Mercruiser 3.0L Rough Shut Off

[choppstixxx]

So just the other day on my 2000 Sea Ray 180 w/a Mercruiser 3.0L I noticed maybe 1 outta the 4 times I shut the engine off it had kinda a rough shut off. This has never been the case in the past handful of times I have used the boat. I am wondering if this could be caused by mixing grades of gas? I have never let the tank get lower than 1/2 a tank and have usually filled it with around 5 gallons of 93/89/90 (e-free). I am going to stick with 90 (e-free) from here on out, but is there anything else that could cause these types of symptoms I should look for? Thanks

 

[Lazy Daze]

Mixing gas won't cause this. Nothing to worry about there.

Sounds like you're having "dieseling" or "run-on". Aside from a mechanical issue (carb), which could certainly be the case (get it cleaned/rebuilt)... if the gas has sat for a while, it starts to lose it's octane, which can cause this issue. Try adding octane boost. Do you use a stabilizer on a regular basis?

A dirty/sooty cylinder can create hot spots, as well - which will cause this issue. That can, usually, be cleaned with additives. At 19 years old, any of the mechanical issues or sooty issues are fair game to be the problem.

 

[choppstixxx]

Mixing gas won't cause this. Nothing to worry about there.

Sounds like you're having "dieseling" or "run-on". Aside from a mechanical issue (carb), which could certainly be the case (get it cleaned/rebuilt)... if the gas has sat for a while, it starts to lose it's octane, which can cause this issue. Try adding octane boost. Do you use a stabilizer on a regular basis?

A dirty/sooty cylinder can create hot spots, as well - which will cause this issue. That can, usually, be cleaned with additives. At 19 years old, any of the mechanical issues or sooty issues are fair game to be the problem.

I haven't used any stabilizer or octane boost before since taking ownership, what brand/type should I use?

 

[Lazy Daze]

This is one of those subjective areas, and there really isn't a "best" one out there. But I've been using Startron for 15 years and have been very happy with it. I would suggest ALWAYS using a stabilizer - it's cheap insurance. For octane boost - whatever they have on the shelf at Autozone will be fine. Once you get this issue sorted out, and use stabilizer regularly, you shouldn't need to use the octane boost again... meaning, I would only use the octane boost for one tank to see if it helps.

 

[paulswagelock]

This is a typical issue for the 3.0. Try letting it idle for a minute before shutting off. I had that engine and it did that 50 percent of the time.

 

[choppstixxx]

This is a typical issue for the 3.0. Try letting it idle for a minute before shutting off. I had that engine and it did that 50 percent of the time.

That's what I read too, the more I think about it I probably tried to shut the engine off too quick the one time it happened. I am buying a bottle of Star Tron before this weekend and will pop some in the tank before I head out, gonna just stick to 90 e-free since there is a gas station on my way to my boat that has it. Thanks again everyone!