Hi,

Before turning the engine off, run at idle in neutral for 30 seconds, then shut the engine down. It gives the engine time to cool, so that the fuel doesn't continue to ignite when it's turned off. This seemed to be the fix for my 5.0 doing the same.

Good luck,
John

 

The last post worked for me most of the time when my 3.0 would diesel. I also found high test solved the issue but that was when gas was less expensive.

 

Have you been using an additive for ethanol? One of the things that ethanol can do is lower the usable octane rating of the fuel.

Next, try running a triple dose of a cleaner (Merc makes a product called Quickleen) to try and get rid of any carbon deposits.

Running the blower all the time may also help. But, adding octane or running the blower (and idling for a minute) are band aids. They may end up being the solution, but the root cause is usually related to a "dirty" fuel system and cleaning it (and be proactive in the future by using additives on a regular basis) is advisable.

 

I've been adding StaBil in the amount recommended on the bottle. Do you suggest something different? I'll try the Quickleen. Thanks.

 

I appreciate your response. I've been using 89 Octane, simply because the previous owner said that the boat seemed to run better with it. Thanks. (If gas prices get any higher, I'll have to give up eating!)

 

Thanks. It only seems to happen when I pull the family on a tube, then shut it down to retrieve them. Maybe the added strain is causing the engine temp to reach a dieselling point?
Hot enough for you in the Queen City? We're getting scalded over here near Washington.

 

That would be my guess CCM. Running the engine hard on a hot day causes the engine to be hot enough to ignite with the ignition turned off. I would try the previous suggestion of letting the engine idle before shutting it down.

 

I looked at your signature. Not sure if you are injected or carburated. Are you sure your choke is open all the way when you shut down? Assuming you are carburated.

 

Stabil (red formula) is fine for stabilization, but it does nothing for ethanol. You want either the Marine (green/blue) formula or the Startron (my personal preference) brand. The carbon deposits (that I mentioned above), by the way, give the fuel a "hot point" to continue igniting - more so than the surrounding "metal" of the carb.

 

Check your idle speed rpm, should be about 650 at idle, I have noticed my 04 185, 4.3L does not return to idle speed when hot, you can lift the cover and push on the linkage and get it down close, then it will shut down properly. Letting it idle a while helps sometimes but not always. In my case I think I need to adjust the throttle cable a couple turns so it will push the throttle lever to the closed position. Check the idle speed, you may find that you are at 900 to 1000, at that rpm you will diesel.

 

Carburated. Not sure about choke. Will check. Thanks.

 

Thanks. Off the top of my head, I think it's idling under normal, non-tubing conditions @ 800, but I'll check.

 

Hi ChooChoo,

Yes, very hot in the Queen City! We are surviving well, though! Good luck getting your issue resolved.

Thanks,
John

 

Thanks for the Startron tip. I'm still learning.

 

Letting is cool is the best tip. Idle speed is also important. And if you can, check your timing too if the first don't work. It is another cause for what you are experiencing. But on my 3.0 which has the same behavior, the timing was perfect. My idle speed is a bit higher than spec, otherwise it takes forever to warm up at the dock, or risk stalling it. Letting it cool for 30 seconds is the best tip, as when I do that, it always shuts down nicely.

 

I had the same problem as you with a 5.7 I checked timing, idle was at 650 rpms would let it idle for about a minute after docking it still would diesel. Found out the fuel dock I bought 2 fills from had water in their tanks they are shut down replacing tanks. Ran the tank down to about 1/8 full added Startron filled up at an ethanol free fuel dock about 4 weeks ago haven't had a problem since.

 

Thanks. Will definitely have the timing checked.

 

Thanks. I'll start using Startron.

 

I experienced dieseling in my 5.0 a few weeks ago. Turns out that I wasn't in neutral when I shut off the engine.

 

One more possibility - same basic solution; this was happening w. our old boat. It turned out that the blower was almost dead; really didn't vent engine space well. However, I discovered Sea Foam in the middle of dealing with the issue; I add it to the gas on a regular basis.