Failure phenomenon
A 2012 Volkswagen Tiguan car, equipped with CAWA engine, has a cumulative mileage of approximately 140,000 km. The car was repaired in other repair shops due to idling jitter, weak acceleration during driving, and abnormal lighting of the engine failure lamp. The maintenance personnel used the failure detector to detect the failure information that the cylinder 4 misfires, and thus judge the car The fault was a single-cylinder misfire fault. For this reason, the ignition coil was replaced, and the spark plug and injector of cylinder 4 were replaced, but the fault remained; the maintenance personnel measured the cylinder pressure of cylinder 4 with a cylinder pressure gauge, which was about 9 bar (1 bar= 100 kPa), which is about 1 bar lower than the cylinder pressure of other cylinders, but still in the normal range. At this point in the diagnosis, the maintenance staff had no diagnosis ideas, so they requested technical support from the author.
Troubleshooting
I rushed to the site and tested it. When the engine was idling, I could feel the slight vibration of the vehicle while sitting in the car. Putting my hand on the engine felt a little jitter, but it was not as obvious as the jitter caused by a complete misfire of a certain cylinder. Using a fault detector to read the engine misfire data in the engine data stream (Figure 1), it is found that the number of misfires in cylinder 4 (recording the number of misfires per 1000 revolutions) is sometimes 0 times, sometimes continuously rising. Put your hand on the tail throat of the exhaust pipe, and feel that the exhaust is not uniform and continuous, but is pouring out one after another as the engine shakes. Comprehensive analysis of the above phenomena, it is preliminarily judged that the working power of cylinder 4 has decreased.
The four major elements for the normal operation of the engine are good cylinder compression, sufficient ignition energy, proper air-fuel ratio, and correct ignition timing and fuel injection timing. Since the car has a single-cylinder misfire failure, it is unlikely that there will be problems with the "correct ignition timing and fuel injection timing", and the injector has been replaced, and the "proper air-fuel ratio" is unlikely to have problems. Therefore, I decided to check the two elements of "sufficient ignition energy" and "good cylinder compression" first.
This car uses an independent ignition coil. The easiest way to measure is to use a COP (Coil on plug, the ignition coil is placed on the spark plug) probe. It uses the principle of electromagnetic induction to measure the secondary ignition waveform of the ignition coil without disassembly. Connecting the COP probe (Figure 2), the measured secondary ignition waveform of the ignition coil of cylinder 4 is shown in Figure 3. The range between the two scales in Figure 3 represents the combustion time, which is only 885.7 us, which is less than 1 ms. Compared with the combustion time of other cylinders, it is close to 2 ms. It is suspected that the insulation performance of the ignition coil of cylinder 4 is reduced, and the ignition energy is insufficient after the high voltage leakage, which shortens the combustion time.
After adjusting the ignition coils of cylinder 1 and cylinder 4, the abnormal ignition waveform is transferred to cylinder 1, which shows that the ignition coil of cylinder 4 is indeed malfunctioning. After replacing the ignition coil, the ignition waveform returned to normal, but there was still misfire in cylinder 4. The fault has not been eliminated, and further inspection is required.
Considering that there is a one-way valve in the connecting pipeline of the cylinder pressure gauge, the measured value is the cumulative cylinder pressure, which cannot accurately reflect the mechanical state of the cylinder, so the author decided to use the pressure sensor (WPS500) of the pico oscilloscope to re-measure the pressure of each cylinder .
First measure the cylinder pressure when starting (ie the cylinder pressure when the starter drives the engine to run), disconnect the injector wire connector under the intake manifold, remove the spark plugs of the 4 cylinders, and connect the pressure sensor (Figure 4) , Measure the cylinder pressures of cylinder 1~cylinder 4 in sequence (Figure 5). The specific measurement results are listed in Table 1. It can be seen from Table 1 that the pressure in the compression stroke of cylinder 4 is about 1.5 bar lower than other cylinders, and the vacuum at the end of the work stroke is about 100 mbar higher than other cylinders (1 mbar=100 Pa).
Enlarge the pressure waveform of cylinder 4 (Figure 6). The wave peak in Figure 6 corresponds to the compression top dead center. The crank angle between the two compression top dead centers is 720°. Use the angle ruler to divide it into 4 parts, respectively Corresponds to the 4 strokes of the engine.
The end of the work stroke corresponds to the lowest pressure, which is about -430 mbar, and the rising slope pointed by the red arrow and the falling slope pointed by the green arrow are asymmetrical (the falling slope is steeper). It may be that the cylinder is not tightly sealed, causing leakage during the compression stroke. The air pressure accelerates and drops during the work stroke.
In order to further analyze the fault, the author measured the cylinder pressure of cylinder 4 when the engine is idling (Figure 7), so that you can see the angle of valve opening and closing. It can be seen from Figure 7 that the crank angle at the time the exhaust valve opens (pointed by the red arrow) is about 33° before the bottom dead center of the work stroke, and the crank angle at the time the intake valve closes (pointed by the green arrow) is about the intake 60° after bottom dead center of stroke. Measure the cylinder pressure of cylinder 1, and compare it to find that the opening time of the exhaust valve and the closing time of the intake valve of cylinder 1 are the same as those of cylinder 4. Since cylinder 1 is working normally, it indicates that the cam, hydraulic tappet and rocker valve of cylinder 4 The components of the transmission group are working properly. Connect the pressure sensor to the oil dipstick pipe and measure the pulsating pressure of the crankcase when the engine is idling (Figure 8). It is found that the pulsating pressure of the crankcase is uniform, indicating that there is no obvious wear between the piston and the cylinder wall of each cylinder.
A comprehensive analysis of the above measurement results can draw the following conclusions.
(1) The cylinder pressure of cylinder 4 is too low, and the vacuum at the end of the work stroke is too large. It is inferred that the cylinder 4 has a bad seal.
(2) The valve opening and closing angles are normal, indicating that the valve drive unit is working normally.
(3) The pulsating pressure of the crankcase is normal, indicating a good seal between the piston and the cylinder wall of each cylinder.
So far, it is suspected that the intake valve or exhaust valve of cylinder 4 is not closed tightly, and there is a slight leak. This requires the cylinder air leakage measuring instrument (Figure 9), which inflates the cylinder through the spark plug hole. The air leakage can be judged according to the difference between the pressure gauge 1 and the pressure gauge 2, and then the position of the air leakage sound The leakage point can be judged, such as the intake valve leakage, and the leakage sound in the intake manifold can be heard at the throttle.
The specific use method of the cylinder air leakage measuring instrument is as follows.
(1) Turn the cylinder piston to be measured to compression top dead center.
(2) Screw one end of the adapter hose into the spark plug installation hole, and connect the other end of the hose to the cylinder air leakage measuring instrument.
(3) Connect the air source and turn the pressure regulating valve until the pressure gauge 1 shows the appropriate input pressure (maximum 7 bar).
(4) Observe the difference between pressure gauge 1 and pressure gauge 2, and calculate the air leakage. For example, if the reading of pressure gauge 1 is 100 psi (1 pis=6.895 kPa), and the reading of pressure gauge 2 is 90 psi, the air leakage is 10 %. Generally, air leakage within 10% is considered normal.
(5) Use a funnel (to amplify and hear more clearly) to listen to the sound of air leakage at different positions and analyze the air leakage point.
After repeated measurements, it is found that when the cylinder 4 is charged, air leaks from the spark plug hole of the cylinder 3, that is to say, the cylinder 4 and the cylinder 3 blow by. Why is this? When the piston of cylinder 4 is at compression top dead center, the piston and valve states of cylinders 1 to 4 are shown in Figure 10. At this time, the piston of cylinder 3 is at the bottom dead center of work, and the exhaust valve is in the open state (exhaust valve). Open in advance). Assuming that the exhaust valve of cylinder 4 is not tightly closed, gas can enter the cylinder through the opened exhaust valve of cylinder 3. Install the spark plug of cylinder 3 to prevent gas from leaking from cylinder 3, and then remove the front oxygen sensor. The air leakage sound can be clearly heard at the installation hole of the front oxygen sensor, which shows that the exhaust valve of cylinder 4 is indeed not tightly closed. .
After disassembling the engine, it was found that there were two black spots on the exhaust valve seat ring of cylinder 4 (Figure 11). It is inferred that the exhaust valve did not make contact with the blackened position after it was closed. Install the exhaust valve of cylinder 4 on other cylinders and find that it can be closed tightly; install the normal exhaust valve on cylinder 4, but it still does not close tightly, indicating that the failure point is the exhaust valve seat ring of cylinder 4. Why does the exhaust valve seat ring of cylinder 4 wear unevenly? Further inspection found that the gap of the corresponding valve guide was too large.
Troubleshooting
After replacing the cylinder head assembly and the ignition coil of cylinder 4, the engine was running back to normal and the malfunction was eliminated.
Fault summary
(1) When the piston of cylinder 4 is at compression top dead center and the piston of cylinder 1 is at exhaust top dead center, the exhaust valve of cylinder 1 is also open at this time, so why can't you hear the sound of air leakage in cylinder 1? ? The author believes that this is related to the structural layout of the exhaust branch pipe. If the spark plug of cylinder 3 is installed and the exhaust pipe is completely blocked from the three-way catalytic converter, the air leakage sound should be heard in cylinder 1.
(2) The meaning of the secondary ignition waveform of the ignition coil is shown in Figure 12.
(3) There are many reasons for low cylinder pressure, such as timing deviation; valve train failure of the valve train, such as the wear of cam peach tip, loose rocker roller bearing, etc., resulting in insufficient air intake and cylinder sealing bad, link bending and the like . The cumulative cylinder pressure measured by the cylinder pressure gauge is low, which can only indicate that the mechanical part of the engine may be faulty, but does not indicate that the cylinder is not properly sealed.
