3.0 idle rpm

[putback]

1985 merc 3.0 alpha 1. runs great above 1100 rpm. Anything less it will shut off within minutes It shuts down like you turned the switch. The farther below 1100 rpm I set idle the worse it is. New ignition components. Carb kit, all passages & vents clean & clear, needle looks fine. Can find no vacume leaks. What else can I look for?

 

[Scott Danforth]

below 1100 is your idle circuit and the choke circuit on the carb. I would lean toward the idle circuit being plugged.

 

[Bondo]

Ayuh,.... Ditto what Scott says,....

 

[airshot]

Something else you might check is your compression, my '83 was having problems idling and wanted to load up and stall and found a blown head gasket between 2-3 cylinders. Did a complete head job over the winter and now she will idle below 600 without any issues at all. I would still take the others advice but while your workin on it a compression test is simple and easy to do and it will let you know the health of your engine.

 

[putback]

Yep, compression checks out & almost no blow by out the vent. Runs all day perfect over 1100. Slow ride or trolling may go a minute or 15 then shuts off like a switch. No cough, sputter, or slow down. On the rare occasion it may troll half an hour. Stalls and starts right up. Does'nt stall going into gear, but I don't like doing it at 1100! Best I can see I have the carb spotless. Every passageway flows rattle can carb cleaner freely. Out of desperation I changed out the coil & condenser, seen them act up like this on cars back in the day. very little carbon on the plugs. Tried to find a vacuum leak with ether, no no &no. Somebody slap me up side the head on this one!!!!!!

 

[Rick Stephens]

I had a 3.0 L that didn't like to idle. Hooked a vacuum gauge to it and found under 1100 it was shakin the needle like a Miley Cirus dance. Valve guides were so worn the motor couldn't idle. Easy to check anyway.

Rick

 

[putback]

Thanks, didn't think of that. I'll do it tomorrow. I'm wondering about the oil pressure sending unit. This has a light not a gauge. After it stalls I have no starter power for a few seconds until the oil light comes on. Red light comes on it starts right up. The original manual notes that. What I'm wondering is doe's this system have low oil pressure protection? Say if the sender goes below 20 lbs it shuts of electricity but not low enough to throw the light? No mention of that in the manual. It used to be fairly common for oil pressure senders to go weak. Also used to be normal for an engine to run much lower pressure at idle than at speed. This is all pointless unless there's a low pressure cut of it the system somewhere, anybody know?

 

[fishrdan]

If it's shutting off "like a light switch" I would look at the wiring or an electrical issue. Check for loose connections or something shorting to ground (like the shift interrupt wiring). I had something similar, cruising along on plane then _____________ engine cuts off like someone turned off the ignition. Well it was, a bad ignition switch.

I'd start jiggling/wigglin all the wires in the ignition system, even removing and cleaning any suspect looking terminals. If it had a spade connector, pull and push back on to make sure it's making a good tight connection, just make sure you know what wire goes back on which terminal. A bad crimp connection could cause this also. loose/corroded. When I rebuilt my boat, I found several sketchy crimps from the manufacturer...