2007 Optimax 175 blowing 20 amp fuse

[chopperdog]

Greetings,
2007 Mercury Optimax, 175 hp, on a 2007 21' Trophy.
Took the boat out today. Has been running great past couple months.
Long run at Idle, then slowly accelerated. Just as boat began to plane, engine quit cold.
Cranks over, but no combustion.
4 fuses are in the engine compartment, 1 is a 5 amp, other 3 are 20 amp.
1 of the 20 amps was blown.
The fuel ball could not be pumped to point of being firm. Like fuel just flowed through it.
I thought this may be a fuel issue, with a faulty gauge indicating 1/2 tank? I do think the gauge is faulty (short?).
Came home (towed).
Interestingly, at home, the fuel ball worked normally.
replaced the 20 amp.
Cranked it over, and fuse immediately blew.
This may be important. When I turn the battery switch on, I hear 'white noise' for a second. The stereo is not wired through the accessory switch. If batt is on, stereo works.
could this be a short, and the cause of my problem today? If so, seems like this would have happened before.
Thanks for the help. You all know how frustrating this stuff can be.
Wife and kids still had a great day, so the stress level remained low.
Thanks again

 

[chopperdog]

I realize I may have to hunt down a short.
I wasn't sure if the fuel ball issue was indicative of particular issue, like with a shorted fuel pump.
thanks again.

 

[chopperdog]

No Title

The fuse that is blowing is connected to a Red/White wire, and a Red/Yellow wire.

Reading a wiring diagram (2001 model), it looks like the Red/Yellow connects to the Ignition Coils and Coil Drivers.
and the Red/White connects to the Main Power Relay.

Trim works, engine cranks, lights work, all electronics work.

Thanks

 

[bakkeb]

I recently had the same problem. After a bit of deductive reasoning, turns out it was a failing coil pack that was the culprit. Here are a couple of troubleshooting steps that may help.
- Get a bunch of 20amp fuses on standby, you'll need them. (Recommend grabbing a bunch for free from a junk yard and keeping spares on your boat.)
- If you have a NMEA-2000 gateway ensure that your alarms are enables and check smart gauges to see if any sensors like Throttle Position Sensor or Water flow sensor are not tripping. You can check this by disconnecting them with power on which triggers an alarm that will go away once plugged back in.
- Disconnect any unnecessary devices or electronics from your switch/acc circuit. Test engine, inspect fuse.
- Try to confirm if the fuse is blowing when switch is turned on to ACC or when it turns over.
- Disconnect large round CAN BUS cable at engine and crank over engine from solenoid. Inspect fuse. If it’s blown it is likely under the hood and not part of the switch or ACC. Reconnect round CAN BUS connector. Replace fuse.
- Disconnect all Coil packs from harness and test key, inspect fuse. If fuse holds and is not blown connect 1 coil pack at a time and test it by turning on key and inspecting fuse. Continue this process for each coil pack one at a time. My Optimax 250 blew the fuse on the 5th cylinder so we changed out the coil and tested it again and it held. Problem solved!