run on

[Joecef]

Hey guys, so I recently bought a new boat (new to me). she runs good. Big difference from my old 2.3omc junk. I bought a 2001 Tahoe Q5L with a 4.3 mercy in it. So my question is, the other day I was out and shut her off, and she ran on for about
5 sec. when it finally shut off it was a bad clunk.

I went and fished for about 20 min. Went back to start it and just clunk. So I tried a few times, thought maybe starter jammed up, just kept clunking. then realized it prob dumped a bunch of fuel into a cylinder and now hydro locked...... I let it sit for about 1 hr to 1 1/2 hrs, tried to start it clunked, then after
the 2-4 tries it started up. it did this once, so is there anything is should do in the new year? carb rebuild? when I bought it I ran high test fuel and a can of seam foam as well.

 

[Walt T]

When it ran on it ran backwards it sucked in water via the exhaust, You have water in your cylinders. Go out to your boat in your pajamas and remove the spark plugs and then crank the motor to eject the water in the cylinders. Don't put it off because you're in the middle of a pool game at your favorite bar with that kinda cute 275 lb babe who has been makin eyes at you all night

 

[Joecef]

When it ran on it ran backwards it sucked in water via the exhaust, You have water in your cylinders. Go out to your boat in your pajamas and remove the spark plugs and then crank the motor to eject the water in the cylinders. Don't put it off because you're in the middle of a pool game at your favorite bar with that kinda cute 275 lb babe who has been makin eyes at you all night

hahahahahahahaha
ya I ended up getting it going after sitting for a hr or so in the water. how do I prevent this from happening again?

 

[Walt T]

At this point all I can think of is bad gasoline. Try non ethanol gas go to pure-gas.org and fill your boat with non ethanol fuel. It's what I do on my boat

 

[GA_Boater]

Were you running hard and shut off as soon as you reached the spot? Let it idle for a few minutes to cool down a little, then shut it off.

 

[Joecef]

Were you running hard and shut off as soon as you reached the spot? Let it idle for a few minutes to cool down a little, then shut it off.

I was going alone at maybe 25-30 mph slowed down and let it run for a few sec then shut it off. Probably should have let it run a little longer

 

[Bt Doctur]

Too lean a fuel mixture
Too hot a engine temp
Too far advanced timing or too retarded timing
Poor fuel quality
Too high an idle

 

[Scott Danforth]

When an engine runs on...that little caugh at the end is the motor running backwards sucking in water

Bt Doc listed the probable causes. Fix the issue to prevent hydrolocking

 

[KD4UPL]

I had a boat that used to keep running after you turned it off. I never found the real cause, I ended up selling the boat. But, I did notice it was mostly after shutting it down hot. If I let it idle several minutes before I turned off the key it would stop just fine.

 

[aimlow]

Too lean a fuel mixture
Too hot a engine temp
Too far advanced timing or too retarded timing
Poor fuel quality
Too high an idle

This 100% sums up the cause. Could be one single item, or more likely a combination. Memorize this list.

 

[Joecef]

Thank you, in the spring i will start with checking timing and also empty the fuel tank and refill with new fuel.

 

[Scott Danforth]

seriously, hope you changed oil, etc. that water went into your crank case

 

[Joecef]

seriously, hope you changed oil, etc. that water went into your crank case

yes sir, all fluids where changed

 

[achris]

Well documented... Earlier Gen+ 4.3s are notorious for running on. Merc tried many things, including a new ignition module that retarded the timing right back at shut off, and a solenoid on the carb that completely closed it when power was removed from the ignition... Nothing really cured it. The only advice is to let the engine idle for a couple of minutes before shutting it off. If you shut it off and it started running on, immediately flip the key back on and let it cool for another minute.

Chris....

 

[Lou C]

That’s one problem I never had with my 4.3. I don’t know if the Quadrajet makes it less prone to run on or some other factor. I would look carefully at the plugs, if the center insulators are white you might need one heat range colder plugs. What carb does this engine have if it is a carb model? Idle speed?

 

[achris]

That’s one problem I never had with my 4.3. I don’t know if the Quadrajet makes it less prone to run on or some other factor. I would look carefully at the plugs, if the center insulators are white you might need one heat range colder plugs. What carb does this engine have if it is a carb model? Idle speed?

If you had a Quadrajet, it was not a Gen+... Gen II and Gen+ (4.3) engines were only supplied with Weber 4bbl carbs (or MerCarb/Rochester 2bbl). The previous generation (non-balance shaft) did have Rochester 4bbl carbs and are not susceptible to the run-on problem.

Chris......

 

[aimlow]

That’s one problem I never had with my 4.3. I don’t know if the Quadrajet makes it less prone to run on or some other factor. I would look carefully at the plugs, if the center insulators are white you might need one heat range colder plugs. What carb does this engine have if it is a carb model? Idle speed?

This makes sense. It's just a 350 Chebby missing 2 cylinders. However, carbs were problems, because they continue to deliver fuel even with the ignition off. Often opening the throttle kills the engine..cool air, higher effective compression which creates "drag".

 

[Lou C]

If this was not common with the pre vortec engines like mine but it is common with the newer ones then something must have changed to cause this either in the cyl head design or the carb used. OMC and Volvo used the vortecs but with the Holley 2 or 4 bbl vs the 2 or 4 bbl Rochesters as used on the pre vortecs. I have not heard of those engines having a running on issue...

 

[aimlow]

There was never a true "Weber" carb..it was either a rebranded Q'jet or AFB/Edelbrock. Seems the later engines used a "swirl port" head with a ramp in the intake port. Has to be a mechanical/temperature issue to be auto-ignition.

 

[H20Rat]

If this was not common with the pre vortec engines like mine but it is common with the newer ones then something must have changed to cause this either in the cyl head design or the carb used. OMC and Volvo used the vortecs but with the Holley 2 or 4 bbl vs the 2 or 4 bbl Rochesters as used on the pre vortecs. I have not heard of those engines having a running on issue...

Might have just been bad luck, but my experience has been completely flipped. My '94 4.3 pre-vortec 4 barrel would diesel itself 100% of the time if you came off plane and shut down immediately. Didn't take much time at idle to avoid it, maybe 20 seconds or so. My current '04 4.3l has never dieseled.