Occasional run on.

[rotorhead511]

2011 Bayliner 175
Mercruiser 3.0
40 hrs

I get occasion run on from my 3.0 when I shut it off.

I'm trying to get more data on when but I think its mostly after pulling a tube with a bit of high idle.

I think the idle rpm is supposed to be 700 but sometimes mine is at 1000. I think that might be when I get the run on.

Still gathering data but what you all think?

 

[Alumarine]

High idle might cause dieseling. It will also cause premature wear to the lower end.
Try letting it idle for a minute before shutting it down.

 

[nola mike]

Yeah, the dieseling is the engine running hot, especially after working hard after pulling a skiier. Advanced timing, high idle could contribute. Definitely let idle for a few minutes before shutting down and see if the problem persists. Dieseling is bad news.

 

[harringtondav]

Yeah, the dieseling is the engine running hot, especially after working hard after pulling a skiier. Advanced timing, high idle could contribute. Definitely let idle for a few minutes before shutting down and see if the problem persists. Dieseling is bad news.

Yes. My 4.3LX did this early on. My spec. idle is around 650 rpm. I fixed mine by turning in my idle mixture screws 1/8 turn each outing until it stopped the dieseling. With a 1000 rpm idle, I'd turn yours back to 700 rpm max using the idle adjust screw that stops your carb throttle arm. Your dog clutch will thank you. Before tweaking your idle mixture screw/needle, note the screwdriver slot position and turn it in until it seats, counting the turns to the closest 1/8 turn. Then back the screw/needle back out to home. Factory settings tend to be 1 1/4 to 1 1/2 turns out from seat. Too tight causes a lean idle mixture resulting in hard starting and engine falter when shifted into gear. Too far out gives you a rich idle mixture, and possibly your engine run-on.

 

[danhenke]

My manual states to turn on blower, allow engine to use for several minutes until idle returns to 700, then shut down. I’ve read here that it can also cause water to get in the motor because the engine runs backwards when dieseling occurs.

ive have it happen a few times. I also use the highest test gasoline I can find. Since reading the manual, and changing to better fuel, it hasn’t happened since.

good luck

 

[Scott Danforth]

the problem with dieseling or run-on, is that last gasp you hear is the motor running backwards and sucking in water from the exhaust.

adjust your timing to spec
run good fuel
let idle for a bit

you also need to find out why your idle is that high. that will kill your outdrive.

 

[kenny nunez]

If your carburetor is one of the models with no idle stop screw and no mixture screws then you should only use premium fuel like danhenke said. And as Scott said “dieseiling” is really bad and in some cases cause a fire if the flame arrestor should come loose.

 

[rotorhead511]

Excellent info, thanks all.

I'll start working it.
Will start with checking the timing.
Then do better at letting it return to proper idle before shutting down.
Then adjust idle followed by mixture if it needs it.


I should be able to find timing spec in the manual, right? Will check it when I get home.

 

[danhenke]

Excellent info, thanks all.

I'll start working it.
Will start with checking the timing.
Then do better at letting it return to proper idle before shutting down.
Then adjust idle followed by mixture if it needs it.


I should be able to find timing spec in the manual, right? Will check it when I get home.

at least the damn distributor is close to the front of the motor on mercruisers! I hate leaning over belts and running gear to adjust my timing!

 

[Cortes100]

Our 2008 3.0L TKS, did the same thing when new. Occasional run-on after a long hard run. Tried premium fuel which didn't change anything. I was able to lower the rpm by like 50 rpm. Let it idle for 30 seconds, and ALWAYS ran the blower when the engine was running. Never had the issue again for the next 7 years.