Dec 1, 2018
Vehicle comes in with a burning engine trouble light and a noticeably poor running engine. Garage business gets to work and as with any diagnosis with a poorly running engine, you first want to know whether it is a cylinder specific or a more general problem. With cylinder specific you quickly end up with ignition, compression and injection, while with engine you generally go more in the direction of problems with, for example, mixture formation, EGR valve, fuel pressure. A divorce that is very important to make at the beginning of your diagnosis. The error codes help with this, they give a clear direction to cylinder 6. After clearing the error codes, the engine runs well again for about a minute, and then goes into error again on cylinder 6. The engine goes noticeably at 1 leg walk less. So far everything is straight forward. Similarly, the next step. It is likely that there is a cylinder specific problem so the focus goes to cylinder 6. Ignition and injection are switched and the compression is measured, errors cleared and… still misfire on cylinder 6 after about 30 seconds of running.
The question is what really happens in that engine? Is there really a misfire in the engine? Is there anything else in cylinder 6 that could be problematic? The garage chooses to present the car with the practical support of GMTO.
Once at GMTO, we get to work. The error codes that were previously read by the garage and now also by us are:
0629 (mercedes code) equal to p0300 misfire (description from the tester)
0645 (Mercedes code) equal to p0306 Ignition ignition cylinder 6 (description from the tester)
The misfire counter in live data is then very relevant to view. You do not see the cause of the problem there, but you can clearly see on which cylinder the problems occur and to what extent.
Cylinder 6 jumps out and runs straight from the start. At 96 misfires the cylinder is noticeably switched off. The engine ECU thus registers misfires. Time to see what really happened. Because with Live data, the interpretation of the engine ECU is always in between, but I want to interpret it myself. This can be done very well by measuring with a scope. It is important to use an accurate scope that can record many measurement points within a short time. The more accurate, the better the diagnosis can be made.
In the first measurement we make, we first roughly record the problem, and later make specific measurements on the problem. In this first measurement we measure the crankshaft signal on channel 1, the ignition control signal of cylinder 1 on channel 2 and we let the scope convert the crankshaft signal into a speed line. This line then graphically shows what happens to the speed over time.
It is remarkable that before the 21st second the engine tries to get the speed to a stable idle speed, but it goes wrong at about 21 seconds. The engine starts running irregularly and cylinder 6 is switched off. We are now going to zoom in on the run-out calculation to see where exactly the misfires take place.
What is striking is that there are no misfires until the ECU intervenes and the control actually stops on cylinder 6 and real misfires occur that are also clearly visible in the measurement. Where does the info in the misfire counter come from in Live data? This is not visible in the measurement.
Before we jump to the conclusion, first the recipe for a good run-out measurement. There is a lot to see with a good run-out measurement. Not only whether a cylinder has no work, but also whether a cylinder, for example, only partially has work, or whether several cylinders occasionally have no work. It is certainly also very practical for diesel engines, which often look at the correction per cylinder. Pay attention; this correction per cylinder only indicates what the control device has done to get the engine running smoothly again. It does not show where the problem really is. A run-out measurement does. For a good measurement it is important to leave the car overnight so that everything is properly cooled down. The measurement must then be made in the first 20 seconds that the motor is running. So don’t drive in first. Why? All controls are then still warming up and not active. Think of the mixture control, knock control, lambda control, run-out / speed control. The engine really shows its true nature now. If there is cause for misfire then it will happen now. Of course there can be exceptions, which again depends on every situation. But especially if there are misfire errors and they are not visible during a run-out measurement, it may be because the ECU is leveling them out. It is then important to take a cold measurement.
Our car showed no misfires during the cold measurement. Only the misfire counter showed something. And the result of the misfire counter was eventually reflected in the measurement. That led to an indirect conclusion; The engine ecu doesn’t quite understand. Is the software broken? No that is unlikely. But the software is no longer completely in line with what is happening in the engine. This can be due to contamination or minor mechanical changes due to, for example, wear. Something that a software developer can hardly anticipate. It can be solved by making the margins in the software a bit wider. No sooner said than done, with the Xentry tester from Mercedes we load the latest software for the engine ecu and the problem is gone. For all 2009 Mercedes models, everyone can do this in their own workshop via Pass Thru. In addition to the run-in measurement, that is therefore quite interesting to delve into if you have not already done so …
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