Engine Overheated and now boat wont start, need help. Possible Causes?

Stagger

Member
Jul 30, 2020
60
Rochester NY
Boat Info
260 Sundancer 2003
Engines
5.7L Mercruiser 350 Mag MPI Bravo III
My Engine is a Mercruiser 350 Mag MPI. I have a raw seawater pump that is leaking, the new one just got delivered today. Previous to yesterday I had not had any overheating issues. Took the boat out yesterday, the overheat alarm went off i anchored and let it cool for several hours. I started it back up, and tried to bring it back home. It did fine at speed, temp was a little high but no alarm was going off and temps were staying consistent..... Until i got to the no wake channel between the lake and the bay. Once i lowered my speed, the overheat alarm went off immediately. I could not kill the engine, as I was in a thin channel with rock walls on either side and a ton of boat traffic. In retrospect I should have backed out of it, and waited for a tow, but i didnt. So I went through the channel, and limped the boat maybe a half mile to a safe spot in the bay to anchor in the dark and wait for the engine to cool. After several hours of waiting, i tried to start the boat, and it wouldnt turn over. It felt like it was the battery, it sounded like it was trying to turn over but just didnt have enough power. So I called out a friend with a jump box, and tried again. Same result, either nothing or it sounded like it was almost going to turn over and didnt. Still thinking (hoping!) it was the battery, I got towed in and hooked to shore power for the night. Tried again this morning, same result. I was exhausted today as i didnt get to my slip until 4am last night waiting for the tow, so i didnt do any troubleshooting. Tomorrow or tuesday I plan in pulling the plugs and taking a look at the cylinders. What are the possible causes for this? Possible repairs? Has anyone else experienced this? Im really hoping that its something that can be easily repaired and not catastrophic engine failure. This is my first boat, and I have only owned it for about 6 weeks so Im still learning. Thank You
 
Depends. If it is hydrolocked, trying to start it repeatedly could have bent some internals. That is catastrophic. Pull all plugs and see if it will spin over, and if water shoots out some of the plug holes. Let’s go from there.
 
Question-you say that you ran it until the alarm went off. Do you not have gauges or watch them? What temp did it get too? And I suggest you get a qualified mechanic to do this work or assist you.
 
Question-you say that you ran it until the alarm went off. Do you not have gauges or watch them? What temp did it get too? And I suggest you get a qualified mechanic to do this work or assist you.
I do have gauges, it was about 180°-190° at speed, and when I slowed down to hit the no wake channel, the temp jumped to 220° maxing out the gauge. But like I said in my post, it was unfortunately not a safe spot to stop the engine, so I had to limp through it
 
It does sound to me as if she is hydrolocked, especially after the comment about needing a new raw water pump. If you go down there today don't be fooled if the engine turns over.

Pull the plugs and get the water out. Get some Marvel Mystery Oil and inject it into each cylinder Then rotate the engine again. If you're good with motors great, you will save some boat bucks. If not get a mechanic and get the ball rolling. Let the mechanic know about the raw water pump and have him replace it. Also might be a good time to change out the risers and manifolds. Whatever you do, get the water out!

Also be prepared to be changing the oil at least once if not twice.

I'd be about 90% sure this all stems from the bad raw water pump. Once it failed the only thing moving water was the forward movement of the boat. When you slowed for the No Wake you were no longer cooling the engine. She overheated and the damage was done.

Let us know what you find and decide to do. Good Luck
 
It does sound to me as if she is hydrolocked, especially after the comment about needing a new raw water pump. If you go down there today don't be fooled if the engine turns over.

Pull the plugs and get the water out. Get some Marvel Mystery Oil and inject it into each cylinder Then rotate the engine again. If you're good with motors great, you will save some boat bucks. If not get a mechanic and get the ball rolling. Let the mechanic know about the raw water pump and have him replace it. Also might be a good time to change out the risers and manifolds. Whatever you do, get the water out!

Also be prepared to be changing the oil at least once if not twice.

I'd be about 90% sure this all stems from the bad raw water pump. Once it failed the only thing moving water was the forward movement of the boat. When you slowed for the No Wake you were no longer cooling the engine. She overheated and the damage was done.

Let us know what you find and decide to do. Good Luck
Thanks, as soon as I get out of work im going to pull the plugs and try to evacuate the cylinders, and pray upon all prayers that the engine is no catastrophically damaged
 
I do have gauges, it was about 180°-190° at speed, and when I slowed down to hit the no wake channel, the temp jumped to 220° maxing out the gauge. But like I said in my post, it was unfortunately not a safe spot to stop the engine, so I had to limp through it
It’s too late now, but for future reference, that motor should run within a few degrees of 160°

Your 180-190° while cruising is a big red flag and the motor should have been shut down immediately, before encountering the point of no return in the narrow channel.

Don’t be surprised if this turns out to need head gaskets and cylinder heads rebuilt or replaced from cracks or excessive heat.
 
It’s too late now, but for future reference, that motor should run within a few degrees of 160°

Your 180-190° while cruising is a big red flag and the motor should have been shut down immediately, before encountering the point of no return in the narrow channel.

Don’t be surprised if this turns out to need head gaskets and cylinder heads rebuilt or replaced from cracks or excessive heat.
Yea... I definitely learned my lesson, and if this happens again will take a different course of action. I signed up for boatus tow service yesterday, so if im ever in a predicament like this again, I can just get my boat towed back to the slip without worrying about the expense of it. Im hoping the head gaskets and cylinder heads are good, but i guess we will see. Thanks for the response
 
This is why I won't go without twins ever again.

One of mine overheated years ago due to a failed riser gasket. I was lucky but I do have great dock neighbors. I got away with replacing the risers and manifolds. I hope you get away from this that easy
 
Update: Pulled the plugs, evacuated all the water. Sprayed fogging oil in the cylinders. I ran out of daylight. Tomorrow I will compression test the cylinders, get some new plugs, put it back together and pray my best prayer that it starts, and if it does go on to whatever other repairs it need from there. If it wont start, I will probably bring it to my mechanic and see if it can be repaired and salvaged, or if I should start shopping a new short block. Thank you everyone who has helped me so far with all the great info. Also the new raw water pump i ordered came in yesterday, so I will be installing that tomorrow
 
Pull the all the plugs and see if it will turn over
I did check the oil with the dip stick, levels and color looked fine. No milkiness
the sooner you get her started and heated up to dry out the moisture the better. That’s why they say 48 hours, your trying to beat the rust starting internally in your motor.

good luck!
 
If you had water in your cylinders, It’s very likely you have water in the oil. (Bet on it). It isn’t obvious from the dipstick. It will be obvious from the first couple of drips from your remote drain. Was the water on both sides, or just one side of engine?
 
Like I said in a earlier post, be ready to change the oil once if not twice! And change that pump! That one part in the root of all the work you are doing now!
 

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