02 SENSORS & FUEL INJECTORS
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Most of you know what an O2 sensor is and how it performs on your vehicle's exhaust system. The logic behind the writing of this page is to keep the explanation simple, while at the same time, emphasizing the importance of this device and how in conjunction with your ECU, they together can produce a masking effect of malfunctioning fuel injectors.
The O2 sensor is a chemical measuring device threaded into the exhaust system, usually before the front catalytic converter, with lead wiring to ground, the engine ECU, and in newer vehicles, a heater wire is incorporated. In newer vehicles, a second O2 sensor may be installed downstream of the catalytic converter, so as to measure converter efficiency. For lack of a more detailed explanation, it looks much like a spark plug. It's purpose is to measure the O2 (oxygen) exhaust emission content from the engine fuel combustion, and compare that exhaust O2 level with ambient (outside) air O2 content. The O2 level variation is measured as a voltage, and it is that voltage which is sent to the ECU. On the basis of that voltage reading, the ECU makes injector pulse adjustments to control fueling, and hence control exhaust emission, and engine performance.
In the ideal, theoretical world of petrol combustion, the ratio of air to fuel is considered to be 14.7:1. That is 14.7 parts air to 1 part fuel. This is commonly known as the stoichiometric ratio. It is also referred to by way of the greek letter "Lambda", and a Lambda reading of 1, is the ideal air/fuel mix. At an O2 sensor reading of 1, the O2 exhaust emission is theoretically ideal...meaning the air/fuel ratio is not too rich ( lambda reading less than 1), and not too lean (lambda reading greater than 1). At this "theoretically perfect" combination, the air/fuel mixture is totally burned (all the oxygen and fuel), exhaust levels of harmful hydrocarbon, carbon monoxide, and nitous oxide, are at a minimum, and the residual exhaust is ported thru the exhaust system, and into the atmosphere. NOTE: The Lambda reading is not to confused with the O2 sensor voltage reading. They are two different readings.
The O2 sensor measures the presence of O2 in the exhaust gases, (which under ideal conditions should be about 0.1 - 0.3%) and transmits the reading, thru a voltage signal, to the ECU. The ECU has the ability, again through its' mapping design, to "interpret" the voltage signals from the O2 sensor, and determine if total combustion is occurring. If total combustion does not occur, then more fuel, or less fuel, is delivered thru the injectors and into the cylinders, such that the O2 exhaust emission level stays at 0.1 - 0.3%. In actual practice, this cycling correction occurs several times per second, such that adjustments are being made continuously, with injector pulse width, and the reading swings in a very tight band, on both sides of the Lambda reading of 1.
If air/fuel ratio corrections are necessary, (and they constantly are), the ECU does so by increasing or decreasing the pulse width (the time open) of the injectors. If lambda readings are greater than 1 (a lean mix), the ECU increases the injector pulse width open time (adding more fuel) , and if lambda readings are less than 1 (a rich mix), the ECU decreases the injector pulse width open time (adding less fuel).
It is in fact an extremely well engineered and very effective system to keep harmful exhaust emissions within exhaust pollution standards, and keeping your engine performance at its maximum. By measuring the O2 content of the exhaust gas, we can likewise evaluate the exhaust content of hydrocarbons, carbon monoxide, carbon dioxide, and nitrous oxide. There is a definite chemical relation between these 5 gases. If one goes up or down, the others vary in a specific combination. However, monitoring (and therefore controlling) is done by evaluating O2 in the exhaust stream.
Let's look at an example of performance if one or more injectors are not performing at 100%. To keep it simple, let's look at an in-line 6 engine.
In a "perfect" engine... all injectors should flow the same exact cc volume of fuel...as shown below...
At this level...we assume there is a 14.7:1 air/fuel ratio and a lambda exhaust reading of 1 (total combustion).
NOTE: The use of 100cc flow rate in the examples is for illustrative purposes only. It makes reading the tables, and the variations easier to understand.
Injector #1...........100cc
2..........100cc
3..........100cc
4..........100cc
5..........100cc
6..........100cc
Total....................600cc
At this level of overall engine performance, we expect the volume of air/fuel ratio to be "almost" totally burned, an O2 exhaust content to be in a range of .1 to .3 %....and harmful exhaust gases at a minimum and within pollution standards.
Now let's look at what "we think" might happens if 2 injectors are "dirty" and don't flow 100cc as the others do.
Injector #1............90cc
2............90cc
3..........100cc
4..........100cc
5..........100cc
6..........100cc
Total....................580cc
In this example, the O2 sensor will read a lean mixture, as injectors #1 and 2, are underfueling, and there is excess O2 in the exhaust. We might think the ECU adjusts pulse width (increasing open time) to restablish cc flow rate to 600cc, as per below.
Injector #1..........93.33cc
2.........93.33cc
3.......103.33cc
4.......103.33cc
5.......103.33cc
6.......103.33cc
Total..................600cc
The ECU "again we think", has adjusted the injector pulse width on ALL injectors, such that the total cc flow volume returns to 600cc, and the exhaust lambda reading returns to 1. While this appears to be a somewhat better overall performance than our first example, it is NOT what happens.
Why not? Not, because you must remember what the O2 sensor is reading. It is reading O2 exhaust content. Injectors (therefore cylinders) #1 and #2 are still underfueled. As a result of their underfueling, the O2 exhaust content still reads a lean overall mixture, as there is still unburned O2 present in the exhaust, from underfueling of cylinders 1 and 2. The ECU continues to increase injector pulse width, until the O2 exhaust content reaches the level that says stoichiometric level is achieved. That is when cylinders 1 and 2 are at stochiometric.Injectors 3 thru 6 become irelevant, as their O2 exhaust content has reached the 0.1 to 0.3 % O2 content, and regardless how much additional fuel is added to #3 thru 6, no additional O2 will be burned, as there is no more available. Unfortunately, at this point, cylinders 3 thru 6 are now overfueled.
So, the "actual" end result of fueling is something approximating the following...
Injector #1................100cc
#2................100cc
3#................110cc
#4................110cc
#5................110cc
#6................110cc
Total flow.................640cc
While the O2 content is now acceptable, (according to the O2 sensor...all O2 present has now been used), the HC (hydrocarbon) exhaust content is high due to unburned fuel in #3 thru #6, and the CO (carbon monoxide) exhaust content is likewise high. The vehicle fails emission standards, suffers from poor performance, poor fuel econony, and hard starting. If a vehicle fails emission testing, or if a vehicle is tested with a gas analyzer, either the owner, or the technician, must have the analytical ability to properly interpret the results, and make corrections. Absent the skills to do so, leaves the owner with trial and error fixes.
This is the exact reason why relying on the O2 sensor and the ECU to properly mix the air/fuel ratio is insufficient. The O2 sensor/ECU will only perform as designed, when the air/fuel delivery to each cylinder is balanced. That is, every cylinder is receiving the same exact amount of air/fuel mix. The O2 sensor and the ECU cannot give you that information. It is the exact reason why injector cleaner tank additives may not fix injector problems. Injectors must be tested "off the vehicle" on a bench test, ultrasonically cleaned and flow tested, so that every injector can be visually inspected and tested for uniformly of fuel flow. Only then can the O2 sensor/ECU perform as designed, and only then will your engine perform at its maximum potential.
Now...let's see what happens when we go into "OPEN LOOP OPERATION".
Open loop operation eliminates the O2 sensor reading from having any effect on fueling. With these readings and corrections "turned off", the fueling is supplied by the "mapping tables" located within the ECU. The tables are prewritten referencing "look-up" charts, which have been calculated to give different air/fuel mixes based upon certain engine operating modes. One mode being while the vehicle is cold and idle, another mode at WOT (wide open throttle).
Because WOT is vastly more important than idle, we will concentrate on fueling in "open loop" at WOT.
At WOT, more fuel is required. That is, the ratio of air/fuel needs to be rich, which is an air/fuel ratio less than 14.7 to 1. This is required to keep HP up, and engine temperatures down.
At WOT, the referencing charts are written to provide the extra fuel requirement...and depending on the engine...something on the order of 10-12% additional fuel, or about a 13.5 to 1 air/fuel ratio.
With that said, let's take a look at operation of the injectors at WOT.
Again, we will use a 6 cylinder in-line engine, and also assume, as we did previously, that 100cc flow is correct.
At cruising speed, and "just prior" to WOT (still in closed loop)...we have the following...
Injector #1............100cc
#2...........100cc
#3...........100cc
#4...........100cc
#5...........100cc
#6...........100cc
Flow......................600cc
At WOT...we go "open loop", O2 sensor readings are ignored, and the ECU adds the additional required fuel, and we get...
Injector #1...........110cc
#2..........110cc
#3..........110cc
#4..........110cc
#5..........110cc
#6..........110cc
Flow.....................660cc
This is how it should be. Extra fuel had been added, the air/fuel mix went rich, HP is maintained, and engine temperatures stay within limits.
Now...let's look at what happens if two injectors are "fouled" (#1 and #2) and fuel supply is diminished. Remember, that when the vehicle goes "open loop", no O2 sensor info is used, and the fueling is "base mapped", which added a 10-12% rich correction.
Injector #1.....100cc
#2.....100cc
#3.....110cc
#4.....110cc
#5.....110cc
#6.....110cc
Flow................640cc
Injectors #1 and #2 are still underfueled, even with the addition of 10% additional fuel, from the "baseline" mapping. Cylinders #1 and #2 are subject to overheating from lack of fuel requirement, cylinder temps on #1 and #2 rise, and therefor subject to severe damage...such as burnt exhaust valves, or holes in the pistons. !! An expensive engine rebuild is likely, or you have an extensive "learning curve" to do it yourself, or the car goes on the "auction" block, or you sell it off in parts. Not very good options.
Our page devoted to burnt pistons, most likely caused by inadequate fuel delivery from the injectors is at THIS LINK The exhaust valves (in normal engine op) will always run hotter than the intake valves. The intake valves are cooled by incoming cool air, and fuel, as well as heat removed from the valve by contact with the valve seat. The exhaust valves receive no cool fuel or intake air, they get the elevated temps from the cylinder combustion. In certain engine operating modes, and will poor injector fuel delivery, if those cylinder temps get too high, those specific exhaust valves overheat, the valve metal will fatigue, and will distort or crack, and the valve will not seat properly. Remember, valve heat (both intake and exhaust) is also removed by full contact with the valve seat. Once distortion or cracking occurs, a tiny "escape" channel will occur on the exhaust valve, which will now also allow escape of combustion when the cylinder is on the combustion cycle, and in time (which may be very short), the channel will continue to increase with further burning of the valve. The downward "spiral effect" begins, and burning of the valve accelerates.
Preventable with properly tested and ultrasonically cleaned injectors, such that the injector cc fuel flow is equal.
Is your O2 sensor operating "sluggish" from carbon buildup?
Robert Bosch (the original developer of the O2 sensor) recommends replacing the O2 sensors, at appx 50,000 miles. Why so? While I continue to develop a new web page, which will cover multiple diagnostics on O2 failure modes...the following will give an explanation on performance effects from an O2 sensor that has become "sluggish", as a result of carbon buildup on the O2 sensor tip.
The O2 sensor, by design, measures the difference in the amount of O2 present in the exhaust stream, as compared to the amount of O2 present, external of the exhaust stream...that is ambient air, whic is basically the air we breathe.
The specific technology is complex, and not the subject of this article. Rather the point is, the technology of the O2 sensor, has the chemical/mechanical ablilty to do so.
The variation of the presence of the amount of O2 in the exhaust stream, as compared to the presence of the amount of O2 in ambient air (outside the exhaust stream), can be measured by the O2 sensor as a variation in voltage, produced by the variation in the internal exhaust stream O2, and the external ambient O2.
Suffice it to say (and excluding the chemistry)...the following is the correlation...
Voltage produced (by the sensor) on the order of 0.1 volts, indicates to the ECU the engine is lean on fuel. There is too much O2 in the exhaust, indicating not enough fuel.
Voltage produced (by the sensor) on the order of 0.9 volts, indicates to the ECU the engine is rich on fuel. There is too much fuel in the exhaust, indicating not enough O2.
When all sensor devices, and all engine components, are functioning properly, (including the O2 sensor), your vehicle should function as designed. Max HP, best accelleration, maximum fuel economy.
With that instruction, what happens when the O2 sensor is "sluggish" due to carbon on the O2 sensor tip?
Carbon buildup on the tip (the portion of the sensor in the engine exhaust stream), causes the voltage reading to decrease. The result is as follows. The ECU (as a result of the ECU mapping tables) reads a low voltage as a lean running engine [low voltage = low fuel]...( not enough fuel given the amount of O2...as measured by the voltage produced by the O2 sensor).
The ECU ... (based on that low voltage signal)...will increase fuel injector pulse width...to increase fuel delivery...to overcome the excess O2, attempting to reach stochiometric ratio.
The result is...the engine becomes overfueled, as a result of the carboned O2 sensor. And with a carboned O2 sensor, no increase, or decrease in fuel will affect the O2 sensor. It will only read whatever amount of O2 present, can be transmitted/detected thru the carbon buildup.
Bottom line...a carboned O2 sensor tip..creates a (false) low voltage signal to the ECU, indicating (too much O2, and not enough fuel)... the ECU increases injector pulse width, on the basis of the false O2 sensor voltage signal, fueling goes excessively rich, fuel economy is less than satisfactory, the engine may stumble, O2 response time will suffer (causing engine surge), and your HC and CO emission levels elevate.
An O2 sensor CANNOT "balance" injector fuel delivery. An O2 sensor can only provide a signal to your ECU, indicating excess or insufficient O2 in the exhaust, and the ECU will then attempt to eliminate excess/or insufficient O2 in the exhaust stream...by increasing or decreasing injector fuel delivery. If one injector is low on fuel delivery, the O2 content will be high within that particular exhaust stream...the ECU will adjust rich (increasing injector pulse width) to remove the excess O2, and all other cylinders in that exhaust stream become rich. If one injector is high on fuel delivery, the O2 content will will be low within that particular exhaust stream...the ECU will adjust lean (decreasing injector pulse width), to increase O2, but all other cylinders within that exhaust stream, will now be lean.
The ultimate fix, would be an individual exhaust manifold for each cylinder, with each manifold monitored by its' separate O2 sensor (each of which sends a signal to the ECU to individually control injector pulse width). Obviously, a cost prohibitive solution. THEREFORE...the current solution is to have balanced fuel delivery from the injectors.
The available solution...have the injectors tested, and ultrasonically cleaned, to restore a balanced fuel delivery.
Page updated December 8, 2016.