: what causes engine 'run in' after you turn off the ignition



steck
07-28-11, 09:54 AM
this is normally followed by a loud squeching noise just before it finally quits !

always turns heads..but for the wrong reasons !


a '73 fleetwood 472..

CADforce69
07-28-11, 10:35 AM
Was A/C on? I have a car (not a caddy) that used to do this only when I stopped engine with A/C compressor on. Ive also seen cars doing that for not using spark plugs with the correct power specifications. Are you using AC R46N type?.

outsider
07-28-11, 10:58 AM
If you mean the car is "dieseling" ie: trying to continue running after it's shut off...

could be a vacuum leak or timing is off. My 69 does this because of the timing and my distributor is still stuck so still trying to get that set up.

amunderdog
07-28-11, 11:15 AM
Usually
Advanced timing on a hot motor.
The engine will continue the combustion cycle free of ignition until the cylinders cool down enough to stop.
Thus the term dieseling.
Sometimes it can happen when the fuel in the carb boils.
As other poster pointed out.
When the A/C is on there is a dash pot that raises the idle to compensate. Some times they will continue to run for a moment after turned off.
They term this Run On.
Those are three off the top of my head?

77CDV
07-28-11, 01:44 PM
In any case, you should correct this as it's not good for the engine long term.

sven914
07-28-11, 06:03 PM
On the side of the carburettor are vacuum brakes. When the engine is turned off they slam the throttle valve shut to cut off the air to the cylinders. When you turn the ignition off, the carburettor continues dripping fuel into the intake, so if the throttle doesn't shut completely or fast enough, it allows enough of an air/fuel mixture to keep the engine running for a few more revolutions. If there is a vacuum leak in the lines to the vacuum brakes, they won't work correctly.

creeker
07-28-11, 10:17 PM
On the side of the carburettor are vacuum brakes. When the engine is turned off they slam the throttle valve shut to cut off the air to the cylinders. When you turn the ignition off, the carburettor continues dripping fuel into the intake, so if the throttle doesn't shut completely or fast enough, it allows enough of an air/fuel mixture to keep the engine running for a few more revolutions. If there is a vacuum leak in the lines to the vacuum brakes, they won't work correctly.

I was going to suggest something that you already explained, on my 80 cdv. I was also getting dieseling, I found an old style mechanic who went right to the
small spring on side of carb., and replaced with a different return spring and the problem was solved, the old spring was not pulling back enough on the throttle to
stop the gas flow, maybe yours is the same, start simple.

csbuckn
07-28-11, 11:09 PM
I love how much I learn here.

cadillac_al
07-29-11, 07:43 AM
I was going to suggest something that you already explained, on my 80 cdv. I was also getting dieseling, I found an old style mechanic who went right to the
small spring on side of carb., and replaced with a different return spring and the problem was solved, the old spring was not pulling back enough on the throttle to
stop the gas flow, maybe yours is the same, start simple.

I was going to say it was idling too high too. Do you use the 4 barrel? Sometimes the secondary throttle plates will stick open a little causing the idle to stay high. A little spray and a little driving will get it loosened up again. Could be the return spring too as creeker said; especially if it isn't in the stock location. If the car is idling nice and smooth and low it should turn off easily. good luck.

outsider
07-29-11, 07:45 AM
would that spring problem cause dieseling only when hot? or at all times?

sven914
07-29-11, 10:58 AM
Dieseling shouldn't happen on a cold engine because there is no ignition source.

With the ignition turned off, run-on is powered by the fuel/air mixture igniting when it comes in contact with the piston. Usually there are carbon deposits on the cylinders that glow red hot, that ignite the mixture. These carbon deposits can also cause precognition, because they will ignite the mixture before the spark plug (when the engine is running).

outsider
07-29-11, 11:14 AM
nice, thanks for the info Sven. Dieseling has happened before on my 87 due to vacuum leak and also happens now on my 69, which I'm guessing is due to timing being off due to the distributor being rusted in place.