This morning I went. After turning on ignition, engine vibrated several times and turned off, and then again could not start. At first I thought it was fuel pump pressure relief valve. was broken and fuel injection pressure was insufficient. I pinched fuel line and it looked like truth. It was not as hard as before, so I pinched oil return pipe with pliers and fired again, but it still did not work. It looks like it's about to start, but it just doesn't work. It suddenly occurred to me that a few days ago idle speed was unstable and I was too busy to clean. Could this be an idle engine problem? So, I stepped on accelerator pedal and turned on ignition and it fired right up successfully. It seems there must be a problem with idle motor that was causing idle to not be able to take in any air. Then remove idle engine to find cause.
Removed dead engine
When removing it, it turned out that there was a layer of soot on it. It seems that condition of throttle valve is not much better. You can't tell if an idle engine is good or bad when you take it off. You need to connect plug and turn car key to ON position. At this time, ECU initializes idle engine, and you can see working status of idle engine.
After turning on power, spool moves a little, which is not obvious, and spool rotates with motor. After turning off key, idle motor is reset and spool falls out straight. It seems that there are no problems with motor itself, but problem lies in movement of spool driven by motor.
Idle engine disassembly
After unscrewing spool, a problem was discovered, it turned out that guide block on motor housing was worn out, and spool rotates with motor, due to which ECU cannot accurately control position of spool. So everyone probably doesn't understand it very well, let's first look at following picture:
Basic screw principle
The above figure shows a ball screw, principle of operation is similar to rotation of screw, valve core is similar to screw in above figure, motor is equivalent to rotating body outside screw in above figure, and circular motion is converted to screw by linear motion of thread . But premise is that screws cannot rotate synchronously.
Perspective view of an idle engine
The image above shows a perspective view of a non-running engine. There is a thread inside motor rotor. The coil is limited by a restrictive device and cannot rotate, but can move left and right. As rotor spins, filament mechanism will make the spool move left and right.
This is how idle motor works, spool moves back and forth.
Idle motor control principle
The figure above shows principle of idling motor control at idle. There is an air bypass on throttle valve. The idle motor controls flow of air into cylinder using an air control valve core. airflow to control idle speed. Since idle motor is a stepper motor, that is, a motor that can be precisely controlled and controlled by a pulse signal, ECU can accurately control idle speed through the idle motor system.
Mounting position of idle motor on throttle body
The image above is a real photograph of installation position of idle motor on throttle. The hole indicated by red arrow below is air inlet, which is connected to air intake pipe. The red arrow above points towards air intake leading to engine. The mushroom head at top of spool is on green circle, and cross-sectional area changes by moving back and forth to control intake air volume.
The image above shows spool removed. You can see spool guide in red circle. Movement acts as a guide. Now this stop has worn off, causing spool to rotate with motor. The reason for wear of limiter is that grease in idle engine hardens at low temperature, causing spool to stick to engine, and torque of spool is too high, resulting in wear of limiter. Thus, coil rotates with motor, and circular motion cannot be converted to rectilinear motion. Therefore, ECU has lost ability to control idle speed.
Since I don't have a new idle motor on hand and I couldn't find original car, I decided to repair this motor. The idea of repair is very simple, that is, remove remaining grease and re-lubricate, and then re-lubricate engine. The spool is limited. I will not describe the specific operation in detail. The lubricating oil that I use is sewing machine oil, which is liquid and is not afraid of hardening at low temperatures.
After repair and installation, I started car back. Since it was a cold start of car, idle speed was high, but it can be seen that idle speed adjustment has returned to normal, and all maintenance work is over.
The main components of idler engine
The picture above shows a disassembled idle engine, which mainly consists of an engine and a spool. Some car owners are always worried about whether they dare to screw idling engine spool. Today I will tell you in practice: dare! Just screw it back in after screwing it in and it will adjust gap by itself once it's installed.