What are the reasons behind a car engine not turning off?

This blog post will answer the question, ‘What are the reasons behind a car engine not turning off?’, and cover topics like what is meant by a car engine not turning off, and diagnosis and fixes for a car engine not turning off.

What are the reasons behind a car engine not turning off?

These are the following contributing factors behind a car engine not turning off:

  • Defective spark plugs
  • Carbon deposits in spark plug
  • Overheating of engine
  • Idle speed is too high
  • Incorrect ignition timing
  • Carbon deposition in engine valves and combustion chambers
  • Problems with fuel injection

What is meant by a car engine not turning off?

At times it is noticed that even after turning off the ignition key, the car engine does not turn off completely/

This mainly happens due to the air-fuel mixture still being ignited inside the combustion chamber. This also means that the mixture is burning at the wrong point of the combustion cycle thus causing the engine to perform jerky or shaking movements.

The area where the air-fuel mixture mixes are called a hotspot and the primary causes behind this occurrence is using the wrong fuel grade or a type of fuel containing low octane, defective spark plugs, incorrect ignition timing, or heavy carbon deposition in and around valves and combustion chambers.

Diagnosis and fixes for a car engine not turning off

These are the following diagnosis and fixes for a car engine that fails to turn off.

Spark plug inspection

This is a common cause behind the engine still running after the ignition is turned off. It is important to check if the electrodes of the spark plug have a white ring or a bluish ring around them.

The reason is that the spark plugs can get overheated, which is signified by the white rings, and are in operating condition if they have bluish rings. The spark plug insulator should also be checked for defects.

Check for engine overheating

It is an important inspection and we must check if we have sufficient coolant level or if the coolant needs to be replaced. Other components like the radiator cap, thermostat, and water pump need to be thoroughly inspected.

The radiator fan inside the radiator housing should be operating normally and the water pump must not have loose or worn-out belts.

The engine coolant must not have lost its cooling efficiency and should be regularly checked for a potential replacement.

Checking the ignition and fuel injection systems

After taking the spark plugs out, a timing light needs to be used to check for proper ignition timing. A hose needs to be used to check for leaks in the engine intake manifold followed by adjusting the air-fuel mixture to the optimum ratio.

Checking the anti-dieseling valve

We need to check the anti-dieseling valve that is usually attached by the car manufacturer beside the carburetor.

Using a spanner, the wire needs to be disconnected and the valve needs to be carefully separated from the side of the carburetor. 

Connect the live terminal using a jumper lead on the anti-dieseling valve to the positive terminal of the battery and bring the casing of the valve in contact with the negative terminal. If the valve is working, a click sound should be heard.

Cleaning carbon buildup in and around valves and combustion chambers

Carbon deposits usually occur as a result of a rich air-fuel mixture being burned in the combustion chamber or due to the leakage of engine oil into the combustion chamber.

A strong indication of carbon deposition is the overheating of the car engine. Carbon deposits can be cleaned by using a product called Seafoam that helps in decarbonization.

This will aid in cleaning the valves, fuel injectors, intake manifold, etc.

Changing the fuel grade

As discussed earlier, a low-grade fuel can cause dieseling or an engine still running after ignition has been turned off. This is because a low-grade fuel has low octane which is very unstable.

Higher graded fuels have high octane that brings high stability and lesser chances of pre-ignition.

Checking engine idling speed and ignition timing

Higher idling speed or incorrect ignition timing can cause the engine to run even after the ignition has been turned off.

Use the car’s instruction manual to find out the correct idling speed for the car or consult a professional mechanic to set it. Ignition timing should be correctly adjusted too as it can affect the combustion rate of the air-fuel mixture.

Research has shown how fitting an air-bleed valve can help prevent or cure the dieseling of an engine.


This blog post addressed the question, ‘What are the reasons behind a car engine not turning off?’.

We understood what it means for a car engine not to turn off even after the ignition key has been turned off. Furthermore, the different diagnoses and fixes for this problem have been discussed as well.  Please feel free to comment on the content or ask questions in the comments section below.

Frequently Asked Questions (FAQs): What are the reasons behind a car engine not turning off?

What causes my engine to keep running after turned off?

Turning off the engine increases its temperature which burns the residual air-fuel mixture in the combustion chamber and causes detonation. Hence, we observe the engine still undergoing some sort of reaction even after turning off the ignition key.

What causes car engine run on?

This happens mainly in gasoline-powered engines that use a spark plug, this results in the engine drawing fuel from the carburetor and igniting it inside the combustion chamber without receiving any kind of spark from the spark plugs.

What causes a car to run rich at idle?

The major reasons that cause a car to run using a rich air-fuel mixture are as follows:

  • Defective fuel injector
  • Defective oxygen sensor
  • Defective fuel pressure regulator