Author: Xin Bao Auto Service Co., Ltd. of any new HE
Failure phenomenon
A 2012 Volkswagen Phaeton is equipped with a 4.2L V8 engine (using "5-valve" technology, that is, each cylinder has 2 exhaust valves and 3 intake valves), and has a cumulative mileage of approximately 310,000 km. The owner came to the factory to report the engine idling jitter failure. The maintenance technician uses a fault detector to detect that there is no fault code stored in the engine control unit; read the engine misfire data and find that cylinder 7 misfires sporadically; adjust the spark plug, ignition coil and injector of cylinder 7 with other cylinders, and the misfire fault does not transfer ; Use the cylinder pressure gauge to measure the cylinder pressure. The cylinder pressure of cylinder 7 is about 8 bar (1 bar=100 kPa), and the cylinder pressure of other cylinders is about 10 bar. The maintenance technician suspected that there was a mechanical failure in cylinder 7, but could not accurately determine the point of failure. The maintenance plan for dismantling the engine could not convince the customer, so he asked the author for technical support.
Troubleshooting
After receiving the car, test it with a fault detector, and there is indeed no fault code stored in the engine control unit; reading the engine misfire data at idle speed, it is found that the misfire data of cylinder 7 is increasing, which determines that cylinder 7 is not working properly. Due to the built-in check valve in the pipeline of the cylinder pressure gauge, the displayed value is the cumulative cylinder pressure of multiple compression strokes, so it is impossible to accurately determine the mechanical failure of the engine, so the author decided to use pico oscilloscope and WPS500 pressure sensor to measure the cylinder 7 The cylinder pressure is dynamically detected.
1, the spark plug on the cylinder removed, the compressed end of the tube is screwed into the spark plug mounting hole, and the other end connected to the pressure sensor WPS500; WPS500 a pressure sensor for converting pressure signals into electrical signals transmitted to the oscilloscope pico; pico oscilloscope The USB cable is connected to the laptop, and finally the cylinder pressure waveform is displayed on the computer screen . Note: Since the spark plug of the tested cylinder is removed, the tested cylinder cannot perform work. To avoid misunderstanding, the work stroke can be called the release stroke.
First, measure the cylinder pressure when starting, disconnect all the wire connectors of the injectors, and start the engine. At this time, all cylinders will not perform work, and the crankshaft is rotated by the starter. Analyzing the cylinder pressure waveform of cylinder 7 at the time of starting (Figure 2) can find the following three abnormalities.
(1) The maximum cylinder pressure at compression top dead center is 7.8 bar, which should normally be about 12 bar.
(2) The rising curve of the compression stroke and the falling curve of the release stroke are asymmetrical, and the descending "slope" is steeper, indicating that the cylinder pressure in the release stroke drops quickly.
(3) A -600 mbar (1 mbar=0.1 kPa) vacuum belt appears at the end of the release stroke, which should normally be about -300 mbar.
As shown in Figure 3, during the compression stroke and release stroke of the cylinder, the intake and exhaust valves are theoretically closed. The combustion chamber is a closed space, but the piston moves from bottom dead center to top dead center, and then returns to the bottom. Stop point. At the beginning of the compression stroke and at the end of the release stroke, the piston is at the bottom dead center position. The cylinder pressures corresponding to these two positions are the same, that is, P a=P b. If the combustion chamber is not tightly sealed, the piston will leak when compressing upwards, resulting in a lower maximum compression pressure; when the piston is released downwards, the pressure will drop faster due to the reduction of gas in the cylinder, and a larger formation will occur at the end of the release stroke Vacuum. Because the cooperation of the piston ring and the cylinder wall cannot completely seal the combustion chamber, allowing a small amount of gas to leak, a certain vacuum will appear during the release stroke, but if the vacuum is too large, it means that the combustion chamber is not tightly sealed.
In summary, it is estimated that the combustion chamber of cylinder 7 is not tightly sealed. Since the intake and exhaust valves are actually opened in advance and closed later, in order to ensure the rigor of the reasoning, the cylinder pressure waveform of cylinder 7 at idling speed is measured next (at this time, except for cylinder 7, other cylinders can do work normally). Analyzing the cylinder pressure waveform at idle speed, we can see the opening time of the exhaust valve and the closing time of the intake valve. Every time a stroke is performed, the crankshaft rotates 180°, and the piston moves from one dead center to another. Therefore, when the four-stroke engine completes one cycle, the crankshaft rotates 720°. The valve opening and closing time is represented by the crankshaft angle before and after the piston reaches top dead center or bottom dead center. The position indicated by the red arrow in Fig. 4 is the opening moment of the exhaust valve, which is 45° (180°-135°=45°) crank angle before bottom dead center; the position indicated by the green arrow is the closing moment of the intake valve, which is the bottom stop 60° (600°-540°=60°) crankshaft angle after the point. These two angles are consistent with those of other cylinders, and it is inferred that the opening and closing timings of the intake and exhaust valves of cylinder 7 are normal.
According to the working principle of the engine, when the cylinder is at compression top dead center, the intake and exhaust valves are closed, and the combustion chamber is closed. At this time, smoke is injected into the combustion chamber from the spark plug installation hole. The smoking part pushes the fault point backward.
Remove all spark plugs (to facilitate the subsequent rotation of the crankshaft), find a long straight iron wire, and insert it into the spark plug mounting hole of cylinder 7 until it bears against the piston crown; ask another maintenance person to use a tool to turn the crankshaft as the wire When moving upwards, it means that the cylinder 7 is in the compression stroke or exhaust stroke; if the spark plug mounting hole is obviously blowing outwards, it means that the cylinder 7 is in the compression stroke. At this time, slowly rotate the crankshaft until the wire rises to the highest point, so you can find The compression top dead center of cylinder 7. Then use a smoke leak detector to pour smoke into the cylinder 7 from the spark plug mounting hole of the cylinder 7 and find that the smoke is emerging from the spark plug mounting hole of the cylinder 6 (Figure 5). Why is this? The ignition sequence of this engine is 1-5-4-8-6-3-7-2. The power cycle of the engine is shown in Figure 6 (input the ignition sequence of the engine in the waveform overlay tool WOT, and the schematic diagram of the power cycle of the engine is automatically generated) Shown.
Analysis of Figure 6 shows that when the cylinder 7 is at compression top dead center, the cylinder 6 is in a state where the work stroke ends and the exhaust stroke is about to begin. At this time, the exhaust valve of the cylinder 6 is opened (the exhaust valve is opened in advance). If the exhaust valve of cylinder 7 is not closed tightly, the smoke in cylinder 7 will follow the sequence of "exhaust valve of cylinder 7 → exhaust branch pipe of bank 2 → exhaust valve of cylinder 6 → combustion chamber of cylinder 6". The spark plug mounting hole emerges. Of course, the exhaust valves of cylinder 8 and cylinder 4 are also open at this time, and cylinder 4 and cylinder 7 are not in the same row, so it can be ignored. Theoretically, smoke can also emerge from the spark plug mounting hole of cylinder 8, but in reality, the smoke will take shortcuts. The specific cylinder from which the smoke comes depends on the structure of the exhaust manifold. In order to further verify the speculation, when the spark plug of cylinder 6 is installed, smoke emerges from the tail throat of the exhaust pipe, which shows that the exhaust valve of cylinder 7 is indeed not tightly closed.
Remove the engine and transmission assembly, remove the cylinder head of cylinder bank 2, and conduct a light leakage test on the valve of cylinder 7, and it is found that one exhaust valve is clearly transparent (Figure 7), indicating that the exhaust valve and valve seat are closely attached Lax. Further inspection found that the clearance between the exhaust valve and the valve guide was too large, and the valve cone and valve seat were worn unevenly when the engine was working, which eventually caused the exhaust valve to be tightly sealed.
Troubleshooting
After replacing the corresponding exhaust valve, valve guide and valve seat, the engine was running smoothly at idle speed and the malfunction was eliminated.
Fault summary
During the diagnosis process, the author also used a cylinder air leakage measuring instrument to measure the air leakage of cylinder 7 (Figure 8). When the pressure on the left is 5.7 bar, the pressure on the right is 1.1 bar, and the air leakage (air leakage = Leakage pressure/filling pressure) has reached 80%. Because this engine is relatively large, the engine room is almost full, and many parts are inconvenient to remove (such as the front oxygen sensor). It is really inconvenient to listen to the sound of air leakage, so the smoke leak detector is used for detection.
