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I went out this past weekend and tried to get out of the marina in low tide. It was nearly impossible so I really pushed the throttle to squeeze out of there. At one point the engine was "peeing" black mud water and actually shut off. When I finally got out of the marina, I tried to open the throttle up on deeper water and was moving along okay, but then the engine shut off and wouldn't restart. I had spare spark plugs onboard so I changed them and it fired up instantly, but soon enough I experienced the same problem and the engine wouldn't restart. I took the spark plugs out and wiped them clean and the engine started up again, at which point I turned around and headed back into the marina at a slow pace.

Any idea what happened here? I pulled the boat out of the water a few days later and flushed it out and the engine seemed to run okay, but I'm thinking maybe I should change the impeller and lower unit oil.

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In your situation, I would change the impeller and see if you can see any signs of mud clogging things up. If it's easy to get at the thermostat, too, that might be another place to check.

I don't see any reason to change the lower unit oil, but it wouldn't hurt if you want to be sure no mud got in there.

Tim

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I've seen the exhaust cavity in the gearcase or propeller plug solid from mud or weeds before. If the engine can't get rid of exhaust it can't breathe and won't run. This is about the worst thing you can do to an outboard short of running it without any water going to it until it locks up which is possibly what you did. My advice is to drop the gearcase, adapt a garden hose to fit the copper water tube and turn the hose on to flush it out. If your engine is a 1985 Merc it is an inline 6 and does not have a thermostat. You may actually need to disconnect the pee hose from the top of the block and back flush it out through the water tube and exhaust passages. Change the water pump, it may be melted from lack of water. This engine may be so full of mud that the water cover where the spark plugs are and the exhaust plate may need to be removed to flush the mud out of all the passages. That job is not for the novice because you will break half the bolts trying to get it apart.

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Changing the spark plugs gave it time to cool down
and trick itself everything was ok!
TP

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Agree plugs have nothing to do with the symptoms. Run motor on muffs. If you want to check water pump turn hose on very slow and have someone crank the motor while you watch the muffs. If the water pump and impeller are ok you will see the muffs suck in. If they do not you have an issue with one or the other.

I will now open myself for what comes. I ran a 70 hp Yamaha 2-stroke through mud like you mentioned almost every time it was used for about 25 years. I never changed the pump or impeller and for the last 7 years the lower end was welded on. The replacement lower end had whatever pump/impeller it came with. If the motor pees and sucks at intake it is not that problem.
I still have to run in this situation on a regular basis and have not done impeller/pump stuff. Motor is 5 years old now. I do flush every time as possible.

Previous motors were not nearly as tough for this environment. Will not mention names.

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Thanks Fitz this sounds like a good diagnosis. When I pulled the boat out of the water and ran it on muffs with hose water, it spewed out a lot of dark water from the exhaust and eventually ran well on high throttle. I might change the impeller either way since the engine pee was inconsistent depending on the throttle.

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Running engine with muffs at much more than idle is not recommended. Happy to have the other experts correct me on this issue.

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That is very good to know, thank you.

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That's correct. At best, nothing unfortunate happens, but it does not tell you anything about performance on the water with back pressure and load. At worst, the motor self-ignites, runs away, and destroys itself and / or injures someone.

Tim

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Tim - so even though it seemed as though the engine was running better after flushing it with hose water, I could potentially have the same problems if I put it back in the water?

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The motor will possibly run "fine" on muffs, but not so under load. The prop must be engaged and resistance provided for this to be accomplished.
If you can back the trailer to a place where the prop and water pick up are submerged it is easy to put the load on the motor required to test.
All good repair facilities have a place like this as it is as close to running the boat underway as possible.

It is normal for pee stream to increase as rpm do.

Do not run the motor for a long time at above idle. I run my motor for a minute or so at 2000rpm to make sure the thermostat opens.