So this may be common knowledge to some, but for beginners like myself it could be very useful. I recently purchased a boat with a 1995 Mariner 225 EFI. Motor is solid and makes a tremendous amount of power. Recently had some rough idling and rough running at lower rpms. Found my tps sensor to be out of adjustment. Re-synced it and it made a big difference, but still had a good bit of smoke and heavy fuel smell at idle. Did some more digging and read the entire section of the fuel management system in my service manual. Most EFI owners will tell you they smoke a good bit, and mine did especially at low idle speeds.
If you want to remedy this, I tell you what I did and this motor basically doesn't smoke except when it's first started after sitting. Being an older engine, I tested the engine air intake sensor ($20) and the engine block sensor ($15) per service manual using basic tools and a multimeter. Both were wayyyyy off and basically giving the engine erroneous temperature data. Ordered new ones, popped them in a few minutes and voilaaa! The engine runs muchh more cleaner and smoothly. I went ahead and replaced both thermostats as well. So, if you own an EFI and want to try and cut down on the smoke, try this. All sensors, both thermostats, and gaskets cost me $90 shipped and 15 minutes of my time so if it works, great!, if not, well you aren't out a whole lot.
If you want to remedy this, I tell you what I did and this motor basically doesn't smoke except when it's first started after sitting. Being an older engine, I tested the engine air intake sensor ($20) and the engine block sensor ($15) per service manual using basic tools and a multimeter. Both were wayyyyy off and basically giving the engine erroneous temperature data. Ordered new ones, popped them in a few minutes and voilaaa! The engine runs muchh more cleaner and smoothly. I went ahead and replaced both thermostats as well. So, if you own an EFI and want to try and cut down on the smoke, try this. All sensors, both thermostats, and gaskets cost me $90 shipped and 15 minutes of my time so if it works, great!, if not, well you aren't out a whole lot.