2002 4.3L runs on after ignition is shut off

Bawbe B

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Aug 25, 2014
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I have a 4.3L in a 2002 Campion Allante with unknown hours on it. Last year it started to occaisonally run on after the ignition is shut off. Now it does it all the time. By running on (dieseling) I mean the engine with fire on one or more cylinders on it's own. I usually flick the ignition on and off quickly until it finally quits. When it does it lets out a big wheezing sound and there is almost a sweet fuel type smell. Weird, I know!
Mercury says to idle the engine for a few minutes after coming back to the dock. That helps a bit but doesn't eliminate it. Boat was tuned up and the coldest AC Delco plugs available put in (Mercury recommendation).
Any suggestions?
 

Fun Times

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Hi there, welcome to iboats!:)

What is the engine idling at while warm? It's best to get the idle in-between 643-648 RPM. Anymore than that will diesel the engine.

Can you recall what seemed to change from before and after you noticed the dieseling? Fueling station, fuel grade, tune up, fouled spark plugs ever, etc. etc.?
 

seilings

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Oct 4, 2012
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43
My 2006 4.3 Mercruiser had the same problem before. Ends up the impeller need to be replaced.
I noticed if my temperature gauge stays above 175 degree, then most likely it will behave dieseling.
So make sure my temperature is equal or lower than 175 degree, it will shut off normally.

The reason causing my engine cannot go below 175 is because the impeller didn't drive enough water into the engine.

When I watch the pee holes while idling, the two pee holes should have steady water stream come out, and I know the impeller is doing its work fine.
 

Bt Doctur

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Aug 29, 2004
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19,092
hi engine temp, hi idle rpm, poor fuel, incorrect timing can cause dieseking or run-on
 

Bondo

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I have a 4.3L in a 2002 Campion Allante with unknown hours on it. Last year it started to occaisonally run on after the ignition is shut off. Now it does it all the time. By running on (dieseling) I mean the engine with fire on one or more cylinders on it's own. I usually flick the ignition on and off quickly until it finally quits. When it does it lets out a big wheezing sound and there is almost a sweet fuel type smell. Weird, I know!
Mercury says to idle the engine for a few minutes after coming back to the dock. That helps a bit but doesn't eliminate it. Boat was tuned up and the coldest AC Delco plugs available put in (Mercury recommendation).
Any suggestions?

Ayuh,..... That,.... is yer motor runnin', Backwards,.....

That's how dieselin' causes hydro-lock,....
 

Silvertip

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Sep 22, 2003
Messages
28,758
Try buying your fuel at a different location if you are always frequenting the same station/marina.
 

Bawbe B

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Aug 25, 2014
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Hi there, welcome to iboats!:)

What is the engine idling at while warm? It's best to get the idle in-between 643-648 RPM. Anymore than that will diesel the engine.

Can you recall what seemed to change from before and after you noticed the dieseling? Fueling station, fuel grade, tune up, fouled spark plugs ever, etc. etc.?

Thanks for the welcome...
643-648 RPM is a pretty tight range. :) The boat is 2 hours away from me, but I will check on the idle rpm. I know it was set to Mercury specs when the boat was tuned up in June. Using the same fuel grade, but from a couple of different stations.
Problems seemed to have started after a piece of the aircleaner fell into, but didn't pass through, the carb and the wrong plugs were installed. June tune-up saw new, proper plugs, and the timing reset. Engine coolant temp has been steady at the 175 range. (I'll double check exact temp on guage on the weekend)
I'll have to check the pee holes.
Thanks for all the quick replies. Keep them coming.
 

Memorom

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Aug 26, 2014
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I have the exact same problem with my 2003 Bayliner 185 4.3L when it is warm. If I am just cruising around and let it cool down a bit it won't run on. But when I am pulling tubes or skiers for a while and want to shut off in the middle of the lake for a break, it has run on for as long as 30 seconds. Any suggestions? Thanks, Steve
 

Bondo

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I have the exact same problem with my 2003 Bayliner 185 4.3L when it is warm. If I am just cruising around and let it cool down a bit it won't run on. But when I am pulling tubes or skiers for a while and want to shut off in the middle of the lake for a break, it has run on for as long as 30 seconds. Any suggestions? Thanks, Steve

Ayuh,...... Welcome Aboard,..... How 'bout you start a thread of yer own 'bout yer boat,.... Don't wanta Hi-jack Bawbe B's thread,....

Or follow along, 'n use the suggestions provided to Bawbe B in His thread,.......
 

Bawbe B

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Aug 25, 2014
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Memorom has the same problem I have.
Apparently Mercury has put out a TSB about this problem in motors with high hours.
Will look into the suggestions here when I head back to the lake this weekend.
 

Fun Times

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Using the same fuel grade, but from a couple of different stations.
Well there goes that theory...:D

Problems seemed to have started after a piece of the aircleaner fell into, but didn't pass through, the carb and the wrong plugs were installed.
I know some V6's have a reputation of dieseling. If you do a Google search you should find lots of stories about it.
A piece of air cleaner/flame arrestor falling into the carb seems a bit unusual.:confused:

What was wrong with the carb? One thing that would contribute to dieseling is if you had a buildup of carbon on the heads/pistons due to running rich. An inspection camera down the spark plug holes like the one from harbor freight might be helpful to see if you have any carbon buildup. http://www.harborfreight.com/digital...era-67979.html

Dieseling can occur for several reasons:
  • Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for igniting unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, rough metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.
  • A carburetor that does not completely close can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.
  • Incorrect timing.
  • An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to burn.
  • An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turn over and burn more fuel and lock itself into a cycle of continuous running.

Apparently Mercury has put out a TSB about this problem in motors with high hours.
http://www.google.com/url?sa=t&rct=j...74035653,d.cGE

Memorom has the same problem I have.
It's a gray area here as the hosts of iboats would rather all members start up a new topic of their own vs tagging onto someone else's topic as things can become confusing trying to help two or members at one time. It's forum etiquette not to hijack topics. The "Help Tip" at the top of the forum page goes into detail of why iboats feels this way.

Good luck to the both of you, hope you find it soon:)
 

Memorom

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Thanks for the suggestion of the new topic Bondo and Fun Times. I will follow the suggestions in this post and if I still need more help I will respect the forum policy and start up a me own thread. I thought the topic was already covered exactly in this thread and wanted to keep the relevant information in a single topic, to help fellow members in their search in the future. Sorry for missing this in the Help Tips.

Thanks for your help and hopefully some of these suggestions and the Mercury link will help out.

Steve
 

Bondo

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Thanks for the suggestion of the new topic Bondo and Fun Times. I will follow the suggestions in this post and if I still need more help I will respect the forum policy and start up a me own thread. I thought the topic was already covered exactly in this thread and wanted to keep the relevant information in a single topic, to help fellow members in their search in the future. Sorry for missing this in the Help Tips.

Thanks for your help and hopefully some of these suggestions and the Mercury link will help out.

Steve

Ayuh,.... It just gets Crazy Confusin' when everybody is talkin' to 2 different posters, with the same problem,....

I quote the post I'm replin' to, 'n hi-light or bold the comment I'm addressin',...
Many folks, just post their comments, 'n ya gotta figure out Who, 'n What they're replin' too,....

No harm, No foul,... Keep on Boatin',..... ;)
 

Bawbe B

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Aug 25, 2014
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7
Thanks for the suggestions and the actual copy of the Mercury TSB.
I looked into a few things while at the lake this weekend...
Engine temp never went over 175. Idle speed when warm was in the 800 range, so I turned it down to about 650. (using the dash tach, not a more accurate secondary tach). This combined with letting it idle for a minute or so seems to have helped as it didn't run on as long after a hard run and didn't run on at all after a slow cruise. However, it made it hard to keep running when cold. There may be a separate auto-choke issue there.I'll keep playing with it every weekend we are up there until it has to come out of the water.
Thanks again.
 
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