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Mercruiser 30 idle problems at sea

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bradcox00

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"hi i have a 2007 mercruiser 3

"hi i have a 2007 mercruiser 3.0 I/O and i have been having some idle problems. at dock it starts and idles totally fine. perfect actually at about 800 rpms. as i leave the dock and head out it totally runs fine, gets to plane quick and has great acceleration. but after running for a good 5-10 minutes, if i pull it back into the neutral position, it idles terribly, chugs, shakes and almost wants to stall out. and for the first time the other day, the engine still chugged for a good 5 seconds after turning it off and removing the key. I am in new york so i winterize but i do all the necessary steps. there is ethanol in my gas but i run with K100 for water removal. What could it be? any suggestions?"
 
"is it carberated, if so when

"is it carberated, if so when you shut it down you could be dieseling, try letting it idle a few minutes after running at or near WOT (wide open throttle)maybe turn on your bilge blower also, hopefully that helps."
 
"crappy gas can do alot of tha

"crappy gas can do alot of that, thinking about it, I am new here and no where near a marine mechanic so just guessing. I use marine grade sta- bil and seems to help."
 
"Ayuh,....

Sounds like the

"Ayuh,....

Sounds like the Choke is Way outa Wack.....
Sticking On maybe..."
 
"I can't take credit for a

"I can't take credit for all this info, it was from another forum.

Engines may ‘run-on’ when the ignition key is turned off. Normally, it
will take a combination of several conditions to make the engine ‘run on’----such as:

1. Type and formulation of gasoline available in the area.
2. Type of engine compartment ventilation system.
3. Outside air, inside engine compartment air and engine coolant temperatures.
4. How quickly the engine is shut off after running at cruising or higher RPMs.
5. How the boat and engine are operated.
6. Engine idle rpm set too high.
7. Wrong heat range spark plugs or spark plugs with deposits on them.
8. Deposits in combustion chamber.



Carbureted engines, unlike EFI, do not shut off the fuel supply to the engine when the ignition key is turned
off. As the engine is coasting to a stop, air is still flowing through the carburetor into the manifold. This air
flow can still draw fuel from the carburetor, through the intake manifold and into the combustion chamber.
A hot spot in the combustion chamber will cause this fuel/air mixture to burn. The engine now ‘diesels’ or
‘runs on’ although the ignition system is not firing. EFI engines do not have this problem because the injectors
do not inject fuel once the ignition key is turned off.

Suggestion to Minimize ‘Run On’
Items number 1 ,2 and 3 can correct or reduce most of these ‘run on’ problems. Letting any engine idle a
minute or two before shutting it off is desirable. This helps the engine to normalize at idle rpm before being
shut down.
1. Be sure engine idle mixture, idle rpm and ignition timing are correct. On 4 cylinder and V6 or V8 engines
with Thunderbolt IV ignition, setting idle rpm as low as possible can help stop ‘run on’.
2. Adjust throttle cable barrel so that the carburetor’s throttle lever returns against the idle speed stop
screw every time the remote control throttle lever is returned to neutral, idle.
3. Idle engine 1 to 1-1/2 minutes before turning key off after running at cruise or higher engine rpm.
4. Try a different brand of regular fuel or try some premium fuel. I have personally seen brands make a difference.
5. Increase idle mixture by 1/8 turn ‘richer’.
6. Go to next colder heat range spark plugs."
 
"[b]"that has no choke it

""that has no choke it has TKS turn key start on it"

Ayuh,... Then That's what's Broken....

I still haven't ever Seen 1 yet...."
 
"Having same problem (runn

"Having same problem (running on) with a 4.3.
I've been told to "lean out" the idle mixture, not increase it. Also, if possible (and with a 4.3, it's damned near impossible), try and set you mixture while under power. In gear, at correct idle speed, and with timing already set.
I'm also told that the problem may be caused by slightly advanced timing. If everything else fails, try cutting the timing back 1 degree. All of the above is especially true if the boat was previously set up at a lower elevation."
 
Check your exhaust flap thats

Check your exhaust flap thats in your exhaust tube. Found mine burnt and alound water to back up into my manifold. Also check the passages in your manifold.They could be clogged causing the manifold to heat up causing engine run on.
 
"thanks everyone, im going to

"thanks everyone, im going to try everything that you guys posted and lets see what happens. if there are anymore problems i know that i can post them here....."
 
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