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Since Day 1! I've tried higher octane, and has not helped. Has anyone else experienced this? All on-line advice states the engine may be running hot. Still running stock spark plugs, water temp is normal, at least what I think is normal at around 170-180. Very annoying.

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2004 180 4.3GL

BTW, changed the impeller last summer. Surprising how they wear. Definately a once-every-2-season maintenance item. Very easy to R&R.

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2004 180 4.3GL

Here is what I found maybe it will help.

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth 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 sparkplug can retain heat and cause the same problem.

A carburetor that does not close entirely 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 combust.

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 turnover and combust more fuel and lock itself into a cycle of continuous running.

I'm confused You refer to diesels, but then say octane and spark plugs which are indicative of a gasser? Which one is it???

Dieseling meaning when he turns the ignition off the engine keeps chugging along for a little bit with no spark like a diesel.

Ok. What grade fuel are you running? When was the last time you changed the fuel filter? Are you running the oem spark plugs? You are running lean somehow and your motor won't last long in that state! I assume the GL motors were not fuel injected. The same thing happened to me with my high compression ratio jetski once and I think I got ahold of some bad fuel. Try switching your fuel source - maybe you are getting old stagnant fuel from a marina?

Thanks for the replys. What i was really after is, Has anyone else had this problem, and what if anything did they find to cure the problem??

I started new out of the box with 87 octane. Like i said this 2004 4.3GL (carborated) has dieseled from day one. Went to high grade, then backed off to 89 and nothing has helped. I don't get fuel from a marina, I tow and store in the garage. Plugs are oem, fuel filter changed every season. I had the boat out only twice this year so far and next time will check for carb stuck open. I will also double check the plugs to make sure the factory installed the right ones.

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2004 180 4.3GL

I have had it happen before on old cars I have had in the past. But they where up in age compared to your boat. Generally some cleaner to remove the carbon took care of it in the short term. Since though yours has happened since it was new I think it would be a carburetor problem. It’s supposed to completely close when the ignition is shut off to prevent fuel from still going into the hot cylinder. That’s where I think I would start. But like I said I have never had it happen on something new. Has the dealer ever looked at it?

Oh and if your idle speed is set to high it can happen too. If you are at a normal idle speed then I would start with the above, otherwise I would lower my idle speed.

I had the same problem with a 4.3 190 horse/alpha drive motor in my previous Glastron SX190.

87 octane: When shut down after pulling tubes or working hard, the motor would diesel for a while. Very annoying. Let engine idle for 2-3 full minutes before shut down, then it usually didn't diesel much or at all.

89 octane: Same as above, but the engine would diesel for a few seconds after working hard. Let idle for one minute to cool off, and then would shut down OK.

93 octane: I could run the engine and shut it down without any issues.

I chalked it up to a very close tolerance timing and compression setup, where Merc was trying to get the best gas mileage and least amount of emissions from the engine. Wound pretty tight. I didn't want to retard the timing, so I burned the better octane. The people I sold the boat to run 89 octane and let the engine idle for one minute before shutting it down.

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Previous Four Winns: 2003 Freedom 180, 150 Johnson XL, purchased new July 2005, sold Sept. 07
Current Four Winns: 2002 Horizon 170, 115 Yamaha V4, purchased May 2010 from family friend.