Vehicle Engine Run-On

Sputtering and shaking that occurs after a vehicle is turned off is known as dieseling, or more commonly as engine run-on. The engine isn't really running as normal, but the fuel remains ignited without the assistance of a spark. Diesel engines operate without a spark, hence the term dieseling.

An engine can continue to gasp and quake for several seconds before finally stopping. Engine run-on is a perplexing problem for many vehicle owners, but it doesn't always take a professional mechanic to diagnose and repair this type of problem. In many cases, a simple adjustment is all that's needed.

Octane

In most instances, regular octane gasoline is sufficient, but if higher octane gas is recommend for your vehicle, or if it isn't running well on lower octane varieties, try a higher octane grade of gasoline. Generally, if you have a stock engine, higher-octane gas isn't necessary, but if your engine has been modified it could solve the problem. You'll have to dig a little deeper into your wallet, but if it solves the problem, it's worth the expenditure. Upgrading to higher-octane gas is an easy solution for a confounding problem.

Idle Speed

If the carburetor is out of adjustment, the idle speed might be set too high. This is one of the primary causes of engine run-on. You'll need a tachometer to check the speed at which the vehicle idles. If you find it's too high, you'll also need to check the throttle linkage to find out if it's sticking before making any adjustments.

Check the throttle linkage by compressing it while the engine is running. If the idle speed drops while pressing on the linkage, it's probably sticking. Correct the problem by spraying and cleaning the linkage with solvent, and make any necessary adjustments to the idle speed.

If the linkage continues to stick after cleaning, it will require a new throttle return spring. Buy a universal replacement spring, and bend the ends to form hooks. Attach one end to the throttle linkage, and hook the other into the bracket. If the spring is too loose, trim the ends to the proper length, and bend them again. There should be a slight amount of tension when the spring is attached.

Solenoid

If the idle speed is correct, check the solenoid if the carburetor is equipped with one. Have someone turn off the vehicle while you're watching the solenoid plunger. Watch for it to retract when the vehicle is turned off. If it doesn't withdraw, the solenoid will require replacement to correct the problem.

Engine Temperature

A malfunctioning thermostat can cause engine run-on if the vehicle is running hot. Check the temperature gauge, and if the engine is hotter than it should be, replace the thermostat as soon as possible.

Also, check the condition of radiator coolant. It should be checked approximately every six months, and flush and replace it annually. If the coolant looks murky, the radiator should be flushed, cleaned, and refilled right away.

Ignition Timing

An ignition timer set too far ahead can cause engine run-on. Check the timing of your vehicle with a timing light. Make the appropriate adjustments according to the tune-up diagram under the hood, and follow timing light instructions and specifications.

Air-Conditioning Compressor

Some vehicles are equipped with an anti-dieseling relay, and it could be a faulty relay if your vehicle is outfitted with this component. The relay momentarily triggers the air-conditioning compressor when the vehicle is turned off. When the relay is faulty, the compressor doesn't turn on when the engine is turned off. Consult the vehicle manual or a qualified mechanic to find out if your vehicle is equipped with this feature.