What would case the truck to continue running..

[BigSur]

...even when the truck is turned off? I recently swap carbs but this happened before but once i turn the key off the truck knocks and sputters. It did it today for about 20 seconds once the car was off. I don't get it.

Jason

Oh this is on my 87' fj60

 

[fjdemon]

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.


This is what I pulled up for you. I think timing also effects it. Later-

 

[fjdemon]

or a vaccum leak or carbon build up. Many things, just maybe I can get you steered in the right direction

 

[fjdemon]

"Dieseling- are you referring to the engine running on after you shut it down? This is caused by either too high compression ratio or carbon buildup. Since I doubt you have that high of a compression ratio lets go with the carbon buildup route. While your motor is running it produces carbon in the cylinders. Over time carbon can accumulate in the cylinder- on top of the piston and on the cylinder head and valves are the most comon places. This gives you two problems- increased compression ration since more volume is taken up by the carbon in the combustion chamber, and the second involves heat retention and is the most common cause of dieseling. When the motor runs the carbon in the cylinder gets hot. If it gets hot enough it will ignite the fuel mixture before the spark plug does causing pre-ignition and dieseling after you shut the motor off. It happens once you turn off the otor because of the one cylinder which is on the compression stroke as you shut off the key still having a fuel charge in it. Since the fuel pump will pump gas as long as the otor is rotating, regardless if the ignitoin is on, if the overheated carbon can keep ignition the mfuel mixture the motor will run on.
Dieseling can also be caused by improperly set timing.
Yes lean running can cause pinging. If you run lean enough it will backfire too."

 

[Marc311]

Improper timing can contribute to dieseling. However, there may be two other issues as well.

First - some have had issues with their ignition switch going bad. This does not sound like it is the case since it only continues for about 20 secs. That sounds like typical dieseling.

Second - if your fuel cutoff solenoid is working properly, it should also prevent this. Did you ground it previously? I may be wrong, but I think the fuel cutoff solenoid also acts as a anti-dieseling solenoid. With no fuel, there's nothing to ignite.

 

[Godwin]

What type of carb? Had the same problem with a Holley, it did not have a fuel cut solenoid. My solution was to have the tranny in gear and ease out on the clutch while turning off the engine. Problem went away with a switch back to a stock carb.

 

[Cruzerman]

As Jim eluded to, the stock carb has a fuel cutoff solenoid. Check this to make sure it is functioning properly.

 

[alaska60]

It mat be the gas you are putting in the truck. try a higher octane fuel. or a fuel additive to increase octane. this should solve your problem.