MERCURY 30/40 Hp EFI Petrol 4 Stroke


New Member
I have a fault on my Outboard Motor which the Local Dealer cannot find out what is causing it.

I currently have a Mariner 30/40 HP 4 Stroke Petrol EFI which I purchased from new some 9 Years ago. The Engine is serviced once a year and gets very little use approx 10 Hours Per Year .This was verified by the Mercury Dealer here last year who coupled up the disgnostics and said that there was only 80 Hours use on the clock. The Engine is mounted on a 5.1:pissed: Metre Planing Hull Boat.

The reliability of my engine has been hopeless , right from new out of the box, indeed the mention of Mariner to me my heckles come up and all I can think of is problems. I have spent nearly double the amount on repairs to this engine as to what I originally paid for it .

I have a fault currently on the Outboard where fuse No 4 after 15 or 20 minutes running of the motor starts to heat up and up and eventually blows. This is the one which is powered by the main relay and provides power to the ignition coils .

A New Fuse immediately inserted allows the engine to start and run again for 15 or 20 Minutes then the fuse blows again. The Local Dealer has been unable to identify what the fault is . I believe that it may be one of the Coil Packs breaking down under load but how does one identify which coil pack it is ? or could it be something else in the circuit of this fuse?

Any pointers/ clues etc would be most appreciated


Boat Tech

Active Member
If you're still dealing with this then you may want to consider buying a wiring diagram from Mercury Marine for your engine model using your engine serial number so you can chase the wiring colors for the circuit in question.

Also try looking at the colors of wires at the coils, Main Relay, injectors in comparison to fuse No 4 and see if there are some matches in color that might lead to a starting point for you.