350 Mag Run-on Dieseling After Run

[gamarines2]

My motorcycle guys and I were talking at work today about gas prices and octane issues, and I told them the reasons why higher than necessary octane was a waste of money. The rationale for using higher octane only if an internal combustion 4 stroke gasoline engine experiences pre-detonation knocks and that whole thing. Then I got to thinking about this. When I run my 350 Mag on a hot run for awhile and then shut her down without idling for awhile, my motor diesels, which we all know is really bad for the engine life (and I immediately turn the ignition back on if this happens). And I will let it idle until it cools some and then shut her down.

But if my timing is correct and I don't experience pre-detonation symptoms when the engine is running, why would it diesel when I shut it down? Is this a low flash-point issue because of too low octane? Should I bump it up a notch?

Jake

 

[642mx]

Re: 350 Mag Run-on Dieseling After Run

My 4 barrelled 4.3L would diesel too. My local marina technician told me to stop running 87 octane and run 92 or 93 (preferably Shell gas) and it would stop dieseling.

On the first tank of 93 octane it was better, after running 2 or 3 tanks of 93, the dieseling was gone. The motor never pinged on 87, but for some reason I guess marine engines like the higher octane.

On another note, since I've been running the Shell 93 octane, my throttle response is better, it starts easier, and my fuel mileage is better than 4 mpg now. (I just went on a 141 mile round trip and used 33 gallons)

 

[chiefalen]

Re: 350 Mag Run-on Dieseling After Run

Well said bubba.

 

[JustJason]

Re: 350 Mag Run-on Dieseling After Run

the biggest issue with a boat engine dieseling is reversion and having the cylinders suck in water.

as bubba said, the boat was designed to run off of 87. So higher octane is a band aid.

Now the number 1 reason for dieseling in a boat is that idle rpms are set to high.
The number 2 reason is a boat being out of tune.
and the number 3 is shutting down to early (and this is the one watersports guys don't like). because of the increased demand and loads placed on a marine engine the combustion chamber temps get fairly high. This is combustion chamber temps, and has nothing to do with water/cooling tempertature, which is what you see on your dash gauge.
Every boat is a little bit different. and you dont always have to be a watersport guy, but on a marine engine, especially carbed. before you choose to shut down the engine you should plan on letting it idle a minute or 2 before you do to lower those combustion chamber temps, and prevent dieseling.

 

[642mx]

Re: 350 Mag Run-on Dieseling After Run

as bubba said, the boat was designed to run off of 87. So higher octane is a band aid.

Is this why every marina I've seen sells 89 or 90 octane?

 

[JustJason]

Re: 350 Mag Run-on Dieseling After Run

so your saying the marina is now the engineer of the engine? And the guy that pumps the gas (usually half the time into the water tank!! )
knows more than the mercruiser guys???????

they probably (and this is a wild ***** guess) sell 89 to either pad their profits or to make up for the fact that they don't move gasoline as fast as say the exxon down the street. and since gas looses octane naturally over time, if the 89 in their underground tank is 3 months old, i got news for ya, its not 89 octane any more.

i guess if i was a gas station owner, and i was only selling 1 grade of fuel, i'd probably sell mid grade as well. people (like you), still think you NEED it... and you don't.

 

[642mx]

Re: 350 Mag Run-on Dieseling After Run

people (like you), still think you NEED it... and you don't.

I have to run it to keep the engine from running on. And before you say my engine is out of tune, here is whats been done this year: new plugs, wires, cap, rotor, fuel filter, fuel pump, distributer sensor replaced, and the ignition module replaced. Also, the marine technician cleaned and adjusted my carb. He's also the one who recommended the higher octane for my boat.

As a matter of fact, everyone I boat with runs premium with the one exception of a buddy of mine that has a brand new MPI 496 powered Baja (he runs 89 octane).

I called the tech this morning and asked him why the higher octane was better for my application, he told me that the 'underhood' temperature is higher on boats like mine and the 87 doesn't cool the cylinder temperature as well as the 93. Makes sense to me... I'll keep using it.

 

[JustJason]

Re: 350 Mag Run-on Dieseling After Run

Do what you gotta do then brotha!!!

But i do gotta ask ya... what's your base timing and idle rpms? I gotta ask only because Merc says it shouldn't desiel on 87, and if your saying yours does, then there's something on your motor that's not in accordance with what merc says.

 

[642mx]

Re: 350 Mag Run-on Dieseling After Run

Do what you gotta do then brotha!!!

But i do gotta ask ya... what's your base timing and idle rpms? I gotta ask only because Merc says it shouldn't desiel on 87, and if your saying yours does, then there's something on your motor that's not in accordance with what merc says.

I always do what I gotta do.

I have no idea what my base timing is. The distributer has never been pulled or played with. It idles about 800 rpms (according to the factory tach and who knows if its 100% accurate).

93 octane works for me. I don't mind paying a little more to keep my engine from filling up with water.

 

[642mx]

Re: 350 Mag Run-on Dieseling After Run

If you are willing to spend the money on octane, why not spend enough to idle the engine down and let it cool properly?

Who said that I shut it off hot without letting it idle and cool down first?

 

[Pushrods]

Re: 350 Mag Run-on Dieseling After Run

Sounds like this is a RUN-ON problem and is fairly common on RV's and would assume the same would happen with a 4 barrel carb on a boat. The problem I have seen is when the engine is hot and shut off, it continues to run and rattle. This is caused by a over rich mixture in the intake manifold at shutdown and is common in Chevys Engines that need a Carb overhaul. Im not sure of what component in the Carb is the culprit, but do know a rebuild of the Carb in most cases fixes the symptom you describe.

Pushrods

 

[gamarines2]

Re: 350 Mag Run-on Dieseling After Run

Thanks all. The lesson I have taken from this is to not shut off immediately. If I had an octane deficiency, like I once had on a Harley that called for 89, it would diesel after an idle period shut-off and it doesn't do that.

I did; however, enjoy some of the "heated opinions" articulated in this thread.

Jake

 

[chiefalen]

Re: 350 Mag Run-on Dieseling After Run

I think the guy who has his idle at 800 is 150 rpm to mush. 650-700 but i like 600 better.

If it don't idle there than something needs adjusting.

Freekin 23 year old motor in my p.o.s. and i can get it to idle nice at a little over 600 in gear on the water.

And so should everyone.

 

[chiefalen]

Re: 350 Mag Run-on Dieseling After Run

Woa thats news to me "fuel cooling the cylinder walls".

Maybe someone can enlighten me?

Inquiring minds want to know?

 

[642mx]

Re: 350 Mag Run-on Dieseling After Run

The original poster, "When I run my 350 Mag on a hot run for awhile and then shut her down without idling for awhile, my motor diesels..."

But you quoted my post acting like I do that.....whatever...

 

[642mx]

Re: 350 Mag Run-on Dieseling After Run

I did; however, enjoy some of the "heated opinions" articulated in this thread.

Me too. Its amazing how everybody thinks I have a problem. My boat runs great with premium and not so great with 87, I don't see a big deal with this....... but others do for some reason.

 

[jkust]

Re: 350 Mag Run-on Dieseling After Run

I had the same run on problem with my 4.3 Merc this summer. The first few times I used it and having never driven an I/O before, I would just shut it off no matter how hard I was running. It would run-on at least half the shut offs those first few uses albeit I drove it hard also. This is with an engine just past its breakin period so nothing has been adjusted out of factory specs yet. I did not upgrade to premium but like 20 seconds of idle time before shut down makes it not happen. I read that tip in these forums. I plan to use higher octane next year and I can't wait to see if it increases performance as mentioned above. I learned that when you shut down a carb enging only the spark stops, not fuel flow like with an EFI engine. Merc actually issued a service bulletin regarding run-on in 1999 suggesting, among other things, a higher octane fuel.

 

[gatesmoonwalker]

Re: 350 Mag Run-on Dieseling After Run

I have seen this problem twice on two different 350 mags.
Once the throttle cable was intermittentaly sticking at the carb.
The second had the idle set around 775. Turned it down to 600 and cured the problem.