Problems after sensor fail 08' optimax 250hp

[aby n]

Hello all, My buddy is having issues with his motor / boat getting on-pad after having a fuel/water sensor fail.

Motor details - s/n: 8M8022142 1B547028 1250P73HD MERCURY 250DFI L PROXS

So after replacing the sensor, my buddy went out on the water & the motor shut off once he tried leaving the dock.

He took the boat to a mechanic & found a blown fuse, he replaced the fuse, it blew again at the shop, did this a few times, to try & identify the issue & couldnt. He threw in a 30a, got it running & all seemed ok. He turned it off, went back to a 20a fuse & all was ok... Hmm, not my kinda guy, but oh well.

So friend went to the lake & he wasnt been able to get on pad. the motor starts right up, idles fine, doesnt misfire under load, but simply doesnt have the berries to get going like it used to prior to the sensor fail.

He has since added 20 gal of fresh local fuel, as the boat was virtually empty with the fuel leak.

Also, what is the p/n for replacement gaskets on the 2 manifolds on either side of the motor up top. it looks like water deposits have been coming from here. pointed out in pic.

Looking for advice, besides finding a different mechanic!

Could the ecu learned a crazy correction / offset value when running the motor & leaking fuel from the sensor? Is there anything he can reset at home without any special equipment?

We regreased the cable - forward / reverse area, as it was bone dry after the fuel washing any lube away. During this work, we went WOT & it looks like the throttle body is near 100%, i would say ~93%. I wouldn't imagine the throttle being fully pinned, which would cause throttle cable stretch or tb/tps damage.

Is there a low cost diagnostic tool to buy to reset stuff, see if sensors failed, etc?

I dont believe he has replaced any fuel filters yet.

Thanks for any & all advice

[aby n]

i've located the gasket p/n i inquired about from a link in a different thread to European Marine Parts Catalog!!!

this website is awesome.

does overheating the motor cause the thermostat cover to leak or does that gasket just weep over time & simply needs to have its bolts snugged every so often?

[Bob G.]

Not a good idea to put a bigger fuse in an electric circuit to get it to work.

Overheating the motor will cause an overheat fault. Could be compressor or a coolant overheat fault for port or starboard. Will need fault codes read to know for sure.

If the thermostats are leaking, replace the gaskets.

Rinda Technologies Diacom software is good for DIY. Not low cost but works good.

Sounds like the motor needs some TLC. Is the boat/motor new to your friend?

If he is going to do his own maintenance, suggest getting a Mercury manual.

[SilverFox579]

^^^^+1^^^^

[aby n]

Thanks for the input Bob G!

Yeah, i was shocked; no punn intended, to hear that a mechanic simply threw in a bigger fuse vs identifying the short issue.

He isnt new to boating, i think this is the 1st issue he's had with the boat since '08.

Yeah, the rinda technologies stuff looks like a must to have for DIY sorting out.

He didnt say it overheated or ignored alarms, he did say when shallow, he has picked up debris & had to poke the water passages free to get the motor to pee.

Since im in between this, what does the compressor do on a boat motor? Does the compressor run on the odd colored fuse in the original post? is there a fuse for the compressor?

I apologize for my ignorance on the mercury compressor. I will read up on it now, but prefer to get experienced insight vs what google says. When i hear compressor, im either thinking AC or a fancy mercedes benz turbo car!

The people on these boards seem really helpful. Thank you

[Bob G.]

The compressor is an air compressor that runs on a belt on top of the motor and it is water cooled. The water from the compressor goes to the pisser hole. No water from the pisser hole = an overheated air compressor. Water from the thermostats as they open will go through the dump hoses and out of the bottom of the motor.

Sounds like it’s time to replace the impeller, check thermostat operation and replace the thermostat gaskets.

FYI: I’m not a Merc mechanic but do my own maintenance. Don, Joe, Savage and others are the experts and can give you more specific troubleshooting.

[aby n]

Thanks for the prompt reply

Yeah, i just reviewed the air compressor strainer thread... yikes! That looks like a must to check after shallow running.

[Bob G.]

Great idea. I forgot to mention that, I remove mine for inspection once or twice a year.

[EuropeanAM]

Tell your buddy to STOP, STOP STOP trying to run this engine. NEVER, EVER put an over-size fuse in a circuit such as this, as that fuse is there to help protect sensitive electronic equipment (such as the PCM).

Pretty likely that he has a shorted ignition coil that is causing the fuse to blow- AND causing the loss of power. HOPEFULLY it doesn't cause the PCM to blow too.

[aby n]

Thanks for the reply Don.

It wasnt my buddy, but the mechanic he took it too who put in the larger fuse.

When the coils short out, does the white oozing stuff typically appear on the bottom; shielded from view, or is it vissible on the top when looking at the coils? I will ask if he has a small inspection mirror to help him see.

With regards to the pic posted of the odd colored fuse, what circuit is that odd colored fuse protecting? I am assuming it's the odd colored fuse that was changed. Do either of those 2 fuse colors look like oem pieces from mercury? if so, which ones.

Should all 3 of those fuses have 20a fuses in their respective locations? If not, what size / amperage should be in the slots?

I dont think my buddy has a mercury manual for his motor.

Thanks for the help

[EuropeanAM]

Coils may or may not "ooze white stuff" when they fail. There may be absolutely no visible indication, either.

There are three (3) 20a fuses in that holder. I'd have to pull a manual to verify but I thing the remaining is supposed to be a 5a fuse.

Note that there are different circuits that are protected by 20a fuses as well- so there's a possibility that something other than a coil is the problem (odds are with the coil possibility, however).