dieseling

[stubbsboogie]

Hey guys I have a quick question about the boat in my sig.
Merc 4.3 carb.
Sometimes when we are out after we have been cruising for a while, I go to shut the boat down and it diesels. Run on. After it idles for a minute or so it shuts down fine.
It has seafoam in every other tank. Otherwise it runs tip top. A buddy said that it was a hydrolock. I am not sure what that is. Does anyone have any ideas? Thanks.

 

[RogersJetboat454]

Re: dieseling

Any recent carb work done? Whats your idle speed?

 

[stubbsboogie]

Re: dieseling

No carb work. Carbs were done right before I bought it,last Sept, I have the reciept. Oil and gear lube change in Feb. By same dealer.
Not sure of idle I believe it is about 700-800. It idles smoothly and runs perfect. No shakes or anything like that. Only when it is good and hot does it ever do this. And it is not each time although it does seem to be getting a bit more frequent.

 

[erie_guy]

Re: dieseling

Stubbs,

Your buddy is partially correct, in that dieseling can sometimes result in hydrolock. When the engine finally stops turning it can rotate backwards for a fraction of a crank rotation - drawing water back from the exhaust manifold and into the cylinder. Next time you try to start, the water in the cylinder stops on a compression stroke and you are done (i.e. hydrolock). This can be compounded if you have the rubber shutters in the exhaust path removed or damaged.

If you are unlucky, you may have to replace the head or if lucky, only remove the spark plugs and get the water out of the cylinders.

I would try to stop the dieseling if at all possible as it in not in any way good for your engines continued health. You may want to try turning the engine off while in gear (i.e. while loading the engine).

Check to assure you have correct engine timing, correct thermostat rating, and proper engine cooling as a possible causes of this dieseling.

Happy boating.

erie_guy
Port Clinton, OH

 

[fossill]

Re: dieseling

Adjust the idle speed to spec and run a can of carb/combustion chamber cleaner through it, then take it out for a good run. Make sure the engine is timed to spec also. On some tempermental engines, running spark plugs with the coolest heat range available is also an option. Thats from my experience in my auto shop days dealing with that issue for what its worth.

 

[EMC 1810SS]

Re: dieseling

Are you letting it idle for a bit before shutting her down? Since day one, after hard pulling my 4.3 has run on if I pull the throttle back and shut off the key. If I slow down and throw it into neutral and let it idle 45 seconds or so it is fine.

 

[Bondo]

Re: dieseling

A buddy said that it was a hydrolock.

Nope, that's not Hydro-lock, but dieseling Causes hydro-lock...

 

[wired1236]

Re: dieseling

Mine does this too when running a good speeds for a while. Mechanic told me, (and as posted on other threads here too), run higher octane fuel and see if that helps. It worked for me. I also idle for a few seconds in hot weather and sometimes even run the blower. All of these things help but running 92 octane always does the trick. I even ran 89 once when I had to gas up at a marina and sure enough, it did it again when I didn't let it idle & cool down.

And yes, dieseling can cause water to back into your engine!

 

[Jeepster04]

Re: dieseling

Are you letting it idle for a bit before shutting her down? Since day one, after hard pulling my 4.3 has run on if I pull the throttle back and shut off the key. If I slow down and throw it into neutral and let it idle 45 seconds or so it is fine.

Our 4.3 will also diesel if we stop then shut it off right away. I always let it idle for at least 30 seconds and its never done it again. When it does do it I start it back up right away so it doesnt have a chance to run backwards.

 

[stubbsboogie]

Re: dieseling

Our 4.3 will also diesel if we stop then shut it off right away. I always let it idle for at least 30 seconds and its never done it again. When it does do it I start it back up right away so it doesnt have a chance to run backwards.

I do the same thing with ours. When I do this it has no run on.
I have done some further research, it seems like this is a pretty common thing for these 4.3 engines. So what causes these pafticular engines to do this?