View Full Version : Merc 43 Dieseling

Robert Beaty
08-09-2002, 10:11 PM
" 1997 Merc 4.3, 40 hrs, Carburated, new plugs, +87 octane, clean water filter. Recently serviced and the engine now diesels quite badly.

I want to check the timming, and will do so, after I find the wire to ground.

I know that my mechanic removed the carburator while diagnosing a problem with the Electroninc module which he replaced. My boat performs conciderably diferent, where I use to get 54 MPH I only get 45MPH now. Engine runs very smooth.

Any suggettions what I should lok for, I have timing lights, dwel and rpm meters (from my automotive days), I also use to be pretty good with the carb mixture controls. "

08-13-2002, 10:17 AM
" Dieseling is usually caused by hot spots in the cylinders many times associated with carbon deposits or possibly over-hot pieces of metal. Did the mechanic put in the correct temperature plugs? Take them out and have a look, you should know what they should look like from your automotive days.

Did the choke get reattached correctly? An overrich mixture can cause the carbon deposites which result in dieseling. Will probably be accompanied by very black sooty plugs.

Check the timing. Very smooth operation and poor top end might indicate retarded spark. Not usually a cause of dieseling. "

05-05-2006, 06:04 PM
"Assuming that you mean 'run-on' when you say 'deiseling', the explanaiton I got from Mercruiser was that they went to a more aggressive cam and coupled with the Vortec heads, there were some hot spots. They had a sheet of about 7-8 possible ways to "fix" this problem and among them was a solenoid that would mount on the carb and basically be what determines the idle speed but when the key is turned of, releases and closes the throttle plate completely. They also recommended was cooler plugs (one grade), higher octane, a slightly lower temperature thermostat, letting it idle after high RPM use(to cool it down) and shutting it off in gear.

The run-on issue was just one of a long list in 1997 to 2000 that caused a lot of dealers to switch over to Volvo-Penta drives, including the one where I worked.

It works, but I personally think it was a pretty lame way to remedy a design flaw. They should have tested it more and when they found that they had problems, worked out some other way to stop it. Sorry I don't remember more but I don't work for a boat dealer now and wasn't able to make copies of all of the service bulletins. Have your dealer check for the bulletin that specifically deals with this problem- if they don't have it, they can get it from Mercruiser. They should have a full set of these, anyway.

You really need to find a solution to this because of some of the aditional problems that can occur due to run-on. I worked on one boat (a Rinker, IIRC) that rotated backward at the just as it was stopping. This made it injest water through the exhaust and the water went into some of the cylinders and when they tried to start it the next time, they cooked the starter because it was hydraulically locked. Until I had the list from Mercruiser, I gave them my own list of things to do until I got the parts, including removing the spark plugs if it hydro-locked again(so they wouldn't cook another starter). They called on a weekend to tell us it had happened again and while they were frustrated, their whole weekend wasn't ruined like the one when they couldn't use the boat."