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Good day all.
I was hesitant to post this when it happened, as I didn't have a solution. Now that I have worked through this issue, I wanted to share. Mainly incase anyone else has similar issues in the future, whether that be for a Model 242, a Volvo-Penta, Mercury, or whatever.

Was at the lake all weekend last weekend. Boat ran great on Friday, and all day Saturday. We (our boat, and another member's boat on here) even had a great evening cruise with the lake super smooth. My daughter was driving on the evening cruise, and she did get the RPM's up a little high (above 5500 for a few seconds) at one point, but that was in the middle of the drive. Everything was all normal the rest of the cruise back into the dock and ramp. As we were waiting on my wife with the trailer, the Check Engine light started scrolling across the gauges. I looked over all the other gauges, and everything looked normal. Oil pressure, engine water temp, voltage, trim, everything looked as it should.
We loaded the boat up, and I checked everything over again. Outdrive was free of debris, etc.

The next day, we put the boat in the water, and I was wondering if the Check Engine light would be back on. It was NOT. However, I was getting a single beep, maybe 1 second long, every 2 minutes. No other warnings, and again, all gauges normal. I slowly trolled out of the boat ramp area, and once the engine RPM's went up to 1,000RPM, I immediately noticed a shake in the boat. Took it out of gear, and then put the boat in neutral to add throttle (so as to remove the props and outdrive from the equation as the source of the vibration). The boat was still shaking. I opened the "hood" and noticed the engine was running very rough. Again, no check engine light, but still the beep every 2 minutes (or so). After looking things over, trying to isolate to maybe a loose plug wire, or anything simple, and starting and stopping the engine multiple times, the beep went away. Still running rough though, and obvious from idle, all the way up to 1000 rpm. I didn't dare go higher.
After hours of thought, and bouncing ideas off my buddy (Thanks @ACNull) I figured it was either a misfire due to fouled plugs, or possibly water in the fuel. After getting home, and ordering some parts, I put in new AC Delco plugs, new fuel water separator filter, and a full tank of 89 non-ethanol gas, with treatment (seafoam, and stabil). I was crossing my fingers, and fired her up on the garden hose. The engine shimmied a bit at first (maybe 4-5 secods) then smoothed out. All was well now...

I guess I can chalk it up to bad gas. Or more correctly worded, water in the fuel. I don't know if this was from the gas station (new to me, with only 87 octane, with 10% ethanol) or if it was from not completely topping of my tank last winter when I winterized (not my normal routine). Either way, it seems to have worked itself out now, and all is well.

Exactly why I store my boat as empty as I can and fill with a full tank of fresh fuel in the spring.

Since you have catalytic converters, don't put anything with oil in it(Seafoam).

I also own a Rinda Diacom so I can read codes that pop up. Very useful in a case like this.

bruceb58 wrote: Fri Jun 06, 2025 12:13 pm Exactly why I store my boat as empty as I can and fill with a full tank of fresh fuel in the spring.

I try to do the opposite, and fill up completely. I didn't last winter, maybe 5-10 gallons shy of full. That process has never failed me before (15+ years). Not sure if it was my process, or luck of the draw. I have too many "new" variables this past year to place the blame on a single thing at this point.

bruceb58 wrote: Fri Jun 06, 2025 12:13 pm Since you have catalytic converters, don't put anything with oil in it(Seafoam).

Too late... already put the seafoam in it, and Stabil. Neither of those have ever hurt my vehicles engines that have catalytic converters though. I've never heard not to use those before. Have you had any bad experiences with Seafoam in engines with catalytic converters?

"with 10% ethanol"

There is a well-known problem of ethanol sometimes causing such vibrations and rough running. I don't understand the technical chemistry, but the upshot is: Ethanol can cause a layer of water at the bottom of the tank and can cause debris loosening, which then clogs up the fuel system.

My 2 cents: It was a fouled plug. How did they look when you replaced them and when was the last time they were replaced.

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NautiWeasel wrote: Fri Jun 06, 2025 2:03 pmToo late... already put the seafoam in it, and Stabil. Neither of those have ever hurt my vehicles engines that have catalytic converters though. I've never heard not to use those before. Have you had any bad experiences with Seafoam in engines with catalytic converters?

The fact that SeaFoam is 60% oil means I don't want to use it on any engine with catalytic converters or O2 sensors.

It's funny...you used to be able to look up the MSDS for SeaFoam an it was always shown on various auto and marine sites. SeaFoam got tired of people knocking the product based on its contents so they made it all hidden. Basically, it's around 60% oil, 30% Isopropyl alcohol and around 10% Naptha. Naptha is probably the only thing that does any good. You could go to Home Depot and buy Naptha at a 20th of the price. BTW, gasoline already has Naptha in it.

Use a fuel injector cleaner that has polyetheramine in it(PEA) many fuel injector cleaners have it. I use Techron.

Kate wrote: Sat Jun 07, 2025 7:48 am "with 10% ethanol"

There is a well-known problem of ethanol sometimes causing such vibrations and rough running. I don't understand the technical chemistry, but the upshot is: Ethanol can cause a layer of water at the bottom of the tank and can cause debris loosening, which then clogs up the fuel system.

That only happens when the amount of water reaches a level of saturation in the ethanol which causes phase separation. The amount of water needed to do that is HUGE! If you had 10 gallons of 10% ethanol fuel left in your tank, you would need over 6 ounces of water in your tank to cause phase separation.

If you are getting that much water in your tank, you have a bad gasket around your fill cap or a faulty venting system.

bruceb58 wrote: Mon Jun 09, 2025 12:22 am It's funny...you used to be able to look up the MSDS for SeaFoam an it was always shown on various auto and marine sites. SeaFoam got tired of people knocking the product based on its contents so they made it all hidden. Basically, it's around 60% oil, 30% Isopropyl alcohol and around 10% Naptha. Naptha is probably the only thing that does any good. You could go to Home Depot and buy Naptha at a 20th of the price. BTW, gasoline already has Naptha in it.

Use a fuel injector cleaner that has polyetheramine in it(PEA) many fuel injector cleaners have it. I use Techron.

Good to know...
I do use Techron in my vehicles as well, and actually have some on my supply shelf in my shop. I'll drop a couple bottles in the tank on the next fill up.

I did take pictures of my plugs, from several angles. I didn't see any single plug that was very different from the others. Maybe #4 and #5 were a tad bit darker than the others, on the ceramic insulator, but again, nothing that really stood out. I didn't know the last time the plugs were changed, so I did go ahead and put in the same plugs again. Genuine AC Delco 41-110 Iridium plugs. I don't believe these were original, as I think they boat would have come with Volvo plugs from the factory, but who knows. I've got 357.XX hours on the boat, and I've probably put 100 hours on in the past 3 seasons. I hadn't changed the plugs before, which was another reason for me doing so.

For some reason.. it's not letting me add pictures of my spark plugs.

See if this works...

Those don't look terrible to me (maybe just a bit rich), but others will chime in.

When you pulled the plugs, was it after it had been run hard at normal operating temp for awhile? If not and it was after the ngine was sitting at idle, very hard to get anything useful from a plug read.

When I do plug reads on a boat, I go out onto the lake, get up on plane and run for 20 minutes and do a quick shut down being careful not to get my wake to back flow into the engine as I am shutting down and do a plug read in the middle of the lake.

bruceb58 wrote: Wed Jun 11, 2025 12:04 am When you pulled the plugs, was it after it had been run hard at normal operating temp for awhile? If not and it was after the ngine was sitting at idle, very hard to get anything useful from a plug read.

When I do plug reads on a boat, I go out onto the lake, get up on plane and run for 20 minutes and do a quick shut down being careful not to get my wake to back flow into the engine as I am shutting down and do a plug read in the middle of the lake.

Nope. These were pulled right when I got home from the situation I stated in my first post. I idled on them for maybe 10 minutes... guessing? Before that though, yes, it was some WOT runs for a few minutes on a night cruise. Probably about as close as I could get to pulling them after a hard run, but not pulling them with the boat in the water.

Either way... I changed them, and they may or may not have been due. I'll keep these, with the #'s on them, for a backup set with me at the lake if needed. For $80, it wasn't bad money.