Is the intermittent trouble code of BMW serious?

Let's share a case, a BMW X 1(E84), N20B20 engine. Owner's reaction: the car sometimes turns off when waiting for the traffic lights. When the cold car starts in the morning, the idle speed of the car is unstable and the engine is abnormal. The test run is indeed as the owner said, the idle speed is unstable, the mechanical noise is abnormal, and the fault light is on, but there has been no flameout.

Fault diagnosis: use the diagnostic instrument to detect and read the fault code, as shown below:

2D6 1 exhaust activated cylinder bank 1 variable camshaft:% control failure, not in place;

2C58 Boost pressure adjustment: shut down as a follow-up reaction;

2D2F exhaust gas catalytic converter front gas mixture sensor: general failure;

(Automotive Maintenance Technology Network/)

2 exhaust activated cylinder bank 1 variable camshaft:% general fault;

2DAF activated cylinder bank 1 variable camshaft: general fault.

Rich mixture fault code.

Looking at the data flow, the long-term and short-term fuel adjustment is MINUS 34%, and the typical mixture is too rich. When the air flow sensor is idling at 700r/min, the data flow is 16.0kg/h, which is normal between 9 ~ 13 kg/h, but obviously larger.

Use smoke tester to find air leakage, especially between supercharger and throttle, and no problem is found.

Since there is no air leakage, check the quality of the air flow sensor.

Coincidentally, the next day, a BMW 3 Series sedan came, also equipped with N20B20 engine, idling at 830 rpm to read the data stream. The air intake is between 12 and13kg/h. Measure the signal of the air flow sensor, adjust the frequency with the simulator, and read the data flow of 15kg/h on the faulty vehicle. Do the last check, put the air flow sensor in the air and start the engine. At this time, no air passes through the air flow sensor, and the data stream read at this time is 9 km/h, and the fault is obviously detected here, and it is preliminarily determined that the computer version is damaged.

Replace the computer board, the idle air flow rate is 9- 10km/h, the idle speed is stable at the beginning, and the mixture is adjusted normally in the long and short term. However, there are still fault codes related to exhaust camshaft adjustment and turbocharging fault codes.

Because I haven't tried the flameout fault, I handed over the car and let the customer use it first.

For this flameout, my understanding is that the mixture is too thick and the ECU is in an extreme posture, which leads to poor combustion, so the engine jitter, brake pedal and heavy load change can not burn well, so the engine jitter flameout.

We decided to ignore the adjustment fault of exhaust camshaft first. No matter where we are, the timing of opening early and closing late is wrong, which should affect the power and should not lead to flameout. Although there is indeed an abnormal noise at the front of the engine.

After the actual delivery, the car owner called the next day and said that the fault light was on again, but it was not turned off. I always thought that the fault was simple, and all the data and fault checking steps had begun to be forgotten.

Three months later, I got a call from the owner, saying that while waiting for the traffic lights, the car started to shake and stall again, and it couldn't catch fire. Half an hour later, I arrived at the scene and everything was fine. Read the fault code, as shown in figure 1.

Don't worry about fake protection yet. This fault code was found when the computer board was replaced, and the fault code of an oxygen sensor was added, so it was not considered. A fault code with high fuel pressure is worth considering. Insufficient oil pressure will definitely lead to flameout, but what does it have to do with waiting for the traffic lights to step on the brake pedal?

I drove the car back to the factory and started it on the elevator. Suddenly, the engine stalled, and I quickly started it again, but it really didn't start. Observing the data flow, the high-pressure oil pressure is 0kPa. If the high-pressure oil pump is damaged, the system oil pressure at this time should be the pressure of the low-pressure oil circuit, so the problem must be the low-pressure system.

At this time, the low-pressure fuel pump was replaced according to the probability. It is reasonable to test the fuel supply current of the low-pressure fuel pump, and read the fault of low current in the fuel pump control unit.

Replace the low-pressure fuel pump directly.

After a period of trial operation, the fault no longer appears and the car is delivered.

However, the next day, it turned off at the traffic lights, first shaking a few times, and then turned off. But one of them caught fire again, and this time there was no fault of low oil pressure, which indicated that the fuel pump was changed correctly.

Although the fault remains, the concentration of mixed gas is normal, and there is no instantaneous ignition waveform and no record of fire. According to the truth, there should be a fire record if there is jitter!

Seriously think about it, will the exhaust camshaft not be adjusted in place, leading to flameout?

For the adjustment of exhaust camshaft CVVT (as shown in Figure 2), the theoretical requirement is to open early and close late. Compared with the normal trouble-free camshaft, the adjustment is divided into two States: one is early opening and the other is late opening and late closing. At the traffic lights, the owner stepped on the brake pedal to stop. From high speed to low load, a stuck camshaft will affect the exhaust. In the early closing state, some waste gas will not be discharged cleanly, which plays the role of EGR. Theoretically, the analysis will not lead to such a serious impact, because the exhaust stroke is so long and there is not much exhaust gas in the cylinder, but it cannot be completely ruled out. At low speed, the exhaust speed decreases, the exhaust valve closes ahead of schedule, and some exhaust gases cannot be discharged cleanly, which is also the main reason why some cars cancel EGR now. On the contrary, if there is something wrong with the intake camshaft, it will lead to the change of valve overlap angle, which will have a great impact on the engine.

But our fault lies in exhaust, so the presumed fault has little to do with the fault code of exhaust camshaft, but these fault codes do exist, so there must be no causal relationship between them and flameout?

The fault based on the exhaust camshaft always exists, and the turbocharging fault and abnormal noise of the front decided to replace the front transmission assembly.

There is not much description of the process of mechanical maintenance, and some accidents occurred during the maintenance. When looking for the top dead center position, I suddenly found that when cylinders 2 and 3 were about to reach the top dead center, the pistons were stuck and could not be rotated by hand. If there is nothing abnormal in the endoscopic inspection, check back and forth whether there is anything in the cylinder, whether the valve is stuck or not, and whether the guide chain is stuck. After a day, the crankshaft suddenly turned again. I decided not to operate according to the maintenance manual, but to remove the chain and sprocket first.

When loosening the chain, it is found that the guide plate is separated from the left and right when it is removed and lifted, as shown in Figure 3.

The joint between the lower port of the right guide plate and the left guide plate is damaged, which is estimated not to cause the crankshaft to turn immobile.

Two damaged special tools (as shown in Figure 4) are used to fix the crankshaft. Because the tightening torque of the fixing screw of the crankshaft is particularly large, it is certain that it can't stand such a large force and be damaged. Looking for special tools on pictures all over the country, even the omnipotent Taobao failed this time. Finally, I asked a friend to find one in Beijing, but I couldn't bear the torque of100n m plus 270. Until the installation of the mechanical part, we have not found the basis for the manual rotation of the engine, and we suspect the valve, but there is no basis and the evidence for disassembling the engine is not sufficient. Plus it got better later, so I decided to leave it alone.

After replacing the mechanical system, I tested it on the hoist for a long time without any fault. The road test is about 200km, and the braking, acceleration and braking are asymptomatic. At this time, the gasoline light is on, so refuel to about 2/3 of the oil level and continue the test run. Here comes the problem. When braking suddenly, the vehicle stalled and then could not be started. There is no fault code, the high-pressure oil pressure is normal, the air flow sensor does not participate in the work under the starting state, and the primary ignition waveform is normal. It is impossible to measure the cylinder pressure on the road, so we must check the mixture concentration first. At this time, the vehicle cannot be started, and the long-term and short-term fuel adjustment cannot be seen. When the oil pressure is normal, the injector should not be stuck, and it will be broken if it is broken. Another reason is activated carbon canister.

Unplug the intake pipe of the carbon canister solenoid valve, and the truth comes out. The intake pipe is full of gasoline. I got under the car and pulled out the three gas pipes on the activated carbon tank, all of which were gasoline. Connect the pipeline and drive back to the factory.

Remove the low-pressure fuel pump and finally find the problem, as shown in Figure 5.

A is connected to the activated carbon tank, and B is connected to the siphon pump on the left side of the oil tank. According to the truth, the gasoline vapor that should be sucked out of the fuel tank passes through B and C at A and is also connected to the siphon pump on the left, and then sent to the engine. I found the fault point here, and the connector at B fell off, as shown in Figure 6.

During normal driving, when the pipeline falls, the normal gasoline level in the fuel tank has no effect. When the fuel tank is filled with a large amount of gasoline, the deceleration reaches a certain level, and the gasoline flows forward and overflows the height of point B. Under the suction of vacuum behind the throttle, the gasoline is sucked into the intake pipe, and the car stalls due to too much mixed gas. Moreover, the vacuum behind BMW's throttle is very low, so the suction is not very large, and there is not much gasoline inhaled in the pipeline. Moreover, after the flameout, the car stopped quickly, and the place B quickly left the liquid level of gasoline and could be started again soon.

Because the mixture was very thick in a short time, the engine control unit didn't have time to make a judgment on long-term and short-term fuel adjustment in a short time, and the engine quickly returned to normal, so we couldn't make a judgment on the mixture concentration for a long time. Because the data flow was all normal, the solenoid valve of the carbon tank was checked many times, and it was normal here. This fault is closely related to the amount of oil in the fuel tank and the deceleration during braking, and it cannot be tested in the test run under normal circumstances.

The last time I replaced the low-pressure fuel pump, I didn't notice that there was still a tube in the fuel tank, so I disassembled and assembled it, and made a fatal mistake, so that I didn't find the exact fault point the second time.

Looking back to summarize the whole maintenance process, the first time the mixture was thick, the computer board was replaced and the diagnosis process was reasonable. When the oil pressure is low for the second time, replacing the low-pressure fuel pump cannot completely eliminate the fuel pump module. Replace the mechanical system for the third time to solve the camshaft fault code. These are all good, and the vehicles do have these problems. It is still necessary to emphasize the causal relationship between a phenomenon and a cause, and we must be cautious.