HomeTips & GuidesIdle Stop Solenoid: How It Works and Why Your Carbureted Engine Needs One

Idle Stop Solenoid: How It Works and Why Your Carbureted Engine Needs One

If you mention the term idle stop to modern-day car owners, they will think of the start-stop system that shuts off the engine when idle to reduce fuel consumption. But an idle stop solenoid is different. 

The device is critical in old carburetor engines and can cause engine damage if faulty. So, here’s an in-depth look at the function of an idle stop solenoid and why you need one for your old vehicle.

Table of Contents

What Is an Idle Stop Solenoid?

An idle stop solenoid is an electrical device that closes off the throttle to prevent an idle engine from continuously running once you turn off the key in the ignition.

Electronic fuel injection systems, such as port and direct injection, eliminate the need for an idle stop solenoid, meaning you can’t find these electrical devices in modern engines.

These electronic fuel injection systems rely on fuel pumps, which require electricity

Therefore, when you cut off the power by turning off the ignition system, the pump goes off, and no fuel goes into the engine.

Furthermore, injectors in direct injection systems are also electronic devices, meaning they won’t fire when the power goes off.

But carburetors are not electrical devices, meaning they will still run for a short time even after you switch off the ignition.

This situation occurs because the engine runs for a short while before coming to a stop when you turn it off.

The function of the carburetor is to create the air-fuel mixture. So, if it still runs, it will continue with the fuel delivery to the engine. 

And when turning off the engine, you’ll have driven for some time, meaning it will be hot. Therefore, even if the spark plugs don’t fire, the fuel will ignite in the combustion chambers because it will encounter hot carbon.

A carburetor

A carburetor

Additionally, the compression of the air-fuel mixture makes it hot and can ignite spontaneously even when the spark plugs are not firing.

An idle solenoid cuts off airflow into the carburetor, closing the fuel delivery to the engine when you turn off the ignition. And since it is an electronic device, this electric choke cuts off fuel delivery instantly.

How an Idle Stop Solenoid Works

To understand how these solenoids work, let’s take a step back to look at how carburetors work.

Carburetors contain butterfly valves or throttle plates that regulate the amount of air entering the engine. When these valves close, they cut off the airflow, denying the engine fuel, which chokes it to death.

But as stated earlier, the carburetor continues to run for a short time before the engine stops after you turn off the ignition. So, some air will flow in, and air drawn into the engine brings some fuel.

A graphical image showing how a carburetor works

A graphical image showing how a carburetor works

Therefore, the engine will still have the ingredients to fire and run. These throttle blades must be slightly open when the motor is idle to maintain low RPM firing. Therefore, the firing might continue when you switch off the ignition.

Solenoids are coils that function like magnets when energized. Idle-stop solenoids contain a plunger that controls the carburetor’s butterfly valves to regulate the air supply when energized and de-energized.

A solenoid

A solenoid

When you turn on the ignition, the solenoid gets energized and pops out the plunger slightly to open the butterfly valves to the idle position.

But when you switch off the ignition, the solenoid coil gets de-energized, retracting the plunger. This retraction pulls the throttle plates to close, preventing the engine from dying by cutting airflow into the motor.

Why Your Old Carburetor Engine Needs an Idle Stop Solenoid

The idle stop solenoid is crucial in carbureted engines because it stops the dieseling effect. And we are not talking about diesel fuel.

This phenomenon occurs only in carbureted gas or petrol engines. All diesel engines use direct fuel injection systems, including the old ones.

Diesel fuel injectors

Diesel fuel injectors

Also known as run-on, dieseling refers to a situation where a gas engine continues to try to run after switching off the ignition, even with the spark plugs not firing.

This issue usually occurs as bucking, coughing, or clanking for about 30 seconds after turning off the vehicle. It can be embarrassing, but the most significant problem is that dieseling can damage the engine if it occurs repeatedly.

It is vital to note that dieseling is more common when running unleaded fuel in carbureted gas/petrol engines. However, it can also occur when using the more harmful leaded fuel.

So, the device is more common in gas engines built after the banning of leaded fuel in 1975.

Different unleaded gasoline fuels

Different unleaded gasoline fuels

Signs of a Worn-Out Idle Stop Solenoid

Dieseling is the most typical symptom of a worn-out idle stop solenoid, which occurs as coughs or sputters after turning off the ignition. Other symptoms include the following.

  • High engine idle speeds
  • Poor fuel economy
A car with rough idle speeds with the check engine light on

A car with rough idle speeds with the check engine light on

A faulty idle stop solenoid usually damages the engine due to the dieseling or engine run-on effect. This condition disrupts the normal engine cool-down process and can cause costly damage if it occurs repeatedly.

How To Install an Idle Stop Solenoid

The process requires four simple steps.

  • Step 1: After removing the old idle stop solenoid, install the new unit’s bracket using its lock washers and screws (provided in the package).
  • Step 2: Mount the idle solenoid using the fixed bracket, then fasten the jam nut.
  • Step 3: Wire the solenoid using a 12V switched connection from the ignition system. This wire should be 16-gauge minimum.
  • Step 4: Test the solenoid. It should push out the plunger slightly to open the butterfly valves when you switch on the vehicle at the carb idle state. After switching off the ignition, it should retract the plunger and close the valves.

You can adjust the solenoid plunger by turning its hex head to increase the RPM (counter-clockwise) or decrease it (clockwise).

Wrap Up

Folks with modern vehicles can mistake the meanings of the terms dieseling and idle stop solenoids. But if you appreciate old cars and have one with a carbureted engine in your garage, you know what dieseling can do to your motor. 

Therefore, this article might make sense only to those with old vehicles or mechanics trained in the operations of carbureted engines. 

So, if you want to learn more about how these old engines work, check out this electric choke article. And comment below to share your thoughts or sentiments about this piece.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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