I was wondering if more people have this ?
My friends Cobalt 343 with 2 x 525 EFI,s are running very rich at idle ,if the engines are warmed up ,they still causing burning eyes when sitting or standing on the aft deck !!
Both engines seem to do this ,we checked the engines with a Merc tool ,no faults ,nothing ,all good .

Is this just the nature of the beast ,and is Merc just running them rich to let them accelerate out of the hole quick ,or is there a cure for this ?
Or do we have to live with this ?
Merc dealer says everything is fine and is working as it should .

Thx A.J

if i run 91 octane mine will run rich
they like just good old regular fuel

I don,t get how this engine can notice what fuel its running at .. there is no Lambda sensor in the exhaust system to measure what air fuel ratio its running.

If you run the 525's and have some cavitation, or over reving the RPM's it dumps fuel to reduce the RPM.s and "save" the engine. Now you running rich and have smoke or dark stuff on your stearn. Don't worry, it's ok.

The motors done run rich or lean from the octane, its how the fuel combust and the mixture of the fuel. Higher octane fuel burns slower, thus it does not burn as clean as a lower octane, which then soots the transom in some cases. If you have a stock 525hp efi, with stock timing, you should run 87 octane and at the highest, 89 octane. 91 should be used for customers with modified spark and air fuel's.

Earlier 525's were richer than the later ones.

Also, some fuels have more additives that will stick to the transom, etc.

I would first try running 87 octane, if this does not eliminate the problem, then you'll want to send your PCM's in to get updated and possibly modified for your application.

if the thermostats are stuck open or out the engine temp sensor will keep fuel injection in a choke mod and run rich. this is problem i have run into, oil will smell like gas and level will be up. both engines should seem unlikely, but could happen over time.

2004's ???

bingo

Thanks for input you all ! we are going to try higher octane fuel to see if this helps .
The boat is a 2007 and engines also .
The main problem is into idling around the docks ,reversing ,when standing on the rear ,the gasses burn the eyes ...it that bad ,and unburned fuel can be smelled.

I don,t get how this engine can notice what fuel its running at