Mercruiser 3.0 "Run-On"

[MH9371B]

I've seen various posts about this, and wanted to share my experience. My engine always had the "run on" effect after turning the key to the off position. Never failed, just about every single time. I recently started (despite how much gas prices stink) using 93 octane super unleaded fuel. Run-on problems have completely gone away.I know some have said this doesn't matter, which octane you use, and some have said it did. I just wanted to add my 2 cents and see if anyone cared to comment.

[Trent]

Yep..Mercrusier had a service bullitin several years ago. One of the things to do was to go to a higher grade of fuel.

[xltier]

the higher octane makes the fuel so it wont ignite by hot spots on the pistons,causing run on or diesling.

[Limited-Time]

If your 3.0 is a H.O. engine it has a compression ratio of over 10 to1 defiantly high octane range.

[seamegr]

I could be wrong but... I think your run on is caused by the idle speed being too high. My VP 3.0 will do the same thing. I do not believe changing grades of fuel will make any difference.While researching this for myself I came across a reference to a Mercury part to fix this. Maybe someone with some more Merc knowledge can enlighten you.Back in my auto mechanic days some carbs were equipped with an anti-dieseling solenoid. It is connected to the throttle shaft. With the engine running, power is applied to the solenoid and it controls the idle speed. When you shut the key off power is removed, the solenoid retracts and closes the throttle plates. Anti-diesel.Greg

[kozman01]

I'm with Greg S on this one. I just took my 04' Sea Ray 180 w/ 3.0 back to the dealer yesterday. I told him that when I shut her down it would "run-on" afterwards. He suggested letting her idle for approximately 3-5 minutes. I advised him that's what I did. I told him it would "run-on" approximately 50% of the time. I told him the first tank of gas I used was 89 octane. THe boat only has 3.7 hours on it. I also told him that I thought the idle was too high (1000-1050K). He said, yep, that's too high and said he'd take care of that. Should I still use the 89 or use the 91/93?

[RatFish]

Go with whatever the owners manual recommends. 04 3.0L, I'd bet 87 octane.

[kozman01]

Your right on the 87 octane. I had just been aware of the aforementioned Mercury bulletin. I'll just wait for the idle adjustment and put good old 87 in her and see what happens then.

[MH9371B]

My idle speed is right around 700-800rpm, which is what my manual suggests, but I still get the run-on. I will stick with the high octane gas, despite the extra money!

[Big-E]

MH9471B: Mine does the same thing on 87 octane fuel. She idles at like 550/600 in the water and will run on for a second or two after a hard run. Last time it happend i just ran out into the gulf. Didnt do it on the 1st tank of gas iput in her cause i used 89 octane then. I put 87 octane in her after i got about 10 hours on motor and since it has an entire tank of cheap gas it does it. I will fill her up for sunday with mid-grade and see if that helps. later, Big-E