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Old 07-18-2012, 02:30 PM   #1
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Default 3.0 L engine keeps running

All, I recently bought a 2005 Maxum 1800 MX. The first day on the water, the boat would not start right away. I figure the reason was that the season just started and it was the first time, for the boat to be on the water for the year. After a few tries finally the ignition turned on, I took it out of the no wake zone and the boat would go fine at low speed; however as soon as I added speed, the boat would turn off. I figured it had to do with the screw by the air intake and the gas intake. so I played a bit with it until the motor run a bit smoother and it would not turn off when picking up speed. What concerns me now is that once I get to the dock, and turn the key to the off position, the motor keeps on running for a second and seems that itís trying to stay on. A friend told me that this could ruin the boat. Tried to take it to a mechanic, but their schedules wonít allow them to look at it for months. Does anyone know what is going on with the engine? Is it something I could fix myself? Or is it something that I need the mechanic? Should I not take the boat out again until itís fixed or would it be ok to take it out? Thanks,
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Old 07-18-2012, 03:12 PM   #2
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Hello, MyBoat, Welcome Aboard!!!! I believe what yuo're friend might be referring to is "Dieseling":

http://en.wikipedia.org/wiki/Dieseling

Dieseling or engine run-on is a condition that 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 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.
Dieseling is not nearly as common as it once was, because it most commonly occurs in engines equipped with carburetors. The vast majority of 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) can also occur in Diesel engines, when the piston or seals fail due to overheating, admitting engine oil into the cylinder. A structurally failing Diesel engine will often accelerate when the throttle is released, even after fuel injection is switched off.
Some carburetted engines have low-pressure fuel pumps. 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, all-terrain vehicles, and most lawn-and-garden power tools.


Potential causes Dieseling can occur for several reasons:
  • Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for igniting unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, rough 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 completely close 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 burn.
  • 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 turn over and burn more fuel and lock itself into a cycle of continuous running.


IT is my understanding that in some circumstances, the Dieseling could actually cause the engine to run backwards for a few seconds which might introduce inversion as it would potentially suck exhaust water up through the risers and manifolds and into the exhaust ports. I beleive this is very rare, but could be very harmful.

I'm sure someone will pipe in with more info., such as one of our resident hear heads.
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Old 07-18-2012, 08:55 PM   #3
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In addition to shrew's discussion, dieseling can be caused by low-grade fuel. Also, if the boat has been sitting for a while you may have had water separate from the fuel and lower the octane. Low octane fuel will ignite at low compression and doesn't need a spark. Burn the fuel you have then fill with new gas and see if you still have a problem. My experience with cars is that dieseling is caused by cheap gas in 95% of the cases. Be sure to use a stabilizer at the end of season.
If you are detail-oriented guy you can check your fuel for water separation. Get a small clear plastic container that has a good lid and fill it one-third with water. Put it on a level surface and mark the water level. Then siphon some of the gas from your boat into the same container (roughly same amount of gas as water). Shake up the gas-water mixture and let it settle. Check the mark on the side of the container, if the water level is above this mark then you have a bad case of water separation and should consider draining your fuel tank.
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Old 07-19-2012, 05:46 PM   #4
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Thank you all for some great information.
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Old 07-19-2012, 07:07 PM   #5
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i think the boat has a knock sensor which works with the octane senor.

need to get a mobile marine mechanics to hook it up to the computer and find the error code.
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Old 08-15-2012, 08:37 AM   #6
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My 2004 1800 SR3 with 3.0 Mercruiser used to do the "dieseling" run-on if switched straight off - especially after a few miles of fast fun! It rarely happens now as I let it idle for a minute or two before switching off the engine.
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Old 08-16-2012, 12:56 AM   #7
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Engine Dieseling TSB.pdf

There is a TSB from mercruiser on that.

-JP
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