5.7 mercruiser engine run on

[scott8058]

95 bayliner
5.7 mercruiser
alpha one

long time since I’ve been here but having a problem. When I turn my key off the motor tries to keep running. I let it do it for about 2-3 second then turn the key and it fires up. I let it run for about another 30 second and then it will usually shut off like it’s supposed to. When I was getting gas local service station mechanic suggested timing to far advanced. Any thoughts and is this bad on the motor? Oh and it seems more likely to do it after a longer cruise. Thanks in advance

 

[achris]

Is it bad for the engine? YES. Very....

Timing too advanced? Possible, but unless someone has been messing with your distributor, unlikely. Those engines hold their timing remarkably well. On my previous engine, I stopped checking the timing annually after 5 years... It never moved. I then checked it every 3 years, still didn't move. 14 years I owned the engine, timing never moved... (once I sorted the bad dissy ground out )

What DOES cause run-on is low quality fuel. Causes hot spots in the combustion chamber, which fire the fuel charge even with the ignition off. So, either find better quality fuel (good luck with that!) or let your engine 'cool' before shutting off. Let it idle for about 60 seconds, and that should stop it...

Chris..........

 

[Scott Danforth]

Shutting it off in gear helps to.

 

[NHGuy]

Get your idle down, below 775. Also if it has been idled for a long time such as running through a no wake zone or marina it can help to bring the rpms up above 1100 or 1200 for 15 to 30 seconds. Then drop it back to idle and shut it down.. This cools the combustion chambers with the added cooling water flow and unloaded (cooler) engine speed.
And if it still runs on put it in gear as you cut the ignition.

When my 87 350 carb did that, it turned out that there was a failed exhaust manifold water jacket allowing water into the combustion chambers. It caused the compression to increase which resulted in deiselling, also called run - on.

 

[alldodge]

Oh and it seems more likely to do it after a longer cruise.

I would use an IR temp gun to verify your temperature, motor is getting hotter. May need a new impeller and ensure your cooling system does not have any restrictions creating the issue. Give it more time to cool after the run until you figure out what the overall cause

 

[scott8058]

Thanks I had the motor pulled last year and a lot of work done to it and the drive and seals. Nothing internal on the engine so idk if it would need re timed after having being pulled, if that was possibly the problem but it started after that work. This weekend I topped it off with mid grade gas (17 gallons) but still did it. But i have always used 87 gas in it so can’t say yet if that helped. We have a station that has no ethonol gas, should I try that? Dude there always brags about being the only one with it. Engine temp gauge always reads a steady 140 but letting her idle for a good min does help and I’ll try with it in gear as I am coming in to the sand bar

 

[alldodge]

IMO do as others suggest, ensure correct timing, idle is correct rpm and its not running rich. Then check that your not overheating and you should be good.

So far as E0 gas, that's all I use but also don't bother with staibil. You don't need more then 87 octane but higher stuff just cost more. If you get the motor running correctly, there should be no need for anything else to keep it from just stopping when the key is turned off