Run on

bspeth

Senior Chief Petty Officer
Joined
Aug 30, 2013
Messages
758
I have a friend and his boat runs on after shutting off the ignition while forward momentum is sloeing but not stopped,please flip me a few ideas of what to check out. Thanks.
 
Last edited:

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,343
never shut off the motor with forward motion
check engine temp, engine timing, check carb for a lean condition
 

bspeth

Senior Chief Petty Officer
Joined
Aug 30, 2013
Messages
758
How do we check carb for lean condition. 2006 3.0. Thank you.
 

Alumarine

Captain
Joined
Feb 22, 2005
Messages
3,738
Try letting it idle (in neutral) for 30-60 seconds before shutting it off.
What motor?
 
Last edited:

wrench 3

Commander
Joined
Aug 12, 2012
Messages
2,108
The most common cause of run on in vehicles is an idle that is too high.
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Yup, high idle, too much advance, too much heat. Those are your likely main items.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,581
too much advance,
Why would ignition timing too far advanced have anything to do with it since the ignition is turned off? I could see retarded timing contribute because you have to give a little more throttle to compensate for the low idle with retarded timing.
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Well the advance doesn't actually contribute to the dieseling, but it helps create combustion chamber heat. If the advance has been moved after the initial carburetor idle setting that increases engine speed and combustion chamber heat. And a hotter than designed high idling engine with a carburetor keeps getting fuel from the mechanical pump and carb idle supply . Which it continues to spontaneously combust until the engine is stopped in gear or some other way.
If you get dieseling put it into gear or turn the key back on and the motor will resume running, then run it above idle to cool it and try another shut down.
Then adjust it to a the lowest speed at which it will run well in gear.
 
Last edited:

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Since we are talking theory who can explain why lean fuel contributes? Does it burn hotter? I've heard of it but I can't picture the why of it.
 

wrench 3

Commander
Joined
Aug 12, 2012
Messages
2,108
Well, it's a fact that a leaner fuel mixture does give you a higher combustion temperature but from here on out this is strictly my guess. Run on is caused by the fuel igniting from the heat created by the compression. So it would stand to reason that if the combustion chamber was hotter to start with, the compression wouldn't have to produce as much heat to cause the problem.
 

bspeth

Senior Chief Petty Officer
Joined
Aug 30, 2013
Messages
758
Thanks for the input guys,I will have Cosmo check these possibilities out.
 

Fun Times

Moderator
Staff member
Joined
May 16, 2009
Messages
9,118
Here's a story from Don S about a 3.0 TKS dieseling.
http://bocweb.yuku.com/topic/3696#.VP-l58I5Cpo

Let me tell a fairy tale of a 2004, 3.0L Mercruiser engine that destroyed itself.
Once upon a time there was this Marine Mechanic, that shall remain nameless, that had to replace a 3.0L Mercruiser engine under warranty due to a broken valve that destroyed a cylinder wall and piston. After replacing the long block with a brand new one, the engine was very hard to start, hard to keep running till it warmed up and dieseled severely when shut off, even after idling for several minutes and not over 140? temperature.
The timing was spot on, and since this engine had the EST distributor was set at the proper timing per the manual and done while in the base timing mode. Still the same problems. It was obvious the engine was running too lean but with the anti tamper devise on the carb mixture screw, it was impossible to adjust it richer.
Knowing the lean mixture was probably the cause of the engine failure, the MM (Marine Mechanic) decided, against his better judgment, that the problem with the engine would occur again in a very short time if not prevented somehow.
Knowing full well that the California EPA may rain fire and damnation on him for the rest of natural born life and on into the unknown after death, the MM decided to take the matters into his own hands and remove the tamper proof cover over the idle mixture screw. He soon found this to be a very difficult task, but somehow managed to remove it without destroying the carb or the screw. (He later found it would have been much easier to order a new mixture screw and just replace it and go ahead and break the old one off.)
Finally after the success of removing the dreaded seal, he backed the mixture screw out approximately a quarter to a half turn. the engine then smoothed out and ran great, idled smooth and slower without dieing and was even easy to start for the first time in it's life, even cold, and never dieseled again when shut off.
While the MM is worried about the hereafter and the consequences of his irresponsible act against the California EPA and all the other mini EPA's of the world, along with the consequences of his complete disregard for the Mercruiser procedures outlined in his service manual, the dastardly dead was done...............
The customer, however, wondered why his boat now worked perfectly but the poor ole MM was unable to tell him the truth and worries to this day he will be found out and that upon passing he will be sent to the worst part of purgatory for his serious misdoings, but I will......uh, sorry about that, he will be happy in knowing that the boats engine will now live and last for many trips to the great fishing grounds of the owner.

Please understand that this is only a fairy tale and should only be told in the proper company, and hopefully someone will not irresponsibly put this in the Vault or something silly like that as it should NEVER be mentioned again as someone else may try it and also be condemned to eternal damnation of the EPA gods.
I will also NEVER post this again as it could possibly be thought that I may condone such irresponsible actions.
 
Top