Sputtering when killing engine

[Zuplar]

i've been having a problem with the engine now wanting to die when you turn the key off. it seems as though it sputtering still trying to run when you turn the key off. it did this almost everytime when i took it out the first time, but since they i replaced the spark plugs and it did it only once after wards. my question is why would it be doing this? the motor starts up and runs really well, but could it still be a timing issue? the motor is a 3.0l mercruiser. any info is appreciated. thanks.

[fishinpa]

have you verified engine temperature and idle speed setting ?

dwell adjstment (aka) point setting and condition and timing check or adjustment may be in order !

all can affect run on in one way or another

fish

[Mischief Managed]

It's happening because there's enought latent heat in the combustion chamber of one or more of your cylinders to ignite the air and fuel mixture without a spark (like a diesel). Assuming the engine is in a good state of tune, you can make it stop by: idling longer after running at planing speeds to cool the engine down, using higher octane fuel, running some Techron or similar chemical treatment in a tankful of two of gas to clean out carbon deposits.

[nomi840]

What is the recommended fuel octane for your engine and what octane are you using? I believe the condition is called dieseling.

From wikipedia :

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.

Dieseling is so-named because it is similar in appearance to how diesel engines operate - by firing without a spark. The ignition source in a diesel is the compression of the fuel in the cylinder rather than a spark as in gasoline engines. 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. An automobile engine that is dieseling will typically sputter then gradually stop, probably because of an electrical fault.[citation needed]

Dieseling is not nearly as common as it once was because it most commonly occurs in carburetted engines. The vast majority of American, European, and Japanese vehicles manufactured after 1990 are fuel-injected—the injectors and high-pressure fuel pump immediately cease supplying fuel to the cylinders when the ignition is switched off. If the injector is damaged or malfunctioning, a small amount of fuel can enter the chamber and be ignited, causing a sputter or two after the engine is switched off.

Dieseling (in the sense of engine run-on, and disregarding combustible gaseous mixtures via the air intake) does also occur in diesel engines, when the piston or sealing fails due to overheating, and admits engine oil into the cylinder. A structurally failing diesel engine will often accelerate when throttle is released, and even after fuel injection is switched off.

Some carburetted engines have fuel pumps, but they are often low-pressure. They are typically designed only to overcome a loss of suction in the fuel line near the engine due to fuel evaporation in hot weather, to supply sufficient fuel to maintain stoichiometric combustion under heavy load with wide-open throttle, or a combination of the two. Fuel demand is low at idle and there is more than enough manifold vacuum to draw sufficient fuel for combustion even if the fuel pump is switched off.

Gasoline engines that are much smaller than the typical automotive engine are usually carburetted for economic and engineering reasons. Dieseling can occur in such engines. These engines include those installed in mopeds, scooters, small motorcycles, ATVs, and most lawn-and-garden power tools.

Potential causes
This condition can occur for a multitude of reasons:

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.

A carburetor that does not close entirely can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.

Incorrect timing.

An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.

An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turnover and combust more fuel and lock itself into a cycle of continuous running.

________________________________

Hope this Helps !

[Zuplar]

this sounds exactly like the issue. i'm not convinced it's timing because the engine runs really smooth and starts really easily and i know with other engines i have messed with it usually is harder starting when timing is off. the boat has recently had a tune up, so it seems as though most everything is in order. this last time i did run higher octane fuel and like i stated it did do it less. i may try running a fuel additive to help. thanks guys, great info!

[fishinpa]

Ahhh Yeah good ole fix it in a Can !







fish

[Bond-o]

this sounds exactly like the issue. i'm not convinced it's timing because the engine runs really smooth and starts really easily and i know with other engines i have messed with it usually is harder starting when timing is off. the boat has recently had a tune up, so it seems as though most everything is in order. this last time i did run higher octane fuel and like i stated it did do it less. i may try running a fuel additive to help. thanks guys, great info!

Ayuh,... An ole Mechanic's Trick, to deal with excess carbon buildup caused Dieseling,.....

Warm the motor to normal temps,...
In neutral, bump the throttle up to about 1200/ 1500rpms,...
Then slowly pour a soda pop bottle of Water down the throat of the Carb...

It'll Clean the combustion chambers to as Clean as the day it was New...

I often do it to a motor that runs, but is being torn down,... Saves Hours of cleanin' later...

[Don S]

Why do people not put a year with the type engine. 3.0L has been used for ever, with many changes from year to year.

Marine engines seldom build up a lot of carbon, unless they idle all the time. Read this about Dieseling in marine engines.
http://www.4shared.com/file/53242775...unning_on.html

[fishinpa]

Ayuh,... An ole Mechanic's Trick, to deal with excess carbon buildup caused Dieseling,.....

Warm the motor to normal temps,...
In neutral, bump the throttle up to about 1200/ 1500rpms,...
Then slowly pour a soda pop bottle of Water down the throat of the Carb...

It'll Clean the combustion chambers to as Clean as the day it was New...

I often do it to a motor that runs, but is being torn down,... Saves Hours of cleanin' later...

Yeah that one has been around forever !
Any ole wrench has used old remedies.


fish

[PitchFork]

i'm not convinced it's timing

Yeah but it only takes a few minutes to rule it out by getting the timing light out.

[fishinpa]

Yeah but it only takes a few minutes to rule it out by getting the timing light out.

I agree it should be checked along with the other things mentioned above.
Start at square one before you go changing everything or ruling out the obvious.
There is never a bad time to check timing ,dwell or point gap ,carb adjustments or do a compression test.
write it down in your book and keep it all for future reference.

fish

[scutly]

be very VERY careful of that dieseling or run on or whatever ya want to call it. during the last crank stroke when if finally dies, it can actually force the engine to run backwards for a moment. that's a very bad moment. during that moment i sucked water into my cylinders and hydro-locked my engine. TWICE!!!! to high an idle setting was the cause for me.

[Zuplar]

sorry for not posting the year, i thought i had, but the motor is a 1999 model. i do have a timing light so it shouldn't be to big of hassle. it seems like it may also be the idling too high or that i had stopped it right after i had been running it WOT for a bit. i'm probably going to check everything out just to be safe.