When you turn off the key, you expect the car to stop! It can be very frustrating when it doesn't. If the engine continues to run just as before you turned it off - as if you hadn’t turned it off at all - your ignition and fuel system are not shutting down. In that case, your car has an electrical problem that needs to be addressed. Either your ignition switch element or a power relay needs to be replaced. If, however, the engine fails to stop, but instead seems to go into leaping convulsions accompanied by knocking and pinging sounds, what you are experiencing is known as "dieseling."

Dieseling occurs when there is something in the engine combustion chamber that is hot enough to light any residual fuel that enters the engine. It is called dieseling because diesel engines ignite the fuel in the same way, without using electrically operated spark plugs. It’s not usually a problem with modern fuel-injected cars, but used to haunt drivers of cars in the dark ages of carburetors frequently.

If your car has a problem with dieseling. There a few steps you can take that may help you figure out what is going on. It’s possible you can deal with the problem with a few minor adjustments.

Part 1 of 3: Determining if you have an electrical or a dieseling problem

Materials Needed

  • Screwdrivers (Phillips and straight)
  • Workshop manual

Step 1: Determine whether you have an electrical or a dieseling problem. When you turn off the key, does it seem as if you haven’t turned it off at all? Do the instruments and accessories and turn signals still work?

Does the engine run smoothly as though you could just put it in gear and dive away? If so, you are experiencing an electrical problem.

However, if the engine goes into fits of knocking and pinging, continuing to turn but not shutting completely off; then go ahead and look at Part 2.

Remove the bottom half of the plastic shroud surrounding the steering column. Locate the electrical connector to the ignition switch. It may be on the back of the ignition switch itself, or it might be at the end of an electrical pigtail from the switch and be located further down the steering column.

Unplug the switch at the connector and everything should immediately go dead. If this is the case, solving this problem will require replacing the ignition switch element.

Step 2: Check for a power relay. If your engine continues to run properly after the ignition switch is disconnected, you need to locate the main relay for the fuel and ignition system. Your workshop manual should be able to help you find this.

In some cars the relay may be called an ECM relay, a Digifant or a DME relay. While it’s quite rare, a car can continue to be powered up by a sticking main relay. If the car finally powers down after the relay has been removed, replace the relay.

Part 2 of 3: Diagnosing a dieseling engine

Materials Needed

  • Distributor wrench (appropriate for your car)
  • Safety glasses
  • Screwdrivers
  • Spark plug wrench (appropriate for your car)
  • Tachometer
  • Timing light
  • Workshop manual

Step 1: Carburetor, or fuel injected? Does your car have fuel injection or does it have a carburetor? If you don’t already know, check your workshop manual.

If your car was made after 1985 it probably has fuel injection. Dieseling is rarely a problem with fuel injection because the fuel delivery is shut down as soon as the switch is off. Unless it is specially equipped, a carburetor will deliver fuel as long as there is air passing through it.

If you have a fuel injected car that is dieseling, there is a leaking fuel injector in the system that has to be replaced.

Step 2: Check the idle speed. If the engine idle speed is too high, it may cause the car to diesel after it is shut off.

Hook up your tachometer according to instructions and check the idle speed. Most four and six cylinder engines should idle around 850 to 900 RPM. Eight cylinder engines may idle around 600. Check your workshop manual and adjust the idle speed according to the specifications.

Step 3: Check the anti dieseling solenoid. In the late 1970s, with the addition of emission controls, dieseling starting becoming a problem on even new cars. So manufacturers added an electrical shut-off valve to their carburetors and called it an anti-dieseling solenoid.

It’s usually cylindrical, about 1-2 inches long and has a single wire connected to it. That wire is hot when the key is on and opens the valve, allowing it to close when the key is off.

Disconnect the wire and turn on the key. Touch the connectors together and release them. You should hear the solenoid click as it opens and closes. If there is no sound, then the solenoid is inoperative and needs to be replaced.

Step 4: Check the ignition timing. Modern cars do not have adjustable ignition timing, but if your car is old enough to have a dieseling problem, then it probably has adjustable timing.

If the ignition timing is off by a few degrees, it can cause the temperature inside the engine to rise enough to lead to the conditions that cause dieseling.

Check the workshop manual for the timing procedure. It can vary widely from car to car. Some cars set the timing at idle, others set it at a high engine speed. Some manufacturers want you to disconnect certain controls before setting the timing. There is no one procedure that applies to all engines.

  • Warning: Be very careful! You will be working close to rotating engine parts.

Shine the timing light on the timing marks while the engine is running at the specified speed and verify that the marks line up as they should. If adjustment is necessary, that is done by loosening the distributor with the distributor wrench and turning the distributor slightly in one direction or the other depending on which way you want to move the timing.

Step 5: Check the spark plugs. Using an incorrect spark plug can also cause dieseling. Spark plugs are designed to maintain a specific operating temperature, and if the plugs installed in your engine are the wrong heat range, they may cause a hotspot that can lead to dieseling.

Take out a spark plug and compare the part number to the manufacturer’s specifications. Replace them if you find anything wrong.

Part 3 of 3: Coping with Carbon

Materials Needed

  • Engine cleaning fuel additive
  • Premium gas

Step 1: Try a fuel additive. If you have gone through all these steps and have not solved your problem, you probably have a serious buildup of carbon in the engine.

Carbon buildup can raise compression in the engine and can cause hot spots in the combustion chamber that can lead to dieseling. There are a number of fuel additives on the market that are intended to help clean the carbon out of an engine while it is running.

Following the manufacturer’s instructions, put a can of fuel additive into the tank when you fill it up. Then take the car out on the highway and drive at speed for a while, also doing some hard accelerations. The cleaning formula may help to break up carbon deposits and send them down the tailpipe.

  • Tip: At some point while discussing your problem, you will probably encounter some old guy who will tell you to try pouring a cup of water down the carburetor while the engine is running. Thank him politely, but don’t follow the advice. It’s a good way to wreck your engine.

Step 2: Change your fuel. Using higher octane fuel may help to alleviate a dieseling problem that you have not been able to remedy through other means. It’s a treatment, not a cure. Higher octane fuels are more stable and not as prone to the pre-ignition caused by the carbon buildup in the engine.

Eventually, you may grow weary of the extra expense of filling your car up with premium. Or using premium may solve your problem at first but will grow less and less effective as time goes by. The ultimate solution to your dieseling problem may involve some major engine repair. Contacting a mobile mechanic, such as one from YourMechanic, to come to your home or office to inspect your car help you figure out what to do next is easy and convenient.

This article originally appeared on YourMechanic.com as How to Troubleshoot a Car That Won't Turn Off.

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