5.7 Mercruiser Hydrolocking

Hey guys, it's been a while since I was here but need some help. I have searched for a month and not had any luck with my problem. I have owned this 1999 5.7 Mercruiser for 7 years and had no issues other than a starter two years ago. Four weeks ago on a two day camping trip, my boat failed to start on the second day. I assumed it was the starter again, no big issue. I took the starter off to find that there was nothing wrong with the starter. I am not a boat mechanic but to have a pretty broad knowledge of engines and have built several. With no starter problems I tried turning over engine with no luck so I pulled all of the plugs and cranked. The #7 cylinder was full of water and #5 looked like it may have spit a little spray. I have never had a miss from the engine and its never been hot, oil pressure runs about 40. Anyway, I pulled the manifold and gasket off since that seemed to be the most common problem. Everything looked good and I filled the riser up with acetone as I had read and replaced the gaskets. I also did a compression test on all cylinders on the left bank. There were a couple lower than I would like to see but were around 100, definitely not the source of my water.So, I changed the riser gasket and put it all back together. I checked oil and no sign of water in the pan. So, I fired it up with muffs and let it run. I even let the muffs run without the engine on to see if it filled a cylinder. Nothing. So, back to the water we went. I have three kids that ski and wakeboard so it gets alot of start and stop on the water. We had perfect trips for three weeks, usually two days per weekend. Yesterday, after boating all day Saturday and half of the day yesterday, it hydrolocked again. This time, there was water on both banks of the engine. I blew all of the water out and it fired up and ran perfectly..... I am at a loss. How is water getting in both sides? It is running perfectly. I have avoided quick stops(which I have never worried about). It isn't even consistent with setting a long time without being started. Yesterday it had been shut down for maybe ten minutes and filled up at least four cylinders. I do know that about three years ago it ran a little warm one day so I decided to change the impeller. WHen I got it apart there was a small rubber piece that I didn't recognize on top of the impeller. I removed it and changed the impeller and have no issues for three years. After looking at several schematics over the last month I realize that it was a flapper from the exhaust. I have purchased the new ones and will be putting them in tonight but I don't believe that I ran for over three years and never had this happen and then suddenly it backed up through the exhaust twice in a month. I have searched and searched and have seen similar problems with other people but never the solution. It is fresh water only and the exhaust manifold and risers look brand new. We have rented cabins next week on Kentucky Lake and the vacation will be ruined without the boat so I am struggling to find the problem. Any help is appreciated.

Re: 5.7 Mercruiser Hydrolocking

Is it possible that you have added extra weight to the rear of the boat like batteries or extra equipment.
Or maybe you have more people or heavier people in the back of the boat and that is causeing the stearn to sit lower in the water?

Re: 5.7 Mercruiser Hydrolocking

My first thought is the missing flappers, you are probably cutting the throttle during your water-sports. without the flappers, the engine will suck water, especially if your loaded a bit more than normal.

the second thought is your manifolds. post pics of your riser/gasket area. if your in salt water, pretty sure you have bad manifolds

if not one of those two items, you have either cracked block, cracked heads, or another similar issue.

Re: 5.7 Mercruiser Hydrolocking

No extra weight has been added. It has never been in saltwater and I will post pics this evening but they look brand new. The left bank that I replaced the riser gasket on looked brand new inside and out. I am going to replace the flappers this evening but I find it hard to believe that I have run for at least three years without the flapper on at least one side and never had this happen and now it happens twice in one month and I have been VERY careful not to stop quickly in the last month. I have never worried about that until this so I can't believe that is what caused it this time. I am at a loss. I am obviously hoping that it is not a cracked block or head but really don't think that it is because it runs so good and then all of a sudden it is completely full of water. If it were a crack it would seem that it would be more likely if it sat longer and had time to fill up the cylinders. Yesterday we had swam for over an hour(while boat was floating) after running the boat and had no issues. When it locked up it had been shut off for maybe ten minutes. I appreciate the responses.

Re: 5.7 Mercruiser Hydrolocking

just because the block/head/manifold/riser is cracked, doesnt mean it wont run great.

you can do a compression test and a leak down test to determine if you have a crack or gasket leak and where it is coming from.

you could have been on the edge of reversion for the past 3 years with the missing flappers

Re: 5.7 Mercruiser Hydrolocking

Scott Danforth said:

just because the block/head/manifold/riser is cracked, doesnt mean it wont run great.

you can do a compression test and a leak down test to determine if you have a crack or gasket leak and where it is coming from.

you could have been on the edge of reversion for the past 3 years with the missing flappers

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I definitely agree. I am still finding it hard to believe that if it were a cracked head or block that I would be filling up cylinders on both sides of the block so quickly. I have the same argument for the risers. I have already tested one and put a new gasket on it and I am getting water in both sides. The only thing that really makes sense is the flappers which I am putting in tonight. My only issue is that I obviously went so long without them with no issues and now twice in less than a month. I have read where several people have said you don't even need them. I would really like to know that I have it fixed before I run it again. I have been lucky both times that it was completely locked up and didn't bend a rod. Any other ideas is greatly appreciated.

Re: 5.7 Mercruiser Hydrolocking

Just a quick thought. If I remove all plugs and hook muffs up and turn the water on and leave it should I not eventually get water running out if it is risers, manifold, head gasket or cracked block? Maybe even go out every 30min or so and bump the starter to open different valves? I am stumped and would rather not trash my engine. New flappers are now installed.

Re: 5.7 Mercruiser Hydrolocking

Get another set of riser gaskets, the ones with the graphite on both sides. Check that there is absolutely no leftover gasket material on the manifolds or risers. Remove any extra carefully and completely. I'd use a block with emery cloth or take em to be surfaced by a machinist. Make sure the slots are in the right place and gently cross-torque them to only 23 to 32 ft/lbs.

Re: 5.7 Mercruiser Hydrolocking

Is your cam a stock design? How long since timing was checked?

Re: 5.7 Mercruiser Hydrolocking

Here are the pics of the risers on the boat. As you can see, the whole engine looks near new. I will post pics of the risers separated also. I will check timing this evening but it starts and runs perfect and never diesels or spark knocks. New riser gaskets and exhaust gaskets and flappers will all be completed tonight.

Re: 5.7 Mercruiser Hydrolocking

pull the risers off the manifolds, and post a picture of the joints. Risers and manifolds go bad from the inside out. the outside will never show signs of problems until it is well past too late.

if you used the green sierra gaskets - replace them with OEM Mercruiser. the green gaskets are known on iBoats as "Green Leakers"

when they leak, they fill your engine up with water.

Re: 5.7 Mercruiser Hydrolocking

I am using the OEM gray gaskets. I will post pics of the side not finished also. I have already done the acetone test and reinstalled one side. Again, not to be argumentative because I am just looking for some reasonable explanation I am missing, but wouldn't it have to be both risers going bad almost simultaneously to be getting water in both sides of the block? As I said, I appreciate all ideas and I am following all ideas.

Re: 5.7 Mercruiser Hydrolocking

for both sides to go bad, could be both mainifolds/risers are bad, or it could be water intrusion thru the exhaust from the missing flappers.

the acetone test only checks for a crack, it doesnt check for a poor gasket mating surface caused by rust

Re: 5.7 Mercruiser Hydrolocking

JKlowry said:

... If I remove all plugs and hook muffs up and turn the water on and leave it should I not eventually get water running out...

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No. If the boat is sitting properly and you have enough volume and water pressure, it'll just run out the exhaust at the back of the boat. Just like it's designed to do.

Re: 5.7 Mercruiser Hydrolocking

pressure test the cooling system, I bought the harbor freight radiator tester for like $100. found out mine was a head gasket blown, just enough to fill will water after shutting down.

Re: 5.7 Mercruiser Hydrolocking

How would you pressure test if with that tester? Mine is fresh water cooled so it isn't a closed system??? Is there a way?

Re: 5.7 Mercruiser Hydrolocking

TilliamWe said:

No. If the boat is sitting properly and you have enough volume and water pressure, it'll just run out the exhaust at the back of the boat. Just like it's designed to do.

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Thanks TilliamWe. I thought the constant volume and pressure may also push it through the leak if there was one in addition to out the prop.

Re: 5.7 Mercruiser Hydrolocking

JKlowry said:

How would you pressure test if with that tester? Mine is fresh water cooled so it isn't a closed system??? Is there a way?

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mine is fresh cooled also.....if I remember correctly..... I drained the block....got 2 elbows from the hardware store and connected the riser and manifold hoses together on each side.....then disconnected the power steering cooler hose and connected it to the pump with one of the flat adapters......had to hold pretty square in the hose to get the clamp tight enough.

Re: 5.7 Mercruiser Hydrolocking

Here are the pics of my riser apart. I have acetone in it now. I have another problem. The flappers(shutter kit) that they got me are way to big. They measure a little over 6" long. My y pipe is only 4"x3" and they can't seem to find the correct part number. I though I had seen a thread here about that with the correct part number. Any help? THanks

Re: 5.7 Mercruiser Hydrolocking

Your exhaust shutter(s) part number should be 807166A1. I went down the shutter replacement road a little while ago. I tried the "A3"s" but they just didn't fit right.