtures will cause the oxygen sensor element to turn black and sooty just as spark plugs become sooty from a rich mixture. This, in turn, will cause the oxygen sensor to become lazy; then die (there’s no voltage output).

Oxygen Sensor Removal

Run the engine until the exhaust manifold is just warm to the touch. Remove the old sensor using an oxygen sensor socket, purchased from tool suppliers. (We found them priced at anywhere from $7 to $17 at Sears and Amazon.com.)

Beware! Some Sensors May be “frozen”in the threads and the threads may strip as the sensor is removed. In extreme cases,some exhaust manifolds may crack upon loosening the oxygen sensor.

Oxygen Sensor Installation

Before installing a new oxygen sensor, clean the threads in the manifold with an 18mm spark plug thread tap.

Note:A few applications originally used a 10mm thread on the oxygen sensor. The replacement oxygen sensor may provide an adaptor plate allowing the installation of an 18mm thread oxygen sensor. New oxygen sensor threads are usually coated with an anti-seize compound.

Install the oxygen sensor and tighten just “barely snug.” This Reduces The possibility for thread seizure when it comes time to remove the sensor

Always test a new oxygen sensor for correct voltage and operation after installation. (This was outlined in Table 1 in the first installment in this series.)

Oxygen Sensor Analysis

The feedback oxygen sensor circuit is a “built-in analyzer” on how the computer system is performing in terms of getting optimum mileage, better low-speed drivability, and the lowest emissions.

If the oxygen sensor voltage is varying normally, one can be pretty sure the drivability checks will be OK. If the oxygen sensor is not working correctly, further checks must be made.

• O2 Sensors, Normal and Lazy: Normally, on most vehicles, the O2 voltage must vary from 0.2-0.8 volt to confirm “closed loop.” The feedback loop is complete, and the computer is commanding an average air/fuel mixture of 14.7:1. In addition, the 0.2-0.8 voltage reading must vary every two seconds or faster.

If the closed loop cycle is longer than two seconds or if “it takes forever” to warm-up, the O2 sensor is “lazy,” and should be replaced.

If the oxygen sensor only develops up to 0.6 volt maximum, replace it.

I recommend an oxygen sensor be tested with every tune-up. (As noted in Part 1 of this series.)

Remember, the tune-up maintenance and adjustments must be at specifications prior to testing an oxygen sensor.

• Rich Mixtures:If The oxygen sensor voltage measurements show a rich condition (0.8 volt fixed), lean the mixture until the oxygen sensor voltage varies. This confirms the computer is working.

On carbureted engines, disconnect the purge valve to see if the rich mixture returns to a normal closed loop mixture (O2 sensor switching from 0.2-0.8 volts). If it does, the purge valve may need replacement. If the rich mixture remains, carburetor repair is needed.

If the vehicle engine has run rich for a long period of time, the oxygen sensor may be carboned up and “lazy,” or quit working altogether (zero volts).

If these abnormal conditions exist, I replace the oxygen sensor along with tune-up repairs and adjustments. This usually provides more accurate data to determine failure causes than trying to analyze causes with dead or lazy oxygen sensors.

• Dead O2 or Lean Mixture, Sensor Diagnosis: If the O2 sensor measurement shows a lean condition, richen the mixture to see if the voltage increases from 0.0 up to 0.8 volts, quickly.

You can perform this test by squirting carb cleaner into the carburetor top or a vacuum port on an FI engine.

If The voltage “pops up” to 0.8 quickly, the mixture is lean. If The voltage stays at zero or goes up slowly, the sensor is “lazy” or “dead.”

• Air Injection: On air-injection equipped vehicles the air management valve must be in the “downstream” position. That means the air management valve routes the pump air downstream of the oxygen sensor.

On engines equipped with “pulse air,” the “pulses” must stop flowing upstream of the oxygen sensor and/or exit to the atmosphere when in closed loop.

“Upstream” means the air injection is routed into the exhaust manifold before the oxygen sensor.

Upstream air injection is used for oxidation of hydrocarbons and carbon monoxide during engine warm-up.

When the engine is warmed up and the oxygen sensor is “hot,” the computer will switch the air injection from upstream to downstream.

The above oxygen sensor tests will not work if the air injection is switched “upstream.”

• Additional Checks:In addition to the above checks, determine other possible cause(s) of rich or lean mixtures. This includes vacuum leaks, “drippy” carburetors, bad oxygen sensors, clogged or drippy injectors, coolant and air temperature sensors out of calibration, bad throttle position sensors, out of calibration manifold air pressure sensors, and/or the fuel pressure out of spec.

The beauty of these checks during a tune-up, as outlined in Part 1 of this series, is that you know what is working or not working before performing carburetor repair or fuel injection diagnosis.

If the engine doesn’t operate in closed loop as discussed above, you must perform some additional checks to determine why the system does not go into closed loop mode.

• New Oxygen Sensor(s): I recommend that a new oxygen sensor be installed to help pinpoint the real cause(s) for not going into closed loop.

After performing the tune-up checks, the diagnostic trouble code charts should be used for additional and accurate diagnosis even if no trouble codes are stored in the computer.

Don’t Try To Save Old Oxygen Sensors: “Leaning it out” to burn off the rich mixture carbon deposits on the oxygen sensor after repair, is not recommended. In this case, the oxygen sensor, after “cleaning,”may change calibration (shift), resulting in poor drivability. Install a new O2 sensor.

The carburetor adjustments may change slightly to just outside of the closed loop “computer window” if you’re using the old oxygen sensor. The performance may suffer and/or the engine may idle rough again, just a few miles after the repair.