Marine Engine Hydrolock

Tim Raymond

New Member
Sep 4, 2019
5
Boat Info
1999 Sea Ray 310 Sundancer
Engines
5.7L Mercs
This thread is intended for marine engine hydrolock discussion. I experienced a hydrolock recently on my 1999 Sea Ray Sundancer 310 with twin screw 5.7L MerCruiser (GM 5.7L EFI 350 CID). I don’t recall exactly what happened during the failure process, but I think it was a high rpm engine shut of that caused the hydrolock. To back track, I had the starboard side water pump impeller fail with an easy repair and it ran great. Then a week later I had the port engine getting hot at cruising speed. I think I shut of the port engine at about 3k rpm. I knew the cause for the hot engine was the impeller since I repacked the starboard side a week prior. After replacing the port side impeller I had an obvious hydrolock situation. Loud engine clunk when trying to start the engine. I didn’t try to force an engine start and I never ran hot for any period of time. After changing the spark plugs and clearing out the hydrolocked water, which was only on top of the cylinders, nothing in the oil, it fired up with no issue. Changed the oil and now the engine sounds great. I have been up to 3.2k rpms and it sounds great. My question is, has anyone experienced a similar hydrolock situation and come out of it without major issues? I have heard the heavier 454’s and 502’s can get away without major issue, but the lighter 350 can not get out of a hydrolock without major issues. I say my 350 is running like a top after a hydrolock.
 
I had a hydro lock on a 350 at idle. The mechanic found a corroded ignition wire that he said seems to have caused the engine to stall for split second and then restart in reverse, sucking in water. Seemed consistent with what I felt at the time. Anyway they pumped out the water, changed the oil and I drove it for 3 more years. Only real damage was the starter was broken from me attempting a restart on the locked engine. I am on the Great Lakes though, so its fresh water. If you are in salt, there is probably more risk of internal engine rust if you don’t do a more robust flushing process.
 
I believe the running in reverse (dieseling) is a carb thing. Not an uncommon problem with a jet pump behind the engine and shutting down hot above idle.
Assume you were at or near cruising speed when you shut down at 3000 rpm? Maybe the lack of exhaust pressure allowed a surge of water to enter when the hull was slowing. A swim platform could easily exaggerate it with water trapped under it?
 

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