4.3 Engine. Post. Knock. Engine continues to try an run

[fredcollett]

2008. 4.3 engine. After a long run on the water or after towing kids on tube , turn off ignition an the engine will knock an clank like it is running on 1 or 2 cylinders. Turn off key does not kill engine. Leave in gear an turn off key. Does not kill engine. What works best is leave engine running in neutral. For. 1 or 2 minutes. Like a cool down period Not practically when u have 2 kids in the lake I know this is a common problem with the MC 4.3. Has anybody got a cure or fix ?

 

[alldodge]

Howdy

Its called run on or most times called dieseling. This is can be caused by a few things but most often due to carbon build up in the combustion chamber. Pull the plugs and make sure you have the correct plugs installed, they are not showing signs of over heating (white deposits) and their gap is correct.

Plugs MR43LTS gap .045

Your timing should be set at 10 BTDC for a carb motor and 8 BTDC for EFI.
To set timing it must be set by placing the motor in base timing

 

[harleyman1975]

Some may disagree but I find that running higher octane fuel will cure this issue. If just running regular try switching to high octane prem fuel. Try adding a can of octane boost if already running high octane prem fuel.

 

[Rick Stephens]

I like occasionally running a little DextronII tranny fluid in my fuel. The high detergents in the tranny fluid help clean off the carbon deposits in the chambers. A full quart in a over 20 gallon tank won't hurt a thing.

 

[Jmunk]

What is the engine idle speed? If it's too high then lowering to the correct idle speed often helps. As stated earlier high octane fuel also helps as well in my experience. Some people chase it and never fully correct it, some people opt to place in gear and shut off when safe to do so.

 

[GA_Boater]

What temp is the engine when you try to shut it down? Carb or FI? Gotta guess carbed motor because it takes fuel to diesel and FI has no fuel with the key off.

 

[Scott06]

Had the same with my old 3.0, decarbonizing the engine with sea foam, as mentioned above on engine temp- new impeller, also cleaned rebuilt the carb float level was low and I think running a tad lean at WOT. Only did it after a good long ski or tube pull, which with a 3.0 was basically WOT, went away after the above.

 

[R055]

My 4.3 does it with 87 octane but not with 92 so I just fill up with 92.

 

[alldodge]

3 octane comments: Only IF running higher octane stops the issue, then your only masking the problem, it hasn't been fixed.

 

[bman440440]

I had run on problems with my 1997 4.3.... even after a complete rebuild(was rebuilt for other reasons not the run on problem) it still did it but I was able to fix it... here is what I did... make sure you get your timing set correctly and also make sure your RPM in gear is set to 650 if that does not work then nest step try a colder set of plugs this works if you have only a minor problem with it (this is where I was able to stop because it fixed the problem)... then final step get the 140 thermostat kit for it and that should fix it... if not then you have other more elaborate issues working there but I doubt it not be fixed by the time you put the 140 thermostat in it by what you say that if you let it run at idle for 1-2 mins that it will shut down properly

 

[R055]

3 octane comments: Only IF running higher octane stops the issue, then your only masking the problem, it hasn't been fixed.

Technically the problem has been fixed if the run on stops because you're no longer risking engine damage.
Also I remember reading the posts of one of the really higher up members here that when he worked at the mercruiser dealership they tried everything to find a fix but couldn't using 87 octane.

 

[bman440440]

here is a link to the mercruiser service bulletin on the problem ---> http://www.marinemechanic.com/merc/distributors/mercurymarine/sterndrive/run-on-dieseling.pdf

 

[achris]

Ok, been following this thread. I was a Mercruiser dealer in the 90s when this problem first started happening.

Merc send us all manner of 'fixes', none of which worked properly. These included a new module with a different idle timing, a carburetor shut solenoid which closed the throttle plate down even further after the key was turned off, different spark plugs, etc etc etc.....

The final solution suggested by Merc was for the engine hot soak to be allowed to dissipate. Ie, let it cooldown before shutting it off.

And that is the bottom line. Modern engines are design to be injected. Marine engine suppliers are still putting carbs on them.... And that is the root cause of the problem.

3 solutions. 1- change out the engine for an injected one, 2-stand off from the person in the water for a couple of minutes to allow the engine to cool, or 3-don't shut off....

Sorry. One more. Live with it and risk sucking water back into the engine.

My personal preference is #3, and it's the safest...

Chris........

 

[fredcollett]

Thanks Guys I will try some of these suggestions in the spring and report back
​Happy New Year Fred

 

[Silvertip]

The key item in all of this is not shutting the engine down until you've let it idle for a minute or so. This was an issue in the early auto emissions days when higher idle speeds came into being. The solution was to install and anti-diesel solenoid on the carb. When the key was turned off the solenoid dropped and the throttle blades in the carb closed fully thus killing the engine. That same theory applies today but with a cable operated throttle system an anti-diesel solenoid may not work. On cars the idle speed was set with the adjustable plunger on the solenoid. On a boat the idle stop screw is used to control idle speed so the solenoid can't drop idle unless that screw was removed. History lesson complete -- just let the beast idle for a bit before shutting down and make sure idle speed is set correctly.