[Kitto]

My father has a 7.4L in his Shabah 210 that we got out of storage, hooked up the muffs and it ran fine. Put it in the water and after running for about 3minutes the high temp alarm sounded, we shut it down right away and pulled the impeller housing. Sure enough the impeller had detonated. We cleaned up the pieces replaced the impeller and hooked it back up. Went to start it and the motor would not turn over, it acted like it was seized up. After several tries we couldn't get any power, almost like we blew a fuse. Got it trailered and now it sits in the garage. Somehow power retuned to the boat later as we went back to the garage and some lights were on and I could turn the key and the alarm would sound but still not able to turn over.

Question we kept wondering about is, if we had the impeller vanes turned the wrong way, would this keep the motor from turning over? Oil level was fine, everything else seemed perfectly fine.

Any thoughts???

[Hudson River Steve]

Remove a spark plug or two to see if you have hydraulic lock. Sometimes when the motor overheats the heads warp and allow cooling water into the cylinders. If that's the case you need to open her up and get the water out ASAP.

[Kitto]

After doing some research I'm afraid that's what happened. I'll keep you posted on what we find out. Thanks.

[Hudson River Steve]

Sorry to hear it. As I said, open up all plus and get the water out ASAP. Spray every spark plug hole with liberal amounts of WD-40 and spin the motor over many times. Next step is to remove heads and assess damage. You might be able to get away with a thicker marine gasket, check Fel Pro.

[99 Mariah 258]

My buddy had a speedboat (Baja I think) with twin Mpi's. Cooked a impeller , the alarm sounded and we shut it down. Then it wouldn't turn over at all. It was hydrolocked. Pulled all the plugs and I think there was water in all cylinders. A lot of water. I thought the motor was trash. We got all the water out , replaced the plugs and changed the oil and it fired up and it still runs to this day(about 175hrs) ..... The big block is tough.

[Motoxxx43]

3min of running then high temp alarm goes off? If you shut it down soon after the alarm came on I highly doubt you have heat damage. Just going off of what you said- no power, like a fuse blown, etc.... First thing I'd be looking at is either the battery or the connection to the battery. Is the terminal corroded? Is it tight?

I have personally seen these motors ran without even a water supply a hell of a lot longer than just an alarm sounding without any heat issues (surprisingly!) I towed in a steaming AND sinking crownline one time. The shop who serviced it didn't attach the hose tight enough on the sea pump-to-motor side. It popped off filling the running vessel and causing it to get crazy hot. Hooked the line up and zero problems after.


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[knightriderz]

Doubt you have any head damage- you most likely fried the flapper in the y-pipe which is why the motor hydro locked .
Follow the earlier mentioned advice regarding pulling the plugs and running lube in the cylinders. (Also fix your impeller)
Then get the y pipe flapper fixed. (Do all this out of the water or motor will hydro lock again if flapper is not fixed)
Then change your oil ASAP (you may want to do this right away as you will have some water intrusion into your crankcase)
May want to change oil right away a second time after running a few hours after repairs above are made.
Oil will look milky after just a bit of running if so you need to change it a second time.
Good lick!

[Kitto]

Update: water was in the cylinders so they drained the oil and went through it. Looks like that was the extent of things. Loss of electrical had to do with a wire coming loose on the Perko switch. Allegedly, nothing wrong with the flappers or anything else. Like I said we shut it down pretty fast so I don't think it got too hot. Thanks for the help.

[Kitto]

Update II: Ok, so things are not as good as they seem. The shop ran the boat for an extended period of time on the muffs, checked the oil and all seemed ok. In preparation for going to the lake on Saturday my dad decides to double check the oil and sure enough it was milky and the oil level was up, so no doubt water in the oil.

So, the question is, would warped heads be allowing water in the cylinder or could the block be cracked?? Could the block crack from cold water entering after we shut it down? The mechanic seems to think there may or may not be flappers in the exhaust and thinks one would have to pull the motor to get to them?? Does anyone know if there is any truth to that? The boat is a 1998 with the 7.4L Merc.

Thanks in advance for all your help.

[Hudson River Steve]

Youe do not have to remove the engine although it is tight back there. You can get to the shutters or flappers by pulling the exhaust pipe elbows and hoses between the exhaust manifolds and the y pipe. The shutters are located in the top of the y pipe.