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Merc 496 "overheat warning" sounds before boat is even started

aafattore

Member
Engine overheated after running for only 20 minutes. Changed impeller (which was needed and missing a few arms) also topped off gear lube. Started boat, water pulled into engine fine (water pumps seem to be working), engine did not overheat although ran a little rough at higher rpm's. Ran engine for 15-20 minutes-alarm continued to sound. We unhooked the wires attached to the gear lube bottle (im told this is where the sensor is) with the thought that it could be a bad sensor....... alarm sounded before engine is even started. We plan to backflush system to purge possible impeller pieces, however this would not explain why the alarm doesn't turn off because the boat is not currently overheating. Any suggestions?
 
Depending on the year (which you did not indicate) it may also have low cooling watr pressure sensor. If impeller pieces are jammed in the hoses, or PS cooler, you could easily have a low water pressure, even with a new impeller and no overheat. You need to backflush and get all the impeller chunks out. Good luck because this can be a time consuming job.
Was the impeller ever replaced prevously?
Opinions vary, but I use every 250 hours or 5 years max.
Some will tell you absolutely every year, which is overkill.
Since its a 496 I assume its a Bravo leg and an engine driven pump?

Rod

Rod
 
Re: 2005 Merc 496 "overheat warning" sounds before boat is even started

Hi Rod-

It's a 2005- sorry about that. This is the first time changing the impeller (150 hours) and yes on the engine driven pump. Not looking forward to the backflush- heard it can be a challenge.
Thanks for your response.

Alicia
 
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Chances are the water tube is collapsing in the bell housing and restricting water flow. All the flushing in the world will not correct this problem. You need to get a scan tool and check the water pressure. If one is not available you can tee into the out put on the water pump some where in the system and hook up a mechanical pressure gauge. Or you can remove the water pressure sensor from the PS cooler and attach a mechanical gauge there.
 
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It went into guardian mode when it overheated on the water for sure. So it sounds like a huge job to determine the issue. I'm bound and determined to fix this thing ourselves yet this sounds complicated! Better get this into the shop huh?
 
If it is the "common" colasped thru transom fitting them the outdrive must come off, the motor must come out and the transom removed from the boat.

If this is not desirable you may be able to install a thru hull pick up as used on a inboard ski boat and many other boat configurations and simply cap the incomming water hose from the transom. This would eliminate the issue once and for all time.

First you must determine if you do have a water flow/clog issue. Impeller pieces may have reached the thermostat......
 
thanks- we're going to backflush since it's likely there are pieces of that impeller (god only knows where) thruout then go from there.
 
You need to diagnose the problem first. Your engine went into guardian mode with the over heat condition not the low water pressure condition. Like I said earlier and I may be wrong but I don't think low water pressure on a 2005 will set you into guardian mode.

If you install the pressure gauge like I said, you will know where the the problem is. It is good to have this knowledge before you have it repaired. If it is in the bell housing by the drive you will need to find a qualified person to do this job for you.

Also, Putting a thru hull water feed in is not repairing the problem. Water needs to pass through this drive to cool it.
 
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If it is the "common" colasped thru transom fitting them the outdrive must come off, the motor must come out and the transom removed from the boat.

If this is not desirable you may be able to install a thru hull pick up as used on a inboard ski boat and many other boat configurations and simply cap the incomming water hose from the transom. This would eliminate the issue once and for all time.

First you must determine if you do have a water flow/clog issue. Impeller pieces may have reached the thermostat......

I have never removed an engine or transom assembly to replace the water feed line. Just the drive and the drive bellows.
 
I agree he needs to find the problem first........it was suggested it may be the thru transom area as a possiblity.

I have done a complete bravo transom rebiuld due to this exact issue.

I do not see how to propperly get the "squish" clamps and the thru hole cleaned out completely any other way. You may be able to get it functional and I mean good to go but with the corrosion biuld up I would rather have it apart. They are not hose clamped on a pass through tune they are "squish clamped".

GOOD POINT on the cooling BUT does the water flow thru the outdrive like a Alpha? or just thru the lower and up and out the water hose to the transom? The alpha generates pressure the bravo does not. If you were to remove the outer water hose from the drive to transom when installing a thru hull pick up this would allow water to flow thru the drive wouldnt it?

I will have to look at a picture breakdown later.........

They did use the pick up on the old TRS drives.

Yes I have a 1987 21 ft Cobalt bow rider, all original.

It is my R&R/drinking barge with a 170 watt per channel two speaker sound system.....Funny though the more you drink the louder it seams to get............lol
 
Ha "SHE" needs to find the problem you mean! My husband has washed his hands completely of the boat! With winter just around the corner here in Chicago, he's got her in storage mode already. He wants to take it to the shop and be done with it however, I WANT TO FIX THIS BOAT! I'm not ready to shut er down just yet-nor do I have $$$ to just dump into it when it may be a simple fix. This boat has been very gently used and I just can't believe it's something major. I will take the advice and go from here. Wish there was a mobile marine mechanic in the area who would be willing to assist me in the fix as I'm up for the challenge.
 
I agree he needs to find the problem first........it was suggested it may be the thru transom area as a possiblity.

I have done a complete bravo transom rebiuld due to this exact issue.

I do not see how to propperly get the "squish" clamps and the thru hole cleaned out completely any other way. You may be able to get it functional and I mean good to go but with the corrosion biuld up I would rather have it apart. They are not hose clamped on a pass through tune they are "squish clamped".

GOOD POINT on the cooling BUT does the water flow thru the outdrive like a Alpha? or just thru the lower and up and out the water hose to the transom? The alpha generates pressure the bravo does not. If you were to remove the outer water hose from the drive to transom when installing a thru hull pick up this would allow water to flow thru the drive wouldnt it?

I will have to look at a picture breakdown later.........

They did use the pick up on the old TRS drives.

Yes I have a 1987 21 ft Cobalt bow rider, all original.

It is my R&R/drinking barge with a 170 watt per channel two speaker sound system.....Funny though the more you drink the louder it seams to get............lol


If you cut the water line in the bell housing it will allow water to flow thru the drive and cool it. If you just plug the line off inside the boat you will burn up the drive. Guaranteed.

If you remove the hose from the transom assembly and use a pipe fitting brush on a power drill you can clean all of the corrosion out. Cover the new line with "acetone" (This will make it slightly sticky) and put "power tune" on the new cone clamp and screw it in tight. Make sure the hose does not spin while tightening and you are good to go.
 
Yes I see your method,

When I did the last bravo I also was replacing the gimbal ring as it was shot so I had to do the complete R&R and the water inlet issue was a secondary fix now that I remeber, it was a couple of years ago.........
 
aafatore,

Here are some pics of my "big fat buick" as it has been called. No speed demon but is very comfortable.
 
Yes it is. He is 12 and he basically stands there and looks for fish ALL day!! Of course he never gets any...........
 
I have a 26'aft cabin wellcraft and three great danes. Causes quite a comotion at dockside. I can feel your pain, I have a mercruiser 350 that the manifolds reisers have been heating up to the point of melting the rubber tubes, flappers etc, but not the engine. So I am watching this post with interest.
 
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Funny- we also have a 2 year old Great Dane. We haven't ventured out with them on the boat yet but it's in the plans for the future. That is, if we can get this boat running properly again. As far as the loss of water pressure, this is probably a dumb thought but if the impeller was put in backwards, were thinking this may be the cause of the water pressure loss. This is a common mistake-so I've read. Guess I should have looked at the OBVIOUS! UGH
 
I'm hearing conflicting stories....... does it matter which way the impeller is replaced. I've heard that if installed in reverse, the impeller will correct itself. Is this correct? :confused:
 
Ha "SHE" needs to find the problem you mean! My husband has washed his hands completely of the boat! With winter just around the corner here in Chicago, he's got her in storage mode already. He wants to take it to the shop and be done with it however, I WANT TO FIX THIS BOAT! I'm not ready to shut er down just yet-nor do I have $$$ to just dump into it when it may be a simple fix. This boat has been very gently used and I just can't believe it's something major. I will take the advice and go from here. Wish there was a mobile marine mechanic in the area who would be willing to assist me in the fix as I'm up for the challenge.

I see you caught the foopah. Was just going to comment on that!!!!!!!!!!! I began to wonder if I needed to make an eye appointment....

Just went through the same thing on my 7.4 Bravo 1. Can be pretty frustrating until that final tada moment and then it's jubulation, then frustration again, because it's usually something simple that I missed the first go around. Besides, when you get it done yourself, there is a huge sense of accomplishment and "usually" you saved some cash in the process. Good luck in finding it, since there is a whole raft of little things to check and it can get frustrating.
 
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You need to diagnose the problem first. Your engine went into guardian mode with the over heat condition not the low water pressure condition. Like I said earlier and I may be wrong but I don't think low water pressure on a 2005 will set you into guardian mode.

If you install the pressure gauge like I said, you will know where the the problem is. It is good to have this knowledge before you have it repaired. If it is in the bell housing by the drive you will need to find a qualified person to do this job for you.


Also, Putting a thru hull water feed in is not repairing the problem. Water needs to pass through this drive to cool it.

Chris- I spoke to the hubby and he still doesn't understand why the alarm would go off before the engine is even started... how does the system know at this point that there any issue? Is that sensor working before the boat is started. Sorry if I'm asking dumb questions. I'm a newby : )
 
Are you saying that the alarms sounds before you start the engine? The alarm may sound when you turn on the key switch because the delay in the alarm horn is no longer working. In a perfect boat, you should be able to turn on the key switch with no alarm sounding for 5-10 seconds. The alarm horn has a delay. The alarm will also sound right away when you turn on the key if the drive oil res is low or if the float is stuck on the bottom.

Excluding the drive lube reservoir. Sounds like you have an over lapping alarm signal. If the delay is no longer working on the alarm as soon as you turn on the key you will get a tone until you start the engine and the oil pressure comes up. If the water pressure is below approx 1lb at that point, the alarm will continue to sound.
 
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