The Stovebolt Site Forums The Shop Area Engine Shop Engine runs on ..dieseling....

Chev 261..... rebuilt stock less than 100 hours...... ran fine..... changed the 3/8 line on the PCV as my curved pipe had kinks.

Engine started fine..... came up to 160 degrees.....BUT... when I turned of the ignition switch the engine mumbled and stumbled and kept on running...popped clutch and it stopped.

Restarted...ran fine... moved it for storage inside a sea container.....when I turned off the ignitionthe same dieseling occured except this time engine seemed to run backward and really farted blue smoke out of the carburator air filter.... now I appreciate why they used to call them flame arresters.

Any ideas where I start looking ...?

Confused...Bob C.

Throttle is open too far at idle, or you have a vacuum leak.

Lower the idle speed- - - -the engine is Dieseling due to running a little too fast on shutdown.
Jerry

Also, check the timing. If it is too slow then the idle speed may be adjusted up to compensate.

Bob,

How are you setting the timing?

What year 261?

Original 261 flywheel (from the year of the truck)?

I've always been told that the timing makes no difference in dieseling since the ignition is off. The plugs aren't firing so the timing should have no effect. Most cases of dieseling I've seen were caused by a hot spot in one or more cylinders, either a piece of carbon or a sharp metal edge that would get hot while the engine is running.

The timing can have an effect. (Only when the switch is on and the engine is running). If the timing is too slow, then the idle screw may be turned up to make the engine idle where it should. This opens the throttle plates a little more than they should be. Bringing the timing up to spec lets the throttle plates be closed a little more.

On more than one vehicle I have seen folks set the timing only to complain it wanted to run on after they worked on it. Set the timing back where it was supposed to be, then set the engine idle speed, and usually cured the problem.

Some of the newer carburetor vehicles the factories started putting idle speed solenoids to bring the idle up while the switch was on, and then to allow the throttle plates to close more when the switch was shut off.

This may not be the case here, just flat may be idled up too fast and the mixture screw a little rich.

But if the timing is slow it can also heat things up and do it pretty quickly.

Dieseling - the ignition can be off

"Dieseling or engine run-on is a condition which can occur in spark plug, gasoline powered internal combustion engines whereby the engine keeps running for a short period after being turned off, due to fuel igniting without a spark."

This condition can occur for a multitude of reasons:

- Built-up carbon in the ignition chamber . , ,

- A carburetor that does not close entirely . . .

- Incorrect timing.

- An engine that runs too hot or too lean . . .

- An idle speed that is too fast . . .

Somebody still needs to explain to me how a non firing spark plug contributes to dieseling. Maybe timing is a valid possibility but I don't see it. Ignition timing is related to when the plugs fire. Key off - no plug firing. Unless there's a carbon build up on the plug(s) that provides the ignition to the fuel/air mix a spark plug would play no role. Ignition timing is irrelevant to dieseling the way I see it. From your link Tim:

"The dieseling phenomenon occurs not because the compression ratio is sufficient enough to cause auto-ignition of the fuel, but a hot spot inside the cylinder starts combustion."

Someone needs to edit the Wiki link to correct timing as a possible cause.

Gotta think beyond the plug firing. The big thing on dieseling is hot engine and enough fuel to fire off the heat.

One way you get enough fuel is set the idle speed up (open the throttle plate) to keep the engine idling where you want it. Also perhaps mixture screw may be too open allowing a richer mixture.

If the timing is too slow, you have to open the throttle plates a little more to get the engine idle speed you want. If the plates are open enough, when the engine warms up, it may allow enough fuel to flow through to be able to fire the cylinder even though the ignition is off.

Also, when the timing is too slow, things tend to heat up quicker and more of the cylinder is exposed to the explosion caused on the compression stroke. The more cylinder that is hot, the easier it is for the engine to diesel when the switch is turned off.

Getting the timing where it should be allows the throttle plates to be closed a little more and less fuel / air enteriing the engine. Also, less cylinder is exposed to the heat during the explosion in the cylinder.

Some manufacturers added a solenopid to open the throttle plates a little when the switch was turned on and then when turnd off allowed the throttle plates to close more shutting off the air fuel mixture.

Does this help a little?

In this case he may simply have the idle speed too high and just bringing thst down may help.

Fuel doesn't matter, air does.

Fuel does matter. Not enough fuel, no dieseling.

Ditto, Roy! When the manufacturers started running higher idle speeds/throttle openings in the mid-1980's to tweak emissions, they also started using a solenoid valve in the idle circuit to positively stop fuel flow. They were called- - - -wait for it- - - -"Anti-Dieseling solenoids"! Run-on without fuel to keep the process going is impossible! That's why a Diesel engine STOPS when the fuel is shut off! We could cheat a little on hydrocarbon emissions tests by retarding the timing and leaning out the mixture, but neither of those procedures had anything to do with Dieseling on shutdown. They did make the exhaust manifolds glow cherry-red on the way to the emissions test station, though!
Jerry

I have also seen poor quality fuel cause dieseling problems too. A friend of mine put what we call "drip" gas in a 1974 Suburban with a 454. He could drive with the switch off after he did that.

He didn't make that mistake again.

OK Roy, I gotta ask, what the heck is "drip" gas?????

I don't know if it's as available as it used to be, but sometimes a well head would put out a liquid that was pretty volatile. Some folks called it "drip gas" because you could let it drip out a valve to fill your can. Right from the well. Some wells had containers to catch this "gas". Others had different names for it.

Wasn't refined or processed, but for many years people with access to it would go gather it up and use it in their vehicles, tractors, for a cleaner degreaser it worked grest. But it is highly explosive.

Hard on valve seats too. May have been hard on the rest of the engine too since it wasn't processed.

In my friends 454, this "drip gas" would fire off without a spark needed. Just warm up the engine and shut it off. Sounded horrible though sitting there running without the spark to set it off. And stunk.

Wow thanks for the many responses.

261 is installed in a Chev 1940 right hand drive Canadian military truck..... engine is from a 1959 5 ton fuel delivery truck.

Truck engine is rebuilt stock.. stock 59 flywheel and starter.....plugs are new AC 45..... dizzy is Pertronix inside...carb is rebuilt YF which runs just super.... stock 216 air filter.

It ran fine before I replaced the PCV pipe with a new one..... may have a small air leak....will check with a unlit propane torch.....

Idle screw may be set too richhhhh as I am still getting some black stain at the exhaust.

Throttle linkage has been sticking and I intended to install a stiffer return spring on the linkage as the throttle doesn't return tight against the idle screw ...

Keep in mind that the throttle linkage has to go right through the floor panel... to the front of the engine..... across to the left hand side just below the water neck/above the fan pulley..... back to the bell crank then up to the carb.....

My bet is 99% on the idling being too fast due to sticking linkage that keeps the carb plate partly open....and the mixture screw too rich.

Thanks for all the help.

On the timing issue....setting up the timing in the tight confined of the semi COE engine bay.... almost impossible to see the marker and position the timing light. Chap in New Hamshire came up with the following idea. On top of the bell housing is a small sheet metal cover.... you remove....set the engine at absolute TDC.....paint a vertical mark on the clutch cover assembly.... run a stiff wire between the two screws that normally hold the flat cover. Now sit back comfortably with the timing light aimed 90 degrees to the wire and the painted mark on the clutch cover. In our case the floor bellhousing cover is not yet installed.... I aim the light and tell my able assistant to rotate the dizzy to the right setting.... tighten up and the job is done.
why is it that GM never documented that approach in there manuals.

Thanks for all the assistance guys it will make next weekend more enjoyable.

Bob C.

For those still a bit fuzzy on carburetors: if there is any vacuum, fuel is supplied automatically.
The reverse (you have fuel, but no air) does absolutely nothing.

Another very important reason why air matters is;
When the throttle is closed and the cylinder can't get all the air it wants (vacuum in manifold)the unfilled cylinder has lower compression pressure.That is why the recommended practice for compression pressure test is to block the throttle open.
If the closed throttle results in lower cylinder pressure then auto-ignition (dieseling)is less likely to occour even with hot spots.
Just for laughs sometime with the engine warmed up good and the vehicle stationary,engine running bring the throttle up to 1/3 or 1/2 and hold it and quickly ,to avoid over revving, cut the ignition off.You may not want to do it many times or the neighbors will have homeland security after you.

Uncle.

Just consider the source, Roy!
Jerry

I know Jerry. But it's usually such an easy fix.