Author: Yu Yao Dongjiang specializing in luxury car plant Ye Zhengxiang
Failure phenomenon
A 2011 Audi A5 Sportback car is equipped with a CDN engine (variable exhaust valve lift) and has a cumulative mileage of approximately 132,000 km. The car’s engine jitters intermittently at idling speed. A slight step on the accelerator pedal will increase the engine speed, and the jitter will disappear; a fault detector is used to detect that the cylinder 4 has misfired; the maintenance personnel have changed the ignition coil, spark plug and injector. After trial run, the fault persisted, and the fault detector still prompted cylinder 4 to misfire; the cylinder pressure gauge was used to measure the pressure of cylinder 4, which was basically the same as the pressure of other cylinders. At this point of diagnosis, there was no maintenance idea, so I drove the car to our factory for maintenance.
Troubleshooting
Use pico oscilloscope , COP probe and pressure sensor WPS500 to measure the exhaust pulsation and the ignition signal of cylinder 1 at the same time when the engine is idling (Figure 60, the green is the exhaust pulsation waveform, and the blue is the ignition signal waveform of cylinder 1). Cylinder 4 is found It did catch fire intermittently.
Disconnect the crankshaft position sensor wire connector, use pico oscilloscope and pressure sensor WPS500 to simultaneously measure the intake pulsation and exhaust pulsation when starting the engine (Figure 61, the red is the intake pulsation waveform, and the green is the exhaust pulsation waveform). The intake pulsation trough of 1 cylinder is occasionally higher, and the exhaust pulsation peak of 1 cylinder is occasionally lower.
Add 1 channel and measure the cylinder pressure of cylinder 1 at the same time (Figure 62, the red is the intake pulsation waveform, the green is the exhaust pulsation waveform, and the blue is the cylinder pressure waveform of the cylinder 1). It is found that the intake pulsation trough of cylinder 4 is occasionally higher. If it is high, the exhaust pulsation peak of cylinder 1 is occasionally low. Further analysis (watch the video case explanation) suggests that the exhaust valve of cylinder 4 is occasionally poorly sealed.
According to maintenance experience, the exhaust valve rocker bearing of this engine is easy to wear, and the failure phenomenon is similar to that of the car. So how to quickly determine the relationship between the failure of the car and the abnormal wear of the exhaust valve rocker arm bearing? Analysis believes that if the exhaust valve rocker arm bearing is abnormally worn, it will be accompanied by abnormal noise. So use the COP probe, the pressure sensor WPS500 and the pickup to simultaneously measure the ignition signal, exhaust pulsation and abnormal engine noise of cylinder 1 (Figure 63, blue is the ignition signal waveform of cylinder 1, green is the exhaust pulse waveform, and yellow is the engine abnormal Sound waveform), it is found that every time around the exhaust stroke of cylinder 4, the abnormal noise of the engine is more obvious, and it is inferred that the exhaust valve rocker arm bearing of cylinder 4 is abnormally worn.
The exhaust valve rocker arm of cylinder 4 was removed for inspection, and it was found that there was no abnormal wear on the bearing surface, but the balls inside the bearing were seriously worn, causing the bearing to loosen. Fix the exhaust valve rocker arm of cylinder 4 (Figure 64), install the dial indicator, shake the bearing up and down, the dial indicator swings a lot; then rotate the bearing and find that the bearing rotates from the position shown in Figure 65a to the position shown in Figure 65b When indicating the position, the swing of the dial indicator is also great.
It can be seen that when the exhaust valve rocker arm bearing of cylinder 4 rotates, there will be two kinds of gaps between the exhaust cam and the exhaust valve rocker arm bearing. When it rotates to a large gap, the valve gap is compensated by the hydraulic support element ( Figure 66), the exhaust valve closes normally at this time; when it rotates to a small gap, the hydraulic support element cannot be compressed, and the exhaust cam will press the exhaust valve rocker arm bearing downward, thereby pressing the exhaust valve downward. The lever arm between the exhaust cam and the support element is much shorter than the lever arm between the exhaust valve and the support element (Figure 67), so a small position change between the exhaust cam and the exhaust rocker arm bearing can be Obtain a large valve lift change, and the exhaust valve will not close tightly at this time. Even if the exhaust valve of cylinder 4 is not closed tightly, but it does not significantly affect the build-up of cylinder pressure, why does cylinder 4 misfire? The analysis shows that cylinder 4 sucks in part of the exhaust gas during the intake stroke, which affects the mixture combustion in idling conditions, causing intermittent misfire in cylinder 4. When the engine speed increases, the air exchange effect becomes better, and cylinder 4 is in the intake stroke. The amount of inhaled exhaust gas is very small and will not affect the combustion of the mixture, so the malfunction phenomenon disappears. This is actually similar to the principle that the valve overlap angle should be reduced in idling conditions, and the valve overlap angle can be increased in high-speed conditions.
Troubleshooting
After replacing the exhaust valve rocker arm of cylinder 4 (Figure 68), the engine was running smoothly at idle speed and the fault was eliminated.
Video case explanation
Disassembly-free diagnosis | Audi A5 single-cylinder misfire of rocker arm bearing loose