Why Does My Engine Keep Running ("Dieseling") After I Turn It Off?

Updated on January 19, 2019
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Tracing the source of a dieseling engine can be difficult at times.
Tracing the source of a dieseling engine can be difficult at times. | Source

When your engine keeps running after shutting it off, you'll hear it firing or coughing. This condition is also known as after-running or dieseling because combustion of the air-fuel mixture happens without the presence of the spark, just like in a diesel engine. For a few seconds, the pistons keep traveling up and down the cylinders. These symptoms usually appear because of a fault that is causing the temperature in the combustion chamber(s) to increase.

Causes of Dieseling

  • Using the wrong spark plugs
  • Spark plug tips covered in carbon
  • Engine overheating
  • EGR Valve Problems
  • Vacuum leaks
  • Idle too high
  • Fuel injection problems
  • Ignition timing too advanced
  • Carbon buildup in the valves or combustion chambers
  • Carburetor problems

Here, we'll take a look at the systems and components behind the potential fault and suggest some tests that can help you find the fault and prevent your engine from potential damage.

I. Problems With the Spark Plugs
II. Engine Overheating
III. EGR Valve Problems
IV. Vacuum Leaks
V. Higher Than Normal Idle Speed
VI. Fuel Injection Problems
VII. Ignition Timing Too Advanced
VIII. Carbon Buildup
IX. Carburetor Problems
Carbonized spark plugs can lead to engine overheating and dieseling.
Carbonized spark plugs can lead to engine overheating and dieseling. | Source

I. Problems With the Spark Plugs

Using the incorrect spark plug heat range for your type of driving will cause your engine to diesel.

  • If you recently replaced your spark plugs, make sure you installed the ones recommended by your car manufacturer. Check your car owner's manual or vehicle repair manual.

However, keep this in mind when replacing your spark plugs:

  • When your drive most of the time around town, make short trips and drive at moderate or low speeds, you'll need a spark plug with a hotter heat range to help burn off carbon deposits that may build up around the plug tip.
  • If most of your driving is done on the highway, you'll need a spark plug with a colder heat range to cool the plug tip more rapidly.

Sometimes, you can even visually check your spark plugs to see if you are using the correct spark plugs for your type of driving. Before doing the next test, make sure you are not supposed to replace the spark plugs yet. Consult your car owner's manual or vehicle repair manual.

1. Remove and inspect one spark plug at a time.
2. Look at the plug's center electrode tip.

  • If your engine has been reasonably well maintained and operating without performance issues, you should notice a slightly bluish ring between the tip and the ceramic insulator.
  • If you are using a plug with a heat range too high for your driving, you'll notice the electrodes burned and worn.
  • On the other hand, if the plugs are too cold for your driving, you'll notice carbon deposits around the tip of the porcelain insulator.

Low coolant will lead to engine overheating.
Low coolant will lead to engine overheating. | Source

II. Engine Overheating

If your engine appears to continue to run after shutting it off, engine overheating may be the cause behind it.

Pay attention the to temperature gauge while driving. If it goes higher than normal, this could be the problem.

1. First, check the usual suspects:

  • Make sure the cooling system has enough fluid. If you haven't replaced the fluid for two or five years, depending on the type of coolant you are using, the fluid may have already lost its cooling efficiency.
  • Check the thermostat, water pump and radiator for proper operation.
  • Verify that the radiator fan is working properly.
  • Check the radiator cap seal for wear or damage; if necessary replace it.
  • Inspect the water pump drive belt for wear, poor adjustment, or damage.

Buildup interferes with EGR valve operation.
Buildup interferes with EGR valve operation. | Source

III. EGR Valve Problems

Another common cause for engine overheating is a stuck-closed EGR valve or carbon buildup interfering with valve operation.

The Exhaust Gas Recirculation (EGR) valve is designed to introduce exhaust gases back into the combustion chambers to lower engine temperatures and, thus, harmful emissions.

But the valve can malfunction:

  • A vacuum leak or electrical circuit malfunction (depending on the type of valve your engine has) can prevent the valve from opening.
  • Road debris stuck underneath the diaphragm can prevent it from moving.
  • A damaged diaphragm may be unable to move.

Still, one of the most common EGR valve failures is due to carbon buildup. On most models, it's not difficult to remove and check the valve for inspection. Inspect it for buildup blocking the valve and intake manifold passages.

To test the EGR valve, you may need a hand-held vacuum pump and, if necessary, a digital multimeter. Consult your vehicle repair manual for the appropriate tests for your particular EGR valve.

Check every vacuum hose for leaks
Check every vacuum hose for leaks | Source

IV. Vacuum Leaks

Vacuum leaks affect engine performance, temperature, and, yes, may cause engine dieseling. For one, it can interfere with the operation of the EGR valve (see the previous section).

Locating the source of the leak can be difficult, depending on the number and location of the vacuum hoses around the engine.

1. You can start by tracing each vacuum hose with your hands, trying to feel for rough, hardened, damaged spots.

2. Check that each hose is properly connected.

3. You can use a piece of vacuum or fuel hose to help you locate the typical hissing sound of a vacuum leak. Put one end of the hose against your ear and use the other end to trace vacuum hoses and intake manifold gasket and throttle body gasket.

Locate the IAC around the throttle body.
Locate the IAC around the throttle body. | Source

V. Higher Than Normal Idle Speed

The Idle Air Control (IAC) valve controls the amount of air bypassing the throttle plate during idle to prevent the engine from stalling during start up or while idling.

In most modern vehicles, the amount of air that bypasses the throttle bore is preset from the factory and controlled by the car computer, so it can't be adjusted.

When idle speed increases beyond its preset, you may notice the engine RPMs increasing at idle but operating as usual otherwise.

Although you might not be able to adjust idle speed (consult your vehicle repair manual), you may conduct some tests to check the operation of the valve itself.

  1. Look for the IAC valve near the throttle body.
  2. Using your digital multimeter (DMM), confirm you have the appropriate voltage signal from the computer. For this, turn the ignition key On but don't start the engine.
  3. Back probe the wires at the IAC valve electrical connector. The voltage signal should be around 10 to 12.5 volts. Otherwise, check the wires for damage or a bad connection.
  4. Unplug the IAC valve electrical connector and measure resistance across the valve electrical terminals. Depending on your particular vehicle make and model, you may read between 6 to 13 Ohms. Otherwise, replace the valve.
  5. Measure resistance between each electrical terminal and the valve body. You should read 10,000 Ohms or more. Otherwise, replace the valve.
  6. Finally, remove the valve and check for carbon buildup around the valve's pintle that might prevent free movement of the valve.
  7. Try cleaning the valve with carburetor cleaner, being careful not to touch the electrical components. If this doesn't solve the problem, replace the valve.

Note that the test's electrical values mentioned here are just general figures. Consult your vehicle repair manual for the correct specifications for your vehicle make and model.

Check the fuel injectors for proper operation.
Check the fuel injectors for proper operation. | Source

VI. Fuel Injection Problems

Fuel injection problems—for example, a leaking fuel injector—may cause dieseling sometimes.

Still, a leaking fuel injector usually shows other symptoms as well. For example:

  • You may find it hard to start the engine.
  • The Check Engine Light (CEL) may blink.
  • A misfire may occur when starting the engine but may go away soon after.
  • Hydrocarbon emissions may increase.
  • Fuel system pressure may be incorrect.
  • Engine performance may be poor.

If you suspect problems in this area, troubleshoot the fuel system.

Use a timing light to check ignition timing.
Use a timing light to check ignition timing. | Source

VII. Ignition Timing Too Advanced

Ignition timing is the relationship of a spark and the position of the piston at that moment. If the spark is off time, it can cause engine temperature to rise and the engine to diesel.

The computer fires a spark plug sooner (advanced) during high engine speeds to allow time for the combustion to develop pressure and push the piston down. Otherwise, the computer fires a spark plug later (retarded).

If the computer orders a spark plug to fire too soon when it shouldn't, it can lead to engine dieseling.

In modern vehicles, ignition timing is controlled by the car computer and is not adjustable. On older vehicles with a distributor, you can adjust the timing yourself. Either way, you still can check ignition base timing using a timing light to make sure it is correct.

1. Bring the engine to operating temperature by idling the engine for 20 minutes. Or run some errands and then park the car at the place you'll be doing your test.

2. Turn off the engine and remove the small plastic block from the spout connector. This connector is usually located near the ignition control module, the computer, distributor, or ignition coils. Consult your vehicle repair manual, if necessary.

3. Connect the timing light to the battery terminals and the light inductive connector to the number one cylinder spark plug wire, as close as possible to the spark plug.

The number one cylinder is the closest one to the front of the engine. If necessary, consult your repair manual.

4. Apply the parking brakes, set the transmission to Park (automatic) or Neutral (manual) and start the engine.

5. Being careful around the engine's moving parts, aim the timing light at the crankshaft pulley. This pulley is the one at the bottom of the front of the engine. Look for the timing marks at the pulley.

6. Check that the correct number of degrees aligns with the timing pointer on top of the pulley or timing chain cover.

You may find the correct base timing for your engine on the Vehicle Emissions Control Information label located on the engine compartment. It may say something like "10 BTDC." Check your car owner's manual or vehicle repair manual.

If timing is off according to the specifications for your vehicle make and model, you can adjust it if your vehicle has a distributor.

  • Loosen the distributor holding bolt and turn the distributor slowly in either direction to adjust the timing mark to the correct degrees while aiming the timing light at the mark.
  • Then, tighten the distributor bolt.

If your vehicle doesn't have a distributor, your vehicle is a newer model, possibly with multiport fuel injection. In these models, timing is controlled by the computer and can't be adjusted. Some models, though, provide a small screw on the computer itself that can be used to adjust timing. Consult your vehicle repair manual.

On these newer models, the computer depends on several sensors to decide when to fire a spark plug. A malfunction in any of these sensors can cause the engine to run on.

So check the operation, connection and wiring of the following sensors, depending on your particular model:

  • Intake air temperature sensor
  • Throttle position sensor
  • Manifold absolute pressure sensor
  • Engine coolant temperature sensor
  • Knock sensor
  • Crankshaft and camshaft position sensors

A malfunction in any of these sensors may trigger the Check Engine Light (CEL). Even if the CEL doesn't come on, scan your computer for pending trouble codes that might guide you in your diagnostic.

Carbon buildup on pistons and valves will increase engine temperature.
Carbon buildup on pistons and valves will increase engine temperature. | Source

VIII. Carbon Buildup

Dieseling can also happen because of mechanical problems. Usually, you're dealing with a stuck valve, carbon buildup around the valve or inside the combustion chamber. Carbon buildup will increase engine temperature. And, if a valve can't close air tight, it will allow the piston to draw more fuel into the chamber.

Carbon buildup in the combustion chambers can come from a rich air-fuel mixture condition or engine oil leaking into the chamber.

You can check the mechanical condition of your engine with a vacuum gauge or by doing a compression test with a compression gauge. Consult your vehicle repair manual, if necessary.

If you suspect carbon buildup at the valves or combustion chamber, you can decarbonize the engine using Seafoam. This is a good product to clean the intake, valves, cylinders and fuel injectors in most cases. Follow the instructions on the product package.

Check the anti-dieseling solenoid for proper operation.
Check the anti-dieseling solenoid for proper operation. | Source

IX. Carburetor Problems

Vehicles with carburetors, especially, suffer from dieseling when the anti-dieseling (fast idle) solenoid is in need of adjustment or malfunctions.

The solenoid is designed to open the carburetor throttle plates when the engine is running and shut the plates when the ignition key is turned off.

Some anti-dieseling solenoids come with a screw adjustment. You may try adjusting and/or testing the solenoid. To test the solenoid, you'll need a digital multimeter or jumper wires.

Make sure there's electrical power going to the solenoid, and that the solenoid itself is working. Remove the solenoid and connect it to battery power with a set of jumper wires. If necessary, consult your vehicle repair manual.

If your carburetor doesn't have the anti-dieseling solenoid, try bringing down the idle speed by turning the speed screw. Consult your vehicle repair manual, if necessary, and check for any overheating issues.

If your engine keeps running after turning off the ignition key, try to troubleshoot and repair the fault as soon as possible. Dieseling can cause serious engine damage if the problem worsens. Check the different systems and components as outlined above. The sections are developed to help you find and repair the fault as soon as possible. Especially, pay attention to those systems you've neglected lately. And consult your vehicle repair manual when necessary. If you don't have the manual, you can buy an inexpensive, Haynes aftermarket copy from Amazon. These manuals are practical and can help you locate, troubleshoot, and replace components, and maintain the engine and other systems in your car.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2017 Dan Ferrell


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    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      7 months ago


    • profile image


      7 months ago

      Have driven an old Mitsubishi Triton that does this. It is likey due to a combination of many of these factors.It has a radiator, but is currently running on air cooling only due to issues with said radiator. It seems to be happy to run like the 2020 Mazda3. Spark ignition sometimes, compression ignition others... If only Mazda knew that Mitsubishi had invented a compression ignition engine that runs on gasoline, 30 years before they did... ;P

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      23 months ago

      I haven't, but it's a rare case. I understand it may happen if oil leaks into the combustion chamber. Time for an engine repair, no doubt.

    • WheelScene profile image


      23 months ago from U.S.A.

      Great read! Very comprehensive article with great photos! Have you ever seen a diesel engine keep running on its own?


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