Computer diagnostics of lambda probe on BMW gasoline engines using DIS
I'm ready to make a bold statement — people know absolutely nothing about the lambda probe. Half of customers trace all their engine problems to the oxygen sensor. "The engine does not idle — I think the lambda is faulty". "My engine is constantly shaking — they told me it was the lambda". “My dynamics have disappeared — I blame the lambda probe”. Clients are allowed to be uneducated, they compensate for this with money. But the problem also affected people providing BMW computer diagnostic services. “I did diagnostics at another service center, they told me that the lambda probe does not change the readings (voltage) — which means it’s time to change the sensor” But in reality the engine just has a strong air leak.
In this article I will try to teach how to diagnose a lambda probe malfunction, and then how to diagnose engine malfunctions based on the lambda probe readings.
First of all, you need to firmly understand for yourself — The lambda probe never has a negative impact on the operation of a healthy engine. Because of it, it will not: shoot into the muffler, have trouble starting or running the engine, fluctuate speed, stall, lose dynamics, etc. Lambda probe — This is a very accurate sensor for the final adjustment of engine operation. To put it simply, a fully functional engine doesn’t even need it, but this is in a vacuum.
In fact, any gasoline engine has various breakdowns, wear, aging processes, etc. All this leads to the problem of poor mixture formation and further combustion. Essentially any engine malfunction — this is just incorrect mixture formation. Fault repair — return of mixture formation to normal. The lambda probe allows you to partially analyze the burnt mixture based on the oxygen level and adjust the engine operating mode. Essentially, this is a gas analyzer that constantly adapts the engine to the changing environment and to faults in the engine itself. If there is an air leak — DME becomes aware of this and adjusts the fuel supply. If the car has climbed high into the mountains, where the air is thin and contains less oxygen — DME becomes aware of this and adapts the fuel supply. A lambda probe will never cause poor engine performance; on the contrary, it helps it and also simplifies troubleshooting.
If we go deeper into the topic, the lambda probe is needed more for the correct operation of the catalyst. The catalyst can neutralize the amount of harmful substances only in a certain composition of exhaust gases. A strong deviation from the composition of the exhaust gases reduces the efficiency of the catalyst or may even break it. But even without a catalyst on the car, the ability to constantly adapt the engine to environmental conditions outweighs the high price of an oxygen sensor.
Not warmed up or not working lambda probe
The first step is to determine the functionality of the oxygen sensor. In 90% of cases, the DME can independently recognize the fault and store the corresponding error. If there is no error, you need to independently check the functionality of the lambda probe using real-time data in DIS.
On a DME Bosch, engine management system, the voltage of a lambda probe that has not yet been warmed up or is not working will always be within 0.45 volts. The voltage can change constantly, but not in a wide range, about 0.4 — 0.5 volts. In this case, the lambda probe integrator is taken as one, and the DME will wait for the oxygen sensor to warm up and turn on.
On the DME Siemens, the voltage of a sensor that has not yet been warmed up or is not working will always be at 0.09 V. The integrator is taken as one, and the DME will wait for the oxygen sensor to warm up.
But if on the DME Bosch engine management system the voltage of the non-working sensor is between the lean and rich mixture (in stoichiometric value), then on the DME Siemens engine management system the voltage of the non-working sensor will be be in the zone of the richest mixture. Therefore, using only one voltage it will not be possible to accurately determine the presence of a faulty lambda probe on the DME Siemens, engine management system, since the oxygen sensor can work and report a very rich mixture, which DME simply cannot correct.
The real-time parameter Adjusting the composition of the mixture with a lambda probe, comes to our aid, which reports on the heating status of the sensor and its participation in engine operation. This status is available for viewing in all DME Sienems, but not in all DME Bosch.
A working lambda probe on a fully repaired engine
On a DME Bosch engine management system, the lambda probe voltage will constantly change in the range 0,1 — 0,9 volts. According to the principle Leaning the fuel mixture — Enrichment of the fuel mixture.
On a DME Siemens engine control system, the lambda probe voltage will also constantly change, but in the range 0,1 — 4,9 volts. According to the principle of Enrichment of the fuel mixture — Leaning the fuel mixture.
Why should the lambda probe voltage constantly change?
The engine ECU independently constantly changes the injection signal by a small value. Usually no more than ± 0.1 ms, and the lambda probe records these changes in mixture formation. The catalyst has the ability to accumulate oxygen. In short — DME first makes the mixture oxygen-rich (so that the catalyst accumulates it), and then oxygen-poor (so that the catalyst uses the accumulated oxygen to neutralize exhaust gases).
The engine ECU has 2 operating modes. With and Without a lambda probe, even on firmware that implies the use of an oxygen sensor.
In the first case, the DME will wait for the lambda probe to turn on (warm up) and constantly change the injection signal within ± 0.1 ms. Because this is how the DME firmware works with adjustment by the lambda probe. The lambda probe may not be working, but if the DME doesn’t know about this, it will still change the mixture, hoping that the sensor will warm up and start working. Before the sensor is turned on, the DME will rely on the multiplying and summing correction values stored in memory.
In the second case, the DME knows that there is no lambda probe (the sensor chip is disconnected) or it is faulty, and will no longer change the injection signal. In this case, either the lambda probe error will be saved, or you will have to generate it yourself. To force the DME to switch to non-lambda operating mode.
Therefore, if the lambda probe does not work, and the DME cannot independently identify the malfunction, then you can independently detect the malfunction — by disconnecting the sensor connector. DME will immediately switch to lambda-free operating mode.
The engine has a weak lean fuel mixture
Let's consider an example when an engine with DME Bosch control system has a lean mixture, for example, due to air leaks.
95% of the incoming air passes through the mass air flow sensor, and 5% through the hole in the corrugation after the air flow meter. In this case, the normal amount of air enters the engine, but the air mass meter reports to the DME that there is less air entering. The injection signal is calculated largely based on the flow meter readings. Of course, other factors are also taken into account, for example: air and engine temperature, but their influence is several times less. Without a lambda probe, we get a lean mixture in the engine.
The lambda probe informs the DME that the mixture is incorrect (lean), and the DME begins to add fuel (increase the injection time). The lambda probe operating mode has a limit on the maximum possible correction; DME can add or subtract 0.5 ms from the injection signal. According to BMW engineers — this is the maximum possible correction for a worn engine that does not require repair.
If DME was able to adjust the fuel mixture without going beyond this limit, then the engine begins to work well, and the lambda probe begins to inform DME about the correct mixture formation (the sensor voltage will constantly change between leaning the fuel mixture — enriching the fuel mixture).
The animation shows that at first the injection signal is between 2.7 — 2.8 ms, and the lambda probe reports a lean mixture. After which the DME increases the injection signal (adds the amount of fuel) until the lambda probe begins to report correct mixture formation. In the example, the correct mixture is between the injection signal 3.2 — 3.3 ms. Lambda probe integrator, becomes greater than one, 1.17.
The engine has a weak rich mixture
Let's consider an example when an engine with a DME Siemens DME control system has a rich fuel-air mixture, for example, due to a faulty coolant temperature sensor.
The sensor constantly reports 5°С to the DME. Even though all other engine sensors are working properly, the DME will still set an increased injection signal for stable engine operation during the warm-up phase. Although in reality this is not required.
The lambda probe informs the DME that the mixture is incorrect and the DME begins to reduce the amount of fuel (reduce the injection signal). The lambda probe operating mode has a limit on the maximum possible correction; DME can add or subtract 0.5 ms from the injection signal. According to BMW engineers — this is the maximum possible correction for a worn engine that does not require repair.
If DME managed to adjust the fuel mixture without going beyond this limit, then the engine will begin to work well, and the lambda probe begins to inform DME about the correct mixture formation (the sensor voltage will constantly change between enrichment — depletion).
The animation shows that at first the injection signal is between 3.5 — 3.6 ms, and the lambda probe reports a rich mixture. After which the DME reduces the injection signal (reduces the amount of fuel) until the lambda probe begins to report correct mixture formation. In the example, the correct mixture is between the injection signal 3.1 — 3.2 ms. the lambda probe integrator becomes less than one, 0.9.
Fuel mixture too rich or too lean
Let's consider an example when an engine with a Bosch DME control system has a too rich fuel mixture.
The animation shows that at first the injection signal is between 3.1 — 3.2 ms, and the lambda probe informs the DME that the mixture is rich. After which the DME begins to reduce the injection signal (reduce the amount of fuel), in an attempt to adjust normal mixture formation: 3.0 — 2.9 — 2.7 — 2.6 — 2.5 ms, but the lambda probe still reports a rich mixture. DME has already reduced the injection signal by an acceptable 0.5 ms (the lambda probe integrator is 0.8), so the error remains.
The error informs you that the DME has reached its maximum regulation limit, but the mixture is still too lean or too rich. After which the DME switches to the lambda-free operating mode, and the integrator is taken as unit.
Lambda probe integrator
Knowing only the lambda probe voltage, it is impossible to know whether the DME is adjusting the mixture based on its readings (whether the engine is overfilled or underfilled with fuel) or whether the mixture is ideal, and the sensor simply informs about the correct mixture formation in the engine (no malfunctions).
For this purpose, the Integrator correction value is displayed in DIS. By which you can find out whether the mixture is adjusted based on information from the lambda probe, and if it is adjusted — then in which direction and by how much.
To describe it more simply — the lambda probe voltage, even with air leaks, will be in the correct range. Simply thanks to information from the lambda probe, the DME was able to adjust the fuel mixture to the correct value. By knowing the value of the integrator, we can know about various faults in the engine. Which, according to BMW engineers, do not require emergency repair. This is why errors are not saved, although there is a small malfunction.
How it works?
- Based on the information included in the DME from various sensors: air flow meter, temperature sensors, throttle valve potentiometer, etc., the required portion of fuel is calculated. This is how the injection signal is generated.
- Fuel is injected and the resulting fuel mixture is ignited (engine operation).
- The lambda probe analyzes the exhaust gases and informs the DME about the amount of oxygen in them.
- DME calculates the integrator value for further correction of mixture formation. If there are no problems with the mixture or the lambda probe does not work, then the integrator will be equal to one. If the mixture is lean, then it needs to be enriched and the integrator value will be more than one. If the mixture is rich, then it needs to be leaner and the integrator value will be less than one.
- The DME multiplies the injection time by the integrator value and obtains the corrected injection signal. If the integrator is equal to 1, then the injection time does not change. If the integrator is less than 1, then the injection time is reduced. If the integrator is greater than 1, then the injection time will increase.
Example: injection signal 3.55 ms, lambda probe reports rich mixture. DME calculates how much to lean the mixture. The result is an integrator equal to 0.8895. The DME multiplies the number 3.55 by 0.8895 to obtain a corrected injection signal of 3.15 ms. The mixture is injected and ignited (engine operation). This process continues indefinitely and allows you to constantly maintain the mixture composition and adapt the engine to the environment.
The integrator only works in conjunction with a lambda probe. If the lambda probe does not work, the DME will not calculate the integrator, but will take it as one. Multiplying a number by one does not change the number. To correct the mixture before the lambda probe warms up, DME calculates and stores multiplying and summing corrections in memory.
DME calculates the integrator down to the millionth value, thereby maintaining very precise mixture correction.
Multiplying and summing correction of the working mixture
To turn on the lambda probe, the sensor needs to warm up to a high temperature. If the heating element in the sensor is working properly, then after starting a cold engine, the lambda probe will warm up in 5 minutes. Otherwise, the lambda will heat up only due to exhaust gases and the time will increase by 15 minutes, or even more. All this time, DME does not know what mixture the engine is running on, and the wrong mixture accelerates catalyst degradation.
Therefore, the DME calculates the corrections in advance (during operation of the lambda probe) and stores them in memory. And while the lambda probe is warming up, DME uses the saved corrections to temporarily adjust the mixture. And after the lambda probe warms up, DME adjusts the mixture in real time, calculating the integrator value. At the same time, the DME constantly updates the multiplying and summing corrections in memory. Based on this data, you can also judge various engine malfunctions.
Summative — idle speed correction
The amount of air entering at idle has the greatest impact on engine performance than the amount of fuel injected. Therefore, based on the readings of the lambda probe, DME can find out about the presence of leaks, a malfunction of the air flow meter, etc. The correction is calculated as a percentage, the maximum mixture correction value ±20%.
Example: at idle the injection signal is 4.4 ms. The lambda probe reports a lean mixture. DME calculates a correction value of +4%. To correct a lean mixture, you need to increase the injection time by 4%. Now the corrected injection time is 4.57 ms.
Multiplying — partial load correction
At higher speeds, so much air enters the engine that the leaks no longer have a strong effect. Much more important — amount of fuel injected. Therefore, based on the lambda probe readings, DME can find out about the serviceability of: injectors, fuel pump, fuel filter, etc. The correction is calculated in ms, the maximum correction value is ±0.5 ms.
Example: the car's fuel pipes are not sealed, causing low pressure in the fuel line. At 2000 rpm, the DME opens the injectors for 6.3 ms, but the lambda probe reports a lean mixture. DME calculates a correction value of +0.15 ms. To correct a lean mixture, you need to increase the injection time by 0.15 ms. The adjusted injection time is now 6.45 ms.
It is not necessary that the summing correction recognizes only air leaks, and the multiplying correction only recognizes the amount of fuel. There can be a huge number of malfunctions, but these are the factors that prevail.
A few words about the second lambda
With the tightening of environmental standards regulating the content of harmful substances in exhaust gases, car manufacturers needed active monitoring of the condition of the catalytic converter. Mainly for two cases:
- catalyst condition monitoring.
- correction of catalyst wear on the engine side.
In the first case, the DME compares the exhaust gas readings between the first and second lambda probe. If the catalyst does not use the oxygen present in the exhaust gas, or does not use it enough, then the DME concludes that the catalytic converter is faulty. The corresponding error will be stored in the ECU memory, and the “check engine” lamp will light up on the dashboard. In this case, the second lambda probe will simply inform the car owner about the need to quickly contact a service station, and the mechanic about the need to quickly replace the catalyst with a new one. If only one oxygen sensor was installed on the car, no one would have known about the faulty catalyst for a very long time, and accordingly the composition of the exhaust gases would be far from environmental standards.
In the second case, the DME also compares the exhaust gas readings between the first and second lambda probes. But already to increase the efficiency of the catalyst or correct its wear/degradation. If the catalyst accumulates less oxygen, but within acceptable limits, the DME will know about this and reduce the amount of oxygen in the exhaust gas. And in the opposite case, if the catalyst is able to accumulate more oxygen than is currently available in the exhaust gas, the DME will know about this and increase its amount by changing the injection signal.