what causes the engine to keep running once you have turned it off?

[battaglino]

I have a 5.7 liter merc. and sometime it just keeps running when I turn the keep off what causes this? Joe

 

[dgopetactical]

Re: what causes the engine to keep running once you have turned it off?

is it dieseling/bucking when you turn the key off or is it actually running?

 

[180shabah]

Re: what causes the engine to keep running once you have turned it off?

Number one cause is carbon buildup in the combustion chamber that get hot enough to ignite the fuel that is drawn in, even without the plugs firing. A couple of free things to check first.
1. Verify timing is correct
2. Verify idle speed
3. Check idle mixture

Unless any of these have been recently changed, they are probably going to be fine, so you will have to clean out the carbon. BG44K is an in tank product that works well, but is expensive. Or if your boat is trailerable you can take it to a dealer or even a JiffyLube for an intake system cleaning. If you go the JiffyLube route, take your muffs and hose with you cause they won't have them.

Good luck

 

[180shabah]

Re: what causes the engine to keep running once you have turned it off?

One more freebie I forgot, after running hard, let the engine idle a while to cool down before you shut it off.

 

[MannyTC]

Re: what causes the engine to keep running once you have turned it off?

Not sure if it is relevant but I had an issue where my engine would not turn off and it was due to a leaky diode which allowed the alternator to keep feeding power to the coil after the key was turned off.

 

[180shabah]

Re: what causes the engine to keep running once you have turned it off?

Hey MannyTC, I think you and ...tactical may have been thinking along the same lines. However, since his problem is intermittant, it is more likely to be dieseling then a defective diode.

 

[Don S]

Re: what causes the engine to keep running once you have turned it off?

Here is a copy and paste of the Merc service bulletin on dieseling.


Mercruiser Service Bulletin
No. 97-17
= Revised July 1999
Engine ‘Dieseling’ or ‘Running-On’
Models
MCM 4 Cylinder, V6 and V8 with Carburetors.
Situation
Under certain conditions, carbureted engines may ‘run-on’ when the ignition key is turned off. Normally, it
will take a combination of several of the following conditions to make the engine ‘run on’. These conditions
include but are not limited to:
1. Type and formulation of gasoline available in the area.
2. Type of boat and its engine compartment ventilation system.
3. Outside air, inside engine compartment air and engine coolant temperatures.
4. How quickly the engine is shut off after running at cruising or higher RPMs.
5. How the boat and engine are operated.
6. Engine idle rpm set too high.
7. Wrong heat range spark plugs or spark plugs with deposits on them.
8. Deposits in combustion chamber.
Carbureted engines, unlike EFI, do not shut off the fuel supply to the engine when the ignition key is turned
off. As the engine is coasting to a stop, air is still flowing through the carburetor into the manifold. This air
flow can still draw fuel from the carburetor, through the intake manifold and into the combustion chamber.
A hot spot in the combustion chamber will cause this fuel/air mixture to burn. The engine now ‘diesels’ or
‘runs on’ although the ignition system is not firing. EFI engines do not have this problem because the injectors
do not inject fuel once the ignition key is turned off.
Suggestion to Minimize ‘Run On’
Items number 1 ,2 and 3 can correct or reduce most of these ‘run on’ problems. Letting any engine idle a
minute or two before shutting it off is desirable. This helps the engine to normalize at idle rpm before being
shut down.
1. Be sure engine idle mixture, idle rpm and ignition timing are correct. On 4 cylinder and V6 or V8 engines
with Thunderbolt IV ignition, setting idle rpm as low as possible can help stop ‘run on’.
2. Adjust throttle cable barrel so that the carburetor’s throttle lever returns against the idle speed stop
screw every time the remote control throttle lever is returned to neutral, idle.
3. Idle engine 1 to 1-1/2 minutes before turning key off after running at cruise or higher engine rpm.
4. Try a different brand of regular fuel or try some premium fuel.
5. Increase idle mixture by 1/8 turn ‘richer’.
6. Go to next colder heat range spark plugs.
7. V6 and V8 only: Install a 140 ?F(60 ?C) thermostat.