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Lancia Beta Sedan Rev: January 19, 2002 |
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The amount of oxygen in the exhaust can be used to determine the relative amounts of fuel and air supplied to the engine. The ideal ratio of fuel and air is the Stoichiometric Mixture, which is proportioned so that all of the fuel burns with all of the air and the exhaust would have only carbon monoxide, water and nitrogen. Real world conditions, of course, modify this ideal so that for maxium power, excess fuel is supplied to ensure that all the air is used for power production. For best economy, excess air is supplied so that all the fuel is burned.
The usual expression of the ratio of fuel to air is Fuel/Air Ratio. For gasoline, the stoichiometric mixture is about 14.7:1 (lb Fuel/lb Air). Another way to look at this mixture is to consider the amount of air mixed with a given amount of fuel:
(weight of air actually used) / (weight of air indicated by stoichiometric balance)
This ratio is named the Greek letter, "lambda", and has the value of one for the ideal case, but in practice can varies between 0.9 to 1.3 depending on such things as engine and fuel delivery system design, fuel characteristics and operating load.
An oxygen sensor is a chemical generator. It is constantly making a comparison between the oxygen inside the exhaust manifold and air outside the engine. A voltage is generated based on this comparison. The output of the sensor is usually between 0.2 and 0.7 volts, with a value of 0.45 volts when the mixture is stiochiometricly ideal.
When the engine has more fuel than needed, all available oxygen is consumed in the combustion process and gasses leaving through the exhaust contain almost no oxygen. The 02 sensor sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts.
It is important to remember that the O2 sensor is comparing the amount of oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.
They can be tested both in the car and out. If you have a high impedence volt meter, the procedure is fairly simple. A VOM with about 100Kohms (50K/volt, 2.5V scale) is about the minimum to avoid affecting the sensor output accuracy. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Insufficient impedence in the voltmeter degrades the measurement because the voltmeter loads down the circuit and absorbs the voltage that it is attempting to measure. Try switching to the next lower scale and see if you get a slightly different value reading which indicates impedance load sensitivity.
The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop computer controller operation. Attach the positive lead of a high impedence DC voltmeter to the oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket to avoid possible voltage drop from engine to chassis. Set your meter to look for 1 volt DC.
Note that many late model cars and aftermarket "Air/Fuel Ratio" gauges use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, the one volt sensor signal on the other and ground if there is a third wire. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire.
When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check how your leads are routed to make sure you won't wrap them up in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the oxygen sensor and (for computer controlled engines) to get the controller into closed loop mode. Closed loop operation is indicated by the sensor showing several cross counts (voltage crossing 0.45V value) per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts.
You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 volts and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high (> 0.45) near 0.45 or steady low (< 0.45)? If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the reading is steady low, add richness by partially closing the choke or adding some propane through the air intake. Be very careful if you work with any extra gasoline, you can easily be burned or have an explosion. If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good.
If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of it's hose and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good.
If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor.
If you are not getting a voltage and the car has been running rich lately, the sensor may be carbon fouled. It is sometimes possible to clean a sensor in the car. Do this by unplugging the sensor harness, warming up the engine, and creating a lean condition at about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that the engine begins to slow down. The extra heat will clean it off if possible. If not, it was dead anyway, no loss. In either case, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.
Use a high impedence DC voltmeter as above. Clamp the sensor in a vice, or use a plier or vice-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If this doesn not happen, the sensor is likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems.
ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always no benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.
Home or professional auto repairs that have used silicone gasket sealer that is not specifically labeled "Oxygen sensor safe", "Sensor safe", or something similar, if used in an area that is connected to the crankcase. This includes valve covers, oil pan, or nearly any other gasket or seal that controls engine oil. Leaded fuel will ruin the O2 sensor in a short time. If a car is running rich over a long period, the sensor may become plugged up or even destroyed. Just shorting out the sensor output wire will not usually hurt the sensor. This simply grounds the output voltage to zero. Once the wiring is repaired, the circuit operates normally. Undercoating, antifreeze or oil on the outside surface of the sensor can kill it.
Almost always, the answer is no. You must be careful to not apply voltage to the sensor, but measuring it's output voltage is not harmful. A cheap voltmeter will not be accurate, but will cause no damage. This is not true if you try to measure the resistance of the sensor. Resistance measurements send voltage into a circuit and check the amount returning.
Sensor location is important for the non-heated EGO (one wire) sensor. Too close to the engine and it will become too hot and shorten the life expectancy. Too far way and the sensor may not stay warm enough to work at idle. Location is not a problem for the thick film heated sensors (TFHEGO) (three wire), and placement in the collector is fine. The heated sensors can be placed almost anywhere; however, placing it too close to the engine will shorten its life. The final collector of the exhaust manifold or header is good.
The reason for taking a local (engine block) ground is to minimize any error (voltage drop) induced by body/block return currents and any voltage offset that might develop from loads like A/C or lights if you used the chassis as ground.
Several companies offer air/fuel ratio monitors that consist of an O2 sensor and a processor/readout box. One example is the Edelbrock device, which the author purchased to assist tuning a Lancia Beta Twin Cam engine with non-stock carburetor, cams and exhaust. The sensor supplied with the unit is a Bosch Part Number E971-9F472-AA. This appears to be a standard three wire conventional O2 sensor. The "little black box" is nothing more than little and black. Based on a description supplied by John S Gwynne, inside the black box there is a surge suppresser, a filter capacitor, one needed and one redundant resistor to control the LEDs brightness, and a LM3914 Dot/Bar Display Driver. The design is straight out of the application books with no creativity. There is also no provision for input signal conditioning/filtering to remove engine/ignition noise. Given the speed of the LM3914's comparators, this can be a problem.
The electronics clearly runs the sensor in the voltage mode drawing only a 25nA biasing current for the LM3914's internal buffer. The sensor voltage and air/fuel ratio have the following correspondence:
| volts (open circuit) | air/fuel LED | Implied Lambda |
|---|---|---|
| 0.250 | 15.0:1 | 1.02 |
| 0.375 | 14.5:1 | 0.99 |
| 0.500 | 14.0:1 | 0.95 |
| 0.625 | 13.5:1 | 0.92 |
| 0.750 | 13.0:1 | 0.88 |
| 0.875 | 12.5:1 | 0.85 |
| 1.000 | 12.0:1 | 0.82 |
From the SAE Transaction pertaining to O2 sensors, this relationship of voltage to LED markings is bogus. It is hard to believe this mode of operation would let the sensor go as low as .8 lambda let alone have this type of linearity. It appears that Edelbrock wanted a product to compete with MSD's O2 sensor, which has only one tri-color LED to indicate the mixture condition. Their gimmick was to add more LED's and who would know if these LED's had any real meaning? They almost flash in a believable fashion.
[Cyberdyne makes an in-dash A/F indicator that lists for $29 in Summit's catalog. Don't pay a dime more. Many of these readout units are labeled with a wide range A/F ratios. Since the standard O2 sensor covers a very narrow range of only a few percent around stoichiometric, this labeling is fraudulent. By the way, as long as standard production O2 sensors are used, all the displays must work the same, so you can use a sensor and a readout unit from different sources.
From the description of how O2 sensors work, and are used in a closed loop engine control system, it appears obvious that all a regular O2 sensor can do is sense rich or lean conditions with reference to the stoichiometrically correct mixture of 14.7:1 (lambda = 1.0). Standard O2 sensors basically measure rich or lean operation. Their response is very nonlinear and can not be used to measure precise A/F. But in fact, that is all it NEEDS to do to keep a closed loop engine controller working well, since in closed loop, the computer wants the mixture to stay as close to stoichiometric as possible (the rich and lean voltage indications of the O2 sensor just keep bouncing signals back to the computer, which in turn keep the mixture very close to 14.7:1)
On the other hand, a system like MSD's, which only indicates rich/lean is plenty accurate, if not precise, because that information CAN be obtained from the O2 sensor. Whether it is worth $150 is up to the owner, seeing as how the same readings can be obtained by a regular O2 sensor and a good voltmeter.
According to the information I have received, only a UEGO (Universal exhaust gas oxygen) sensor can determine accurate AFR readings. Also these sensors cost a LOT of money, and the interface circuitry is extremely complex and expensive.
This is NOT to say that the regular O2 sensor can't be used for tuning. Because it can be very helpful for tuning for emissions, or returning an engine to a previous mixture setting (which you had recorded in O2 sensor volts).
If the voltage is near .5V you're very close to stoichiometric. Higher voltage implies a rich fuel/air mixture and lower voltage means a lean mixture. So to tune the engine, one would want to get a reading that that "bounces" above and below 0.5V, which indicates that the engine is running "close" to stoichiometric. If the meter is consistently on the rich side (and often pegged), try leaning it out and vice versa. The meter doesn't really have the accuracy to properly set any A/F ratio other than stoichiometric.
Interpreting voltages off 0.5V (1) can only be done with temperature compensation and (2) only correspond to a few percent variations off stoich. Only when the voltage is "pegged" can you conclude that the engine is "somewhat" rich or lean.
The lambda sensor is located in the exhaust system. It is an oxygen sensor which measures the oxygen concentration. This makes it possible to constantly monitor the composition of exhaust gases.
The Lambda sensor is a probe having a ceramic body (4) that is fitted inside a housing (1) which protects the ceramic body against mechanical influences and serves for the installation of the sensor. The outer part of the ceramic body is exposed to exhaust gases, while the inner part is in contact with the ambient air. Operation of the Lambda sensor is based on the fact that the ceramic material used becomes conductive for oxygen ions when temperature is 300 'C or higher.
If the concentration of oxygen inside the probe differs from that outside the probe, an electrical voltage is developed between the two surfaces because of the special characteristics of the material used. The strength of the voltage generated will depend on the difference in the concentrations of oxygen in the exhaust gas and ambient air. The Lambda sensor provides feedback to the control unit, This tells the control unit what the engine has done with the air/fuel mixture. The feedback signal is combined with the input from all the other sensors so the control unit can immediately make corrections to the mixture. This provides the best concentration for performance and exhaust emissions. Since the electrical signal of the sensor occurs only at temperature of 300'C or higher, a special circuit in the control unit inhibits any input during engine warm-up. The same inhibition occurs when throttle plate switch signals full throttle operation.