Mercruiser 4.3L carb engine "dieseling"

[ragtopguy]

I have only been through this one season with this boat. All of the previous times that I have filled up the gas, it has been with premium non ethanol gas. The last 2 fill-ups have been with regular non ethanol gas. One time it was about 18 gallons and 6 gallons. The change was due to availability at a different gas location. Yesterday, almost every time the key was turned off, the engine kept sputtering and trying to run. Would the change in octane cause this or possibly something else going on? Is there an octane booster that might help until I can go back to the premium grade.

 

[harleyman1975]

Well you know it doesn't like low octane fuel. Yes add a couple of cans of good "klotz" or "104" or some other octane booster. Also shut engine off in gear until you have a tank full of premium. Also what is the rpms at idle in neutral? Should be around 700 in neutral and 650 in gear. If higher, then adjust idle accordingly. This could cause reversion if it is allowed to continue.

 

[spoilsofwar]

Have you changed anything else? My carbed 4.3 (albeit a Volvo) dieseled when I switched to copper plugs from the factory supplied iridiums. As soon as I went back to iridiums it stopped. The fuel supply was constant all along; 90 octane ethanol free.

Bad gas could definitely cause it, but I'm just trying to make sure you isolate the problem fully before blaming the gas. This engine doesn't require premium fuel, at least not mine, not sure of merc, so 87 octane in and of itself is not the cause.

 

[R055]

4th of July weekend I had a full tank on 92octane and ran great, after using the tank up I had to get the gas from the lake I was at that was 87 octane, my 4.3 diesels with the 87 octane.

 

[Fishermark]

Mercruiser recommmends letting it idle a bit after a high speed run to avoid dieseling. You might try that and see.

 

[Scott06]

Had similar situation with a 3.0 definitely idling aft coming off a tubing or ski run for a minute helped. Also helped to decarbonize the cylinders by stalling engine with sea foam or similar (gm used to make a top engine cleaner), once a year drop it in stall the engine let it sit 20 minutes come back and burn off the carbon this helped a great deal.

 

[NHGuy]

That's a known thing with the 4.3's. Run it at a faster idle before shutoff to cool it down. Then if it diesels turn the switch back,on, it will run. Cool it again and shut it off in gear or while putting it into gear. If it's timed wrong that can be involved, and definitely it needs a nice low idle speed.

 

[thumpar]

You may try it with a 140 degree thermostat too.

 

[R055]

Chances are it will fix itself when he switches back to 92 octane if it didn't diesel with 92 octane.

 

[Bondo]

Yesterday, almost every time the key was turned off, the engine kept sputtering and trying to run.

Ayuh,..... Welcome Aboard,..... As noted above, If, or When it does that, quickly turn the key back On,....

When it diesels, the last Sputter ya hear before it finally dies,..??..??.....
That's the motor spinnin', Backwards,......

The potential to suck in water causin' hydro-lock is Immense,....
That could mean busted crank, rods, 'n pistons,....

It could be as simple as doin' a basic Tune-up,....
An advanced diagnostic tune-up can tell ya the overall general health of yer motor,....

 

[ragtopguy]

Thanks.

 

[FreeBeeTony]

I had a '75 Monte Carlo that did the same thing every once in a while.......every time I replace the dist cap and it stopped......fwiw.

 

[Halibut214]

I know this post is a bit old but still valid as this is not caused by many of the suggested causes and is an ongoing problem with most 4.3 owners. I had it back to the dealer 3 times for this when it was new and they were never able to fix it or even make it better. This has been happening to me since I bought my bought my boat brand new and have never ran anything in it but premium fuel. I finally decided this year I was going to fix the problem once and for all so started searching. I've found a few combinations that might work, lower temp thermostat and next cooler range plug. I will try that first to see what happens. Lowering the RPM only served to stall the boat when putting it in gear.