Failure phenomenon
A 2009 Buick Excelle equipped with an F16D3 engine and a cumulative mileage of approximately 83,000 kilometers. The owner reported that the idling speed of the car's engine was too high, and the engine fault light was on abnormally.
Troubleshooting
After receiving the car, test it out and start the engine. The engine idling speed is about 1 100 r/min, and it runs smoothly. At the same time, the engine fault light on the combined instrument is on. Use a fault detector to detect that the fault code "P0507 Idle Speed High" is stored in the engine control module (ECM) (Figure 1). Read the engine data flow (Figure 2), the actual engine speed (1 058.50 r/min) is more than 100 r/min higher than the set engine speed (940 r/min); the "idling air control position" is 10 counts, Too low, generally about 50 counts; engine load is 30.39%, too large, generally less than 20%; "calculate throttle position" (that is, throttle opening) is 0%, normal, indicating that the throttle is completely closed; The absolute pressure sensor signal of the intake manifold is 48 kPa, which is too large, generally about 32 kPa; the long-term and short-term fuel adjustments are both 1%, indicating that the mixture concentration is basically normal.
Figure 1 Fault code stored in ECM (screenshot)
Looking at the maintenance information, the setting condition of the fault code P0507 is that the actual engine speed is higher than the set idle speed by at least 200 r/min and lasts for more than 10 s. The speed difference in Figure 2 does not reach 200 r/min because the set idle speed is relatively high. When the set speed is reduced to about 800 r/min, the actual engine speed will not decrease, and the speed difference will reach 200 r/min. /min or more.
Figure 2 Engine data flow when the faulty car is idling (screenshot)
As shown in Figure 3, the engine speed of the car's engine at idling speed is controlled by an idle air control (IAC) valve installed on the throttle body. There is a stepping motor inside the IAC valve, which is composed of multiple steel stators and a rotor in a circle. Each steel stator is wound with a coil, the rotor is a permanent magnet, and the center of the permanent magnet is a nut. All stator coils are always energized. As long as the current direction of one of the coils is changed, the rotor will rotate through a certain angle. When each stator coil changes the direction of the current in the proper sequence, a rotating magnetic field is formed, which makes the rotor made of permanent magnets rotate in a certain direction. If the order in which the direction of the current changes is reversed, the direction of rotation of the rotor will also be reversed. The nut connected to the center of the rotor drives a screw rod. The head of the screw rod is a plug, which can be extended or retracted as the rotor rotates, thereby reducing or increasing the cross-sectional area of the idle bypass air passage. Every time the current direction of the coil is switched, the rotor rotates through a fixed angle, and the lead screw extends or retracts a certain distance (called 1 step or 1 count).
Figure 3 IAC valve control circuit and working principle schematic
Let's analyze the engine data flow at idling speed. The throttle is completely closed and the IAC valve count is low, indicating that ECM is trying to reduce the engine speed by reducing the IAC valve count (ie reducing the cross-sectional area of the idle bypass air passage); engine load, air intake The absolute pressure of the manifold is relatively high, and the long-term and short-term fuel adjustments are normal. It is inferred that excess air enters the cylinder to participate in combustion after being measured by the intake manifold absolute pressure sensor.
Remove the air intake pipe at the front end of the throttle valve (Figure 4), block the end face of the throttle valve with your hand, and immediately stop the engine, thereby eliminating the possibility of air leakage in the pipeline behind the throttle valve; block a little idle bypass air intake port with your hand , The engine speed drops immediately, indicating that the excessive idle speed of the engine is caused by the excessive opening of the IAC valve. The possible causes are: IAC valve damage, such as mechanical jamming; ECM damage, such as IAC valve stepping motor forward and reverse Control exception.
Figure 4 Remove the intake pipe at the front end of the throttle
Use a pico oscilloscope and two current clamps (Figure 5) to measure the current of the two coils inside the IAC valve at the same time. It is found that when the ignition switch is turned on, the self-check current waveform of the IAC valve is shown in Figure 6, and it can be seen that the ECM can control the IAC. The valve stepping motor is forward and reverse (that is, it can switch the current direction normally). So far, it is concluded that the IAC valve is damaged.
Figure 5 Connection mode of current clamp
Figure 6 The current of the two coils inside the IAC valve during self-test (screenshot)
Troubleshooting
After the IAC valve was replaced, the engine idling speed returned to normal, and the engine fault light no longer lights up abnormally. The fault is eliminated. Read the engine data flow at normal idling speed, the actual engine speed is 845 r/min, the IAC valve count is 57, the engine load is 19.33%, and the intake manifold absolute pressure is 35 kPa.