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New Mercury V8's - Need Help

Old 03-02-2021, 05:51 PM
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Default New Mercury V8's - Need Help

A friend that I fish with recently repowered his 34' SeaVee with twin 300hp Merc V8's. I've been singing the praises of these new motors for months but we've got a problem in Mercury Land. Recently while cruising at 5,100 rpm the starboard motor started to stumble, lost power, ran very rough and quit. The motor would idle fine and after shutting it down for a short while then ran fine. Had the motor checked by a mechanic, no codes but found one suspect spark plug. All plugs were replaced and motor ran great until today. Ran 50 miles offshore like a top until same motor lost power, stumbled badly and died. Shut the motor down for a short while and fished. Afterwards motor ran perfectly for 10 miles to another fishing spot. Shut down to fish for another hour and a 60 mile run home and again the motor ran perfectly. Motors are brand new, 82 hours on them and are spectacular until they're not. Any suggestions much appreciated.
Old 03-02-2021, 05:55 PM
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Coil issues?
Old 03-02-2021, 06:03 PM
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Originally Posted by Buttapollock1010 View Post
Coil issues?
Our thought also but not sure.
Old 03-02-2021, 06:05 PM
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I’m no expert, but I’d start simple... engine fuel filter. I think I recall mercs having an in-house filter. Or a bad primer bulb.
Old 03-02-2021, 06:22 PM
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Originally Posted by steve841 View Post
Im no expert, but Id start simple... engine fuel filter. I think I recall mercs having an in-house filter. Or a bad primer bulb.
We also thought about fuel issues but why would it run perfectly for 50 miles and then stumble and quit. Bad fuel it would stumble immediately I would think.
Old 03-02-2021, 06:40 PM
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Possibly some caulking from the sending unit came loose and is floating around in the tank and every once in a while gets caught in the fuel pick up tube. When you shut it off it looses suction and starts floating around until it’s picked up again. Happened on a buddies boat so it can happen.
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Old 03-02-2021, 06:53 PM
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I also would look toward a fuel obstruction or blockage. Two tanks or one ? Maybe swap the fuel hoses at the pickups and see if the issue jumps. I would also measure fuel vacuum right before the engine while running.
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Old 03-02-2021, 06:57 PM
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Originally Posted by captbone View Post
I also would look toward a fuel obstruction or blockage. Two tanks or one ? Maybe swap the fuel hoses at the pickups and see if the issue jumps. I would also measure fuel vacuum right before the engine while running.
Two tanks. We were going to run off the one tank in an effort to get home if need be but ran perfectly coming home.
Old 03-02-2021, 11:50 PM
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Try changing the fuel filter on the back of the motor. Some shops are confused thinking these filters 35-8M0106635 300hour change they are actually 100hour service items. There is also a by pass part number from mercury for that filter. I was told they were having issues with it collapsing on the inside.
Also those motors have had a few wire harnesses failures nothing major but I had one fail. The actual harness thats on the motor. Does it start shuddering so bad it looks as if it will fall off the boat? Its the rev limiter kicking in and causing it to shake like that.
i have 4 of the 300v8 right now and have about 1200hrs on them. There is nothing else available that I would choose to run. The motors are so strong and have had hardly any issues with the motors. I consider those things minor considering I have four of them.
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Old 03-03-2021, 02:52 AM
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Sounds like maybe a fuel pump or something else fuel related. I had brand new one here that acted like that and it had a faulty fuel pump right out of the box. At around 5000RPM, it would start to stumble and fall on its face. A fuel pressure gauge confirmed it wouldn’t reach over 35 PSI.
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Old 03-03-2021, 03:16 AM
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Originally Posted by ALL OUT View Post
Possibly some caulking from the sending unit came loose and is floating around in the tank and every once in a while gets caught in the fuel pick up tube. When you shut it off it looses suction and starts floating around until its picked up again. Happened on a buddies boat so it can happen.
I've known two brand new builds have obstructions in the fuel line - one was my own boat which had a tiny piece of unidentified plastic in the fuel line - it would flow to the end of the line where it would block the connector, relax and let the fuel flow again - one was a friend's boat where the culprit was a little piece of work glove in the tank that would get up into the pick up tube, relax when the engine was stopped and sometimes get right back in there, sometimes get back hours later - they got it by pulling the pick up tube immediately the engine started to falter.
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Old 03-03-2021, 04:08 AM
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We had problems like that which were the little fuel filters on engine that collapsed and needed replacing.
Old 03-03-2021, 04:50 AM
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Thanks everyone! I have relayed the information to my friend and we will investigate thoroughly. I will report back with our findings. Thanks again!!
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Old 03-03-2021, 04:57 AM
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For those that are having engine mounted fuel filter problems, the part number for the plastic elbow to bypass that filter is 8M0152433. You will have to install a boat mounted fuel filter in its place and I’d also suggest running the water in fuel harness to a WIF sensor installed in a Racor bowl.

If you’re trying to run the engine mounted filter as well as a boat mounted filter, that’s likely why you’re having collapsing issues. Mercury wants one or the other, not both.

OP, have your buddy get his dealer to check on Mercury Service Bulletin 2021-02 regarding spark plug wire installation.
Old 03-03-2021, 05:09 AM
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I have 2020 300's and they're back in the shop for the third time for oil leaks. They can't seem how to fix the problem. I hope your friend gets a quick resolution. Wish I stuck with the my Old Verado's.
Old 03-03-2021, 05:38 AM
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A clogged filter on the suction side of the HP pump will cause starvation (engine slowing down to stop) at high RPMs, but the engine will usually idle. If the filter is clogged, the problem is not going to come and go.

With no error codes, my vote is loose battery cable, faulty assembly on an electrical connector. Start the engine, shake battery cables/harness plugs (including behind the dash) to see if you can get it to stumble.
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Old 03-03-2021, 06:54 AM
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David,
Question, as I have a 300 v8 coming on new boat soon. I have a fuel water separator installed in boat. Mercury does not want that extra filter? because their is one in motor? Am I missing something?
thanks,
Old 03-03-2021, 12:03 PM
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Interested in hearing about this too. I have the same situation, one filter in the bilge area of the boat along with the standard filter in the motor. There have been a couple of times where I've felt the motor sputter a little when going from idle to high throttle.
Old 03-03-2021, 01:22 PM
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Originally Posted by davidwademarine View Post
For those that are having engine mounted fuel filter problems, the part number for the plastic elbow to bypass that filter is 8M0152433. You will have to install a boat mounted fuel filter in its place and I’d also suggest running the water in fuel harness to a WIF sensor installed in a Racor bowl.

If you’re trying to run the engine mounted filter as well as a boat mounted filter, that’s likely why you’re having collapsing issues. Mercury wants one or the other, not both.

OP, have your buddy get his dealer to check on Mercury Service Bulletin 2021-02 regarding spark plug wire installation.
Thank you for so much advice on this forum. I apricate the fact you are very willing to help us all if possible. This two filter thing is very confusing to me. I have a 2020 BW 330 Outrage and it has both filters with the v8's. My question is why would Whaler do that? I mean its all Brunswick and I would not think there would be too much fighting going on. If Merc is saying "No" and I believe you. Am I confusing the fact I have a water separator and a fuel filer with just two fuel filters? Is that two different things in this case? I do not have an issue with my motors, just always wanting to learn.
Old 03-03-2021, 01:31 PM
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Can you switch coil packs with another cylinder and see if the cylinder problem changes?

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