Hydrolocked Mercruiser 350 MAG MPI

"I have a question about the p

"I have a question about the possibility of reparing a 2001 Mercuiser 350 MAG MPI engine with about 90 HRS. I let the engine overheat Friday to about 200 degrees on the temp guage. I guess my impeller in the water pickup finally gave out but that's a different problem.

When I figured out what was causing the alarm to go off I stopped the engine and sprayed the exhaust risers with water from the transom shower. The rubber fittings between the manifold and the downpipe had got hot enough to start smoking a bit and I was afraid of them burning.

When I tried to restart the engine it never turned over completely, one brief clunk noise and then nothing. I tried once more with the same result.

I called a boat shop and learned of hydrolock caused by pulling water up the exhaust manifold into the cylinders when you overheat the "flapper" in the exhaust. This was somewhat confirmed when I unscrewed the sparkplugs and had water in the front cylinder on both sides but the others appeared to be dry. I wasn't sure if I should be able to turn over the motor by hand or not with the plugs out. I couldn't. So I figured the engine was either scrap or not and there shouldn't be any more damage caused by using the starter. To my surprise it turned over smoothly without any unusual sounds of scraping or rattling. Just a little more water shot out of the sparkplug holes.

I've read in previous posts on cars that now I need to put some Marvel Oil in the cylinders and turn it over a few times before starting it.

Does this mean that I probably don't have any major damage and should just replace the "flapper" and change the oil? (After fixing the cooling problem that caused all this.)
I don't see any water in the oil and it's still golden brown, not burnt looking or smelling. The reason I mention the oil is that the only mechanic willing to work on it soon tried to convince me that the heads were cracked or warped and the main bearings would all need to be replaced. He told me that was the only way water gets into the cylinders. He basically wants me to bring the boat to him Monday to pull the engine, rebuild and send me a bill for $3500. He also told me not to turn the engine over and that he would check it for me. Needless to say I would like to try to fix this myself and wonder if this is something best left to the pros.

I plan to fix the water pump before I run the engine again."

"You need another mechanic.
F

"You need another mechanic.
First,pull the drive, drain the manifolds ,pull the plugs, drain any water from the cylinders, replace the plugs and get the motor fired up.
You can safely run for about 15-20 seconds.The point is to dry the pistons ,rings and bores before they rust up.
Compression test next with the carb wide open.2 adjacant cylinders with low readings are what your looking for.
If you just slowed down when you saw the hi temp and did not just turn the key off, the water should not have gone up the exaust. If the motor dieseld a bit (ran backwards from the heat) that could suck water into the cylinders.
You should check the manifolds and risers for cracks too"

"Thanks,

I agree with the n

"Thanks,

I agree with the new mechanic proposal.

I've drained the manifolds and pulled the plugs but wasn't sure if I should start it up again before fixing the water pump or not since no water flow was what created the problem. But it makes sense to dry the cylinders out before they rust.

Do you think the engine would have gotten hot enough to cause any internal damage without cooking the oil? The temp guage was showing about 200 degrees when I turned off the engine and continued to rise to about 215 before cooling back down.

As I was spraying the exhaust manifolds with water I saw the rubber "jump" like there was a sudden release of pressure. I assume this is when the water went into the engine. I'll check them to make sure I didn't crack one on the inside."

"200 is nothing, 260-280+

"200 is nothing, 260-280+ that's a problem. As for the water in the engine could be from a blown head gasket(not likely but possible) cracked Risers or manifolds etc..

After you get the water out and do as BT suggests, you will want to have someone reputable tear down the engine and have a look and see what's good and bad, if your' still having problems.

ps.. the rubber flappers aren't that good at keeping water out anyway.

pss.. a cyl head could be cracked too.. doubt warped, doubt you did any damage to the bearings either."