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A 1989 Gemini whose engine speed would not increase after warming up.

When I checked the symptoms, there was no problem after warming up, but after 10 minutes, the engine started hunting between 1400 and 2300 RPM even when the accelerator pedal was pressed.

At first, the engine speed rose to nearly 2800 RPM, but the engine was still warming up, so I think the fuel cut-off during deceleration had not yet started. (The engine speed dropped at about 2800 RPM, but this was because the accelerator was turned off. The accelerator pedal was kept pressed after that.)

This is clearly hunting caused by fuel cut-off during deceleration.

Hunting caused by fuel cut-off during deceleration is just my own opinion, and refers to the following condition.

During normal driving, when the accelerator pedal is released, the throttle valve is fully closed, and the idle contact of the throttle sensor (current cars do not have this idle contact, and the throttle sensor signal determines whether the accelerator is fully closed) is turned on. At the same time, the engine brake comes into effect and the engine speed slowly drops while maintaining a certain level of speed.

When the engine brake is on, there is no need to release fuel, so the injector is stopped to prevent fuel from being released.

This is called fuel cut during deceleration.

When this happens, the vehicle speed and engine speed gradually drop.

If the fuel cut continues like this, the engine will eventually stall, so fuel injection begins when the engine speed drops below around 1200 rpm.

In this way, fuel cut during deceleration is a system that activates when the idle contact is ON and the engine speed exceeds a certain rpm.

If the idle contact is stuck and ON while the engine is stopped and the accelerator pedal is pressed, what happens is that when the engine speed is high, the idle contact is stuck and ON, and the engine speed reaches the rpm at which fuel cut begins (this varies depending on the engine, but is generally around 2000 rpm), and the engine is not decelerating, but the idle contact is ON and the fuel cut works.

When this happens, the engine speed drops slowly while driving because the engine brake is active, but when the engine is stopped, the engine speed drops suddenly.

When the engine speed falls below the lower limit for fuel cutoff (this varies depending on the engine, but is generally around 1100 rpm), fuel is injected to prevent the engine from stalling, and the engine speed increases rapidly.

When the engine speed reaches the upper limit for fuel cutoff, the fuel cutoff is activated again, and the engine speed drops.

When this cycle of hunting occurs, I call it hunting due to fuel cutoff during deceleration.

There are two main reasons why this phenomenon occurs when the car is stopped.

The first is when the ISCV breaks and the engine speed increases without the accelerator pedal being depressed, or when air is sucked in by a D-Jetronic engine using a vacuum sensor, or when the idle speed is high due to cleaning the throttle valve with Nissan's electric throttle.

Hunting due to fuel cutoff occurs when the engine speed is high with the accelerator fully closed.

The other is when the signal is always ON due to a malfunction of the idle contact system that determines the idle speed, even when the accelerator pedal is depressed.

In this case, even if the accelerator pedal is depressed, if the idle contact is ON, the ECU will determine that the accelerator is fully closed, so if the engine speed exceeds the upper limit of the fuel cut, fuel will be cut, and if it reaches the lower limit, fuel will be injected.

Then the hunting will begin.

The problem this time is that hunting occurs when the accelerator pedal is depressed, as described below.

The characteristic of this fuel cut hunting is that it does not occur when the engine is cold, but starts after the engine is warmed up.

This is because the fuel cut itself only works after the engine is warmed up.

(Probably, the combustion state is unstable when the engine is cold, so it does not want the rotation to drop significantly when the fuel cut is stopped.)

In this case, the engine revved up without any problems when the engine was cold, but after the engine was warmed up, the hunting began when the engine speed was gradually increased.

It was clearly hunting due to the fuel cut during deceleration.

Thinking that the idle contact was faulty, I unplugged the connector to the throttle sensor.

I thought that if the contacts were just stuck, this would fix the problem, but then hunting started.

However, I had already anticipated this, since the client had already checked it out.

Finally, I started the full-scale inspection.

To be continued one more time