Author: Yu Yao Dongjiang specializing in luxury car plant Ye Zhengxiang
Failure phenomenon
A 2011 Porsche Cayenne 3.0T car, equipped with a CJT engine, has a cumulative mileage of approximately 210,000 km. The car was repaired in other repair shops due to intermittent engine idling jitter. The maintenance personnel used a fault detector to detect that cylinder 4 was misfired; after changing the ignition coil, spark plug and fuel injector, the fault remained and the fault detector still prompted the cylinder 4 Misfire; Use a cylinder pressure gauge to measure the pressure of cylinder 4, which is 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.
Fault detection
Test drive after receiving the car, start the engine, the engine idling intermittently jitters, and the engine fault light on the instrument cluster lights up abnormally. Check with a fault detector and find that the engine control unit (DME) has the fault code "P030400 Cylinder 4-Misfire detected" and the fault code "P130A00 Cylinder closed" (Figure 51); read the cylinder misfire data (Figure 52) , It is found that the number of misfires of cylinder 4 is 916, and the number of misfires of other cylinders is 0. It can be seen that cylinder 4 does have a misfire failure.
Based on the principle of changing from simple to complex, it was decided to check the ignition system first (although the possibility of ignition coil and spark plug failure has been ruled out, the possibility of ignition control signal failure has not been ruled out). Checking the maintenance information, the right side (passenger side) cylinder bank of the car is from front to back (the direction near the front is front) and cylinder 1, cylinder 2 and cylinder 3 respectively, and the left side (driver side) cylinder bank from front to back They are cylinder 4, cylinder 5 and cylinder 6, and the ignition sequence is 1-4-3-6-2-5.
Use pico oscilloscope and COP probe to measure the ignition waveform of cylinder 4 when the engine is idling (Figure 53). Compared with other cylinders, it can be seen that the ignition waveform is normal (multiple ignition control mode is used at idling speed). Since the fault is intermittent, it is necessary to capture the corresponding ignition waveform when the cylinder 4 misfires to judge whether the ignition control signal is normal. Add 1 channel to measure exhaust pulsation with a pressure sensor WPS500 (Figure 54, red is the exhaust pulsation waveform, and blue is the ignition waveform of cylinder 4). When the exhaust pulsation is abnormal (caused by the misfire of cylinder 4), zoom in on cylinder 4. Observation of the ignition waveform shows that the ignition waveform of cylinder 4 is normal, and it is determined that the ignition control signal of cylinder 4 is normal.
Perform a relative compression test (Figure 55, use a starter to drive the engine, do not start the engine, and measure the starting current with a current clamp ), and found that one of the cylinders could not build cylinder pressure at the beginning (no current peak), and then the cylinder pressure gradually decreased Return to normal, and it is inferred that the cylinder 4 has intermittently poor sealing. Measure the cylinder pressure and starting current of cylinder 4 at the same time (Figure 56, the red is the starting current waveform, the green is the cylinder pressure of cylinder 4), and it is found that as the cylinder pressure of cylinder 4 increases, the starting current also returns to normal, thus confirming the cylinder 4 Intermittent poor sealing, possible causes of failure include: the inlet and exhaust valves are not closed tightly; the piston ring, piston or cylinder wall is damaged; the cylinder gasket is damaged.
If the intake and exhaust valves are not closed tightly, the pulsation of the intake and exhaust will be most affected during the compression stroke and the power stroke (under normal circumstances, the intake and exhaust valves are both closed). Simultaneously measure the intake pulsation, exhaust pulsation and cylinder pressure waveform of cylinder 4 when the engine is started (without starting) (Figure 57, the yellow is the intake pulsation waveform, the blue is the exhaust pulsation waveform, and the green is the cylinder 4 Cylinder pressure waveform), it is found that the intake pulsation trough after the compression top dead center of cylinder 4 is higher, and the exhaust pulsation fluctuates abnormally. Assuming that the intake valve is not closed tightly at this time, the piston of cylinder 4 moves downwards to suck in air, and the intake pulsation trough should be low, thereby eliminating the possibility of the intake valve being not tightly closed; assuming that the exhaust valve is not closing at this time Strictly, the piston of the cylinder 4 moves downwards to suck in exhaust gas, which does cause abnormal exhaust pulsation fluctuations. It is inferred that the exhaust valve of the cylinder 4 is not closed tightly. Since the failure occurs intermittently during engine operation, it is inferred that the exhaust valve rocker arm or the exhaust valve spring is more likely to fail.
After removing the valve cover, it was found that the exhaust valve rocker arm bearing of cylinder 4 was abnormally worn (Figure 58), and the exhaust cam was also abnormally worn. Check the exhaust valve rocker arm and exhaust cam of cylinder 5 and cylinder 6 for abnormal wear, but the degree of wear is not as severe as that of cylinder 4. The analysis suggests that the surface of the exhaust valve rocker arm bearing of the abnormally worn cylinder 4 is uneven. If the valve clearance is hydraulically compensated on the concave surface, when the rocker arm bearing rotates to a convex surface, the valve clearance will be too small, resulting in the exhaust valve The closure is lax. So what is the reason for the abnormal wear of the exhaust valve rocker arm and exhaust cam? Taking into account the long accumulated mileage of the car, the pressure loss of the oil after being delivered to the valve chamber is large, so that the exhaust valve rocker arm and exhaust cam are poorly lubricated, resulting in abnormal wear. In addition, it was found that the vehicle's exhaust valve rocker arm support element had no oil holes in the head (Figure 59), so it was decided to replace the exhaust valve rocker arm with oil holes on the support element head to help it improve the lubrication conditions and avoid recurrence. Abnormal wear.
Troubleshooting
After replacing the new exhaust camshaft and the exhaust valve rocker arm with oil holes, the engine was running smoothly at idle speed and the malfunction was eliminated.