Vehicle information:
A new 2013 Grand Touring sedan with approximately 35,000km, LTD engine, and automatic transmission. The car came into the factory for repair due to black smoke from the exhaust and foul tailpipe odor.
Fault diagnosis:
Black smoke and odor are generally caused by inadequate combustion, mainly due to a thick mixture (due to injector leakage, high fuel pressure, too little air intake, the engine ECU detects the wrong oxygen sensor signal, etc.), combustion caused by inadequate. With the KT600 GM22 connector into the engine system, and automatic transmission system, found that there are no fault codes, so into the data flow, analyzed the data against the repair manual, found that the injection pulse width is normal (total speed of 3.1ms or so), the intake air absolute pressure data is normal (no electrical load of 40kPa or so), the airflow data flow is normal, 1578Hz or so, the oxygen sensor data is normal (front oxygen changes rapidly between 0.1 and 0.9), airflow is about 3q/s, check the data of intake, multiple acceleration, and deceleration to check the data of exhaust camshaft, which also shows normal. The only abnormal data is the atmospheric pressure data, which shows 90kPa, while the atmospheric pressure in the south is around 101kPa in summer and 102kPa in winter, and the service manual describes the maximum permissible deviation as 5kPa. The vehicle’s atmospheric pressure sensor seems to be inside the computer board, but the abnormal function of the outside intake absolute pressure sensor will cause abnormal data from the atmospheric pressure sensor.
So first of all, we tested the data of the air intake absolute pressure sensor measured the actual air intake pressure with a vacuum pressure gauge, and found that the relative pressure at the total speed was 46mm Hg. Equivalent to 60kPa, using atmospheric pressure 101kPa minus 60kPa and relative pressure of 41kPa. indicates that the absolute pressure sensor displays a value of 40kP. It is correctly displayed (with a deviation of 14kPa or less is normal) and functions normally. Then detected the front oxygen sensor was not normal, we used the method that is normal with the car and other water temperature to 85 ℃, with the chemical cleaner slowly sprayed into the air intake, the results found that the front oxygen sensor had reported thick “(data has been shown as thick), the front oxygen sensor troubleshooting. Atmospheric pressure sensor is installed in the engine ECU inside, is it a problem?
Since changing the computer board is usually the last option, let’s check the other aspects first, remove the spark plugs, it turns out that the spark plugs are long overdue for replacement, and the jump fires have been severely jumped off the mark, and the intake pressure of the GM lineup may also show 90kPa, but it won’t affect the performance of the vehicle. If only the atmospheric pressure is displayed incorrectly (10kPa lower than normal), while the absolute intake pressure is correct, it can indeed cause a malfunctioning consequence of a rich mixture. Based on the fact that the function of the spark plug has been abnormal, it will lead to insufficient ignition energy and a bad odor in the exhaust gas. Based on the aging of the spark plug, a preliminary maintenance program was drawn up: change the spark plug, clean the three-way catalytic converter, clean the throttle, and clean the intake pressure sensor.
Started right away after talking to the owner. The spark plugs were replaced, the throttle was cleaned and matched, and 2 three-way catalytic cleaners were hung from the brake vacuum pipe with a hanging bottle. Later, I used a piece of wet white paper to hang it at a position about 15cm from the exhaust pipe to check whether there was still black smoke and found that there was no more black smoke discharged before I finished hanging the three-way catalytic cleaner. Once again KT600 into the engine control system to read the fault code, but also no fault code. In addition, I was surprised to find that although the intake pressure was still about 40kPa, the atmospheric pressure had changed to about 100kPa, that is, it became normal. Not sure what the cause is (perhaps due to having disconnected the battery?)
Originally thought that the fault was solved, after a 4-hour interval to fire again, the results of the atmospheric pressure are still showing 90kPa, in the water temperature of 80 ℃, go to smell the exhaust, the results of a foul smell, confirmed that the problem has not yet been solved. Based on the atmospheric pressure again to 90kPa, first went to check the power supply of the intake pressure sensor iron, are no problem, replaced the intake pressure sensor after a few days of testing, returned the visit, and confirmed that the problem was solved.
Case summaries:
- The data flow of the GM lineup is divided into several large sections (engine control, HAV air conditioning and ventilation, and emissions ……), and just looking at the data flow is dozens of pages, which requires a diagnostic technician to have considerable data analysis skills. It is important to have a good grasp of commonly used data, such as the general values of atmospheric pressure in a given area. In this case, we note that the atmospheric pressure is shown as 90kPa, inferring that there is a problem with the intake pressure sensor or the engine ECU, but there are relatively few engine ECUs that have problems, and replacement of the ECU requires online matching to be possible. Comprehensive plants generally should not start with the ECU replacement method and need to use the elimination method to the end to replace the ECU.
- The failure of this vehicle should be intermittent, with the apparent failure occurring only when the engine ECU receives atmospheric pressure of 90kPa.
- The cause of this failure is twofold, first of all, the intake air absolute pressure sensor failure leads to more oil injection, and due to the aging of the spark cold ignition energy is insufficient to lead to incomplete combustion, the oxygen content of the exhaust gas is high, the engine ECU and then thicken the injection, thus producing the phenomenon of exhaust black smoke, odor.
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