30l Mercruiser keeps running for a short time when switched off.

[Salacia]

Hi Folks

It would appear I have the opposite problem to most people on the forum! Every now and then my 3.0l mercruiser 2007 keeps running when switched off. it only does it for a few seconds and coughs splutters and farts then eventually dies. Does anyone have any thoughts?

Thanks

Rob

 

[tpenfield]

Re: 30l Mercruiser keeps running for a short time when switched off.

It is called 'run-on' or 'dieseling' . . . it can be caused by carbon buildup in the cylinders and/or deposits around the spark plugs. Also a high idle setting can cause that to some extent as well. I would check for all 3 of these conditions.

There is some 'stuff' that you can run through the engine that removes some of the carbon build up. I can't recall what it is called, but I did buy some from the GM dealer at one point. Anyway, you can check the spark plugs and the idle setting, then see if you need to do the carbon treatment.

 

[NHGuy]

Re: 30l Mercruiser keeps running for a short time when switched off.

You can also correct for the condition that causes the run on-too. When dieseling occurs it is related to extra heat in the combustion chambers. The cylinders "fire" without spark. I have corrected this with fuel mixture adjustment and I have also been able to temporarily forestall it by running the engine above idle for a short time which increased the cooling water supply.
With my old junkbox cars as a kid I could stop a dieseling motor by turning it off in drive. It's important to not allow the boat to diesel, When it occurs there is a greater chance of water being sucked into the motor through the exhaust, this can cause major engine problems so take steps to prevent & fix the problem.

 

[Bondo]

Re: 30l Mercruiser keeps running for a short time when switched off.

When it occurs there is a greater chance of water being sucked into the motor through the exhaust, this can cause major engine problems so take steps to prevent & fix the problem.

Ayuh,.... That's because the motor actually turns Backwards in it's last breath...

 

[Don S]

Re: 30l Mercruiser keeps running for a short time when switched off.

Dieseling is a problem with the newer 3.0L engines. Usually do to high engine idle rpm, or not allowing the engine to idle for a minute or two after a run before shutting off. Another reason on the 3.0 is lean fuel mixture on the carb. Especially if you have a TKS carb.

 

[mark1905]

Re: 30l Mercruiser keeps running for a short time when switched off.

I have corrected this with fuel mixture adjustment

..by going leaner or more rich?

 

[Salacia]

Re: 30l Mercruiser keeps running for a short time when switched off.

Thank guys some great advice here. NHGuy to mark1905's point was it leaner or richer? I changed the plugs last week and noticed a very high carbon build up which makes sense so will probably run some redex through it as it can't hurt. Would changing the octane of the fuel help at all?

Thanks again

Rob

 

[Bondo]

Re: 30l Mercruiser keeps running for a short time when switched off.

I changed the plugs last week and noticed a very high carbon build up which makes sense so will probably run some redex through it as it can't hurt.

Ayuh,.... Build-up on the plugs is unburned fuel,... Lean it up abit....
They should be cream, 'n coffee brown....

Mechanic-in-a-Can won't fix a carb problem...
It rarely fixes Anything....

 

[Salacia]

Re: 30l Mercruiser keeps running for a short time when switched off.

Ayuh,.... Build-up on the plugs is unburned fuel,... Lean it up abit....
They should be cream, 'n coffee brown....

Mechanic-in-a-Can won't fix a carb problem...
It rarely fixes Anything....

LOL very true I can't say I have had much luck in the past with it. I will give it a tweek and see how we go. Any suggestions on how much I should adjust it by?

Thanks