/I have a 1969 Corvette with a 350 engine and a 1971 with a 454.

The one with the 350 engine keeps running after I turn the ignition off maybe about 4 times a year (i drive it about 2-3 times a month). This did not bother me much since it does not happen often.

My 454 diesels all the time it always smells like unburned fuel and with this one I realized I really need to learn how to stop them from dieseling.

Any advise on how to make it stop?

 

Higher octane fuel and a slower idle.

Idling too fast can cause dieseling. Too low of octane can cause dieseling. Also, large buildups of carbon in the combustion chamber can also contribute to dieseling.

And no, before someone says it, ignition timing has nothing to do with dieseling as dieseling occurs after the ignition is turned off.

Mike

 

If you are running ported vacuum to the distributor, change to manifold vacuum. It usually helps. You may have to lower the idle setting on the carb.

If it's an automatic - turn off the engine in drive. The added load usually stops it.

Higher octane fuel will help.

 

I would have to disagree.

Dieseling is about a very hot combustion chamber and/or something in that chamber that is very hot. Retarded timing creates excessive heat.
Not saying it can't be solved by slowing the idle or using high octane fuel but the cause may not have been addressed by doing either of those.

 

 

 

Yes. I have one. Why do you have coke in your carburetor?

 

An over rich idle mixture combined with an idle speed that is too high will cause an engine to diesel. The rich mixture promotes the buildup of carbon deposits in the combustion chamber and when the engine is good and hot the carbon will glow red and act like a glow plug in a two-cycle model airplane engine; providing the heat needed to promote combustion. It doesn't hurt to give an engine a good workout every once in a while to blow the carbon out..

 

Yes, I missed excessive engine heat, that can contribute to run on. And also correct that retarded timing can cause excessive heat, but so could a bad thermostat. If the engine isn't overheating though, too low of octane, too fast idle, and carbon buildup are the primary causes.

Idle speed is easy, so is better gas. You can clean the carbon out by water or Seafoam down the carb with the engine up to full temp and keeping the engine speed up while you SLOWLY pour the water or Seafoam down the carb. Shut it off immediately and let it set for 10 minutes. Start it back up and race it up a few times. You are going to make a lot of smoke, so be prepared. Now go out and drive it really hard. Most of the carbon will be gone.

Mike

 

garden hose with wide pattern cone. not pouring water in. be holding revs up well above idle. if you hydro lock it, pull the plugs. dont try to crank it out.

 

Water in a glass poured down through the carb is pretty effective for removing carbon build up.

I would not use a garden hose or a high rate of water into the carb. Hydrolock can permanently damage your engine so you don't want too much flow of water into the engine.

I can idle my engine up to 1800 RPM and shut it off and it stops running immediately. So Idle RPM is not a cause of run on or dieseling.

The Cause is something is hot enough to ignite the fuel without spark. High idle may be a contributing factor due to the other factor being present.

Dieseling is typically caused by retarded timing and/or carbon buildup.

If your timing is too advanced you may be getting detonation or you may be getting detonation for a number of reasons. Detonation is going to cause a hot spot or spots in the combustion chamber, these may also a source of ignition after the ignition is turned off.

The specs for timing on our vettes were for emissions, not performance. So it may call for 8* or 10* of initial but something more in the range of 14* to 16* may be more appropriate for cooler running.

My initial runs @ 19*. It's a slightly modified engine with a bigger cam that needs more initial for good idle. I also have modified the travel of the mechanical and vacuum advances to limit total timing advance to the numbers that the engine like. 32* total and 51* with vacuum at cruise.

I would start with cleaning the assumed carbon out, then work with the timing to get things running cooler.
Keep an ear out for detonation, you don't want to run into that either by advancing the timing.

Limit the total with vacuum to 50* to 52*, total without vacuum (Initial plus mechanical) to around 36*.
This would mean if your mechanical advances 20* total then your limit for initial is going to be 16* without limiting the mechanical advance.

 

Engine run-on after shutdown is a function of excessive throttle blade opening at shutdown. This can be interpreted as "high idle speed," but the throttle blade opening can be excessive without the idle speed actually being high. To stop the run-on, you have to reduce throttle blade opening.

The easiest and most common fix is to fix the timing: If timing is retarded, the idle speed screw has to be cranked in to get the idle speed up to a reasonable level. Your initial timing at idle should be in the low- to mid-teens. If you hook up your vacuum advance to manifold vacuum, timing should be up around 30-ish at idle. By running this amount of timing, the idle speed screw can be backed off significantly compared to an engine running 6 to 8 degrees initial on ported vacuum. Then, run the lowest idle speed to produce a comfortable and reliable idle. This will often solve the run-on problem.

If that doesn't do it, simply use the GM method to solve the problem: GM had run-on problems with these cars, and installed anti-dieseling solenoids on them to fix it. The solenoid is the same solenoid used on the A/C cars to increase idle speed when the A/C was turned on. When used as an anti-dieseling solenoid, the carb's idle speed screw is backed off to about 450 - 500 rpm, and the engine idle speed is set using the solenoid in the energized position with ignition "on." When ignition is turned off, the solenoid collapses, closing the throttle blades, preventing the run-on condition.

Lars

 

the hose is so you can regulate a spray cone pattern, almost a fog. like a big injector nozzle. and you practice your spray pattern away from the engine first. you dump even as little as a glass full it can all run into 1 cyl and do bad things.

 

Good info here Lars. Many of the 50's and 60's GM cars had the anti-dieseling solenoids.

Still, I have seen cars that did not originally have this problem develope it and after doing a carbon cleanout stop the dieseling.

Mike

Mike

 

Because one of the two reasons for fuel ignition after shut down was eliminated.

If there is no source of ignition ie. something hot enough, it will stop running after shut down.

Or conversely if there is no source of fuel then the engine will stop running after shut down.

Either will do the trick to prevent dieseling.

I also remember a different type of anti dieseling solenoid. They were nothing more than a needle valve that when energized by the ignition allowed fuel to flow. Then when the ignition went off the needle valve closed and shut off the fuel. No fuel= no run on/dieseling.

 

 

Ideling to high

 

Lars quote

 

The easy way to stop a dieseling engine is to simply load the engine a bit.

If you have an auto trans, turn the engine OFF in-range, THEN put the shift lever in PARK. If you have a manual trans, leave trans in gear (2nd works better than 1st, in this case), let the clutch out just enough to put drag on the enigne, and turn OFF the ignition.

GM actually did similar stuff for cars that had A/C. When the ignition was turned OFF, the A/C was energized for about 3 seconds so that the engine was 'loaded'. That would kill the engine.

These are just 'tricks' to get the engine to die, if it wants to diesel. As Lars stated, it would be best to adjust timing and carb settings so that it wouldn't diesel in the first place. But, in a pinch......

 

Mine was spark plugs that were of the incorrect heat range. Although when I checked the part numbers they were the equivalent of what I replaced them with.