motor wont die when turned off

[Matt_Jones105]

i recently purchased a 2008 sea wave with a 3.0 mercruiser in it and the motor runs great until you try to kill it. When you turn the key off it sounds like it has to lug itself down to die and sometimes it will backfire and you can smell gas. Could it be timed wrong? or pumping to much gas or what??

 

[jkust]

Sounds like good old fashioned Run-On. Run-on is very common on the 4.3's but someone would need to pipe in as to how common it is on 3.0's. First thing I'd try is that when you are ready to shut it down, let it idle for a couple minutes. A lot of times it happens after a long wot run and then you shut it down abruptly and you have carbon build up in the engine and it keeps trying to fire. Carb engines do this because unlike MPI, the fuel flow doesn't shut off instantly. Then the second thing I'd try is to put a higher octane gas in it which will stop the symptom of run on. Even 89 octane can help dramatically. This is THE only time an engine that doesn't require higher octane actually provides a benefit even if it is just masking the problem.

 

[Matt_Jones105]

Yeah I can tell when I let it idle for a minute it does help it. Thanks for the input!

 

[alldodge]

i recently purchased a 2008 sea wave with a 3.0 mercruiser in it and the motor runs great until you try to kill it. When you turn the key off it sounds like it has to lug itself down to die and sometimes it will backfire and you can smell gas. Could it be timed wrong? or pumping to much gas or what??

:welcome: to iboats

run on or dieseling is caused by engine out of tune, idle to high, pour fuel (bad gas) or timing set to high

 

[NHGuy]

Yup, get the timing right, and the idle down. Then if it runs on don't just sit there and let it diesel, either throw it into gear or turn the key back on, it will run again. Allow it to run at a little higher rpm til the temp comes down. Then try to shut it off.
If it still won't stop you can actually shut it off and then shift it into gear which will probably stop the motor. Just be tied off good, or where no one can get bumped by the boat.

 

[thumpar]

All the above and you might want to check the carb also. If it is to rich it can make the problem worse.

 

[Scott Danforth]

many 3.0's run on. just key off in gear and it will die every time.