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

Yes- the key is turned, we hear the normal warning beep, then there's a 5 sec pause, then the steady beep remains on before and after the boat is started. So our question is, how would this alarm know the oil or water pressure before the engine is started? I also wanted to add that the water pressure is good. Do you think it could be the oil pressure sensor that may have gone bad? Perhaps the overheat caused by the faulty impeller somehow fried the oil pressure sensor?
 
aafattore said:
Yes- the key is turned, we hear the normal warning beep, then there's a 5 sec pause, then the steady beep remains on before and after the boat is started. So our question is, how would this alarm know the oil or water pressure before the engine is started? I also wanted to add that the water pressure is good. Do you think it could be the oil pressure sensor that may have gone bad? Perhaps the overheat caused by the faulty impeller somehow fried the oil pressure sensor?
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So your warning alarms delay is functioning. The Alarm is sounding after 5 seconds or so because there is no oil pressure yet. Once the engine starts and the oil pressure comes up and the oil pressure switch goes open and turns off the alarm. That is how it is supposed to work.

How do you know the water pressure is fine? Did someone attach a scan tool and check it?

The water pressure could be fine but the sensor may not giving the ECM (Engine Control Module/ the computer) the correct reading.
 
Docksidemarineservices said:
So your warning alarms delay is functioning. The Alarm is sounding after 5 seconds or so because there is no oil pressure yet. Once the engine starts and the oil pressure comes up and the oil pressure switch goes open and turns off the alarm. That is how it is supposed to work.

How do you know the water pressure is fine? Did someone attach a scan tool and check it?

The water pressure could be fine but the sensor may not giving the ECM (Engine Control Module/ the computer) the correct reading.
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Thanks Chris ... correction, we do not yet know the water pressure, I meant the oil pressure is fine based in the gauge. We are trying to locate a pressure gauge. In the meantime, what are your thoughts to the possibility of a bad oil pressure sensor? My thoughts are that this is obviously all related to the overheat from the bad impeller. Impeller is fixed, we back flushed the system-did not find any of the pieces so ... could this hot engine have maybe fried one of these sensors?? It's just too coincidental that something would have gone out not realted to the overheat. It almost seems like there should be a reset button somewhere. LOL
 
Is the engine still over heating or do you just have alarms sounding?

Until you have checked the water pressure, manually and through a diagnostic terminal, there is no more diagnosing to be done.
 
Hi Chris
Thought I'd give ya an update. Our auto mechanic friend stopped by today and, although he was having issues with his pressure gauge, he noticed (by feel)a gurgling (maybe air) in the hose that goes up to the exhaust manifold on the same side as where the pump is. In addition, for just a few seconds, he removed the hose from the manifold to try to see if water would flow freely. Water came through but not at a steady pace-it would flow, then sputter a bit' flow heavy' then sputter. His guess is that, since we only replaced the impeller alone, there's something not right within the pump housing causing air which would restrict water pressure. Hopefully within the next few days he's going to stop back and pull out the pump and inspect. Fingers are crossed!
 
If it is the brass pump you will have to sand the groves out of the back cover. You will also need to inspect the water lines from the drive to the pump and make sure there are no leaks. Leaks where air can be drawn in.
 
Docksidemarineservices said:
If it is the brass pump you will have to sand the groves out of the back cover. You will also need to inspect the water lines from the drive to the pump and make sure there are no leaks. Leaks where air can be drawn in.
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I will let him know to check the lines from the drive.

You think it would be ok to sand them out instead of buying new back cover plate? Thanks agian
 
Not to ask a question again BUT,

When you said you back flushed the system what exactly did you do? Did the impeller loose any pieces and did you find them all?

Typically when a impellor goes and pieces break off they do not go backwards in the water flow, the go forward towards the motor side of the output hose.

I am not 100% sure how your water flow is but I would suspect that the output water from the impeller housing goes under the motor to a power sterring cooler or possibly some other cooler device, Correct? Did you disconect the water line there as this is where impeller pieces will accumulate.

This past summer I was trouble shooting a similar issue (intermitten overheat) (dufferent boat, 21 ft inboard ski boat (carbed).) but it also has a engine mounted water impeller similar to yours. The issues was the impeller lost several pieces and some where at the next pinch point down stream from the impeller and they were all at the power steering cooler clogging the input. Fortunatly the thru hole in the cooler where smaller than the pieces so they were stuck there.

Not sure if this is relevant but if it is make sure you check.
 
Here is the pic of the pump. The back plate is REALLY grooved! Not sure how you would get in there to sand this. The pulley can be removed but I don't see any way to get into the actual housing. 026.jpg027.jpg
 
kghost said:
Not to ask a question again BUT,

When you said you back flushed the system what exactly did you do? Did the impeller loose any pieces and did you find them all?

Typically when a impellor goes and pieces break off they do not go backwards in the water flow, the go forward towards the motor side of the output hose.

I am not 100% sure how your water flow is but I would suspect that the output water from the impeller housing goes under the motor to a power sterring cooler or possibly some other cooler device, Correct? Did you disconect the water line there as this is where impeller pieces will accumulate.

This past summer I was trouble shooting a similar issue (intermitten overheat) (dufferent boat, 21 ft inboard ski boat (carbed).) but it also has a engine mounted water impeller similar to yours. The issues was the impeller lost several pieces and some where at the next pinch point down stream from the impeller and they were all at the power steering cooler clogging the input. Fortunatly the thru hole in the cooler where smaller than the pieces so they were stuck there.

Not sure if this is relevant but if it is make sure you check.
Click to expand...

I didn't watch him do the entire backflush but I know he disconnected many hoses and I did see him remove the hoses to the PS cooler and saw the honeycomb inside and there was zero debris visible-unless they were small enuf to fit thru those little holes. After the impeller imploded the boat got up to about 260 degrees so maybe the peices disinergrated or melted. I recently posted a few picks of the pump that we just removed (in the pouring rain- impatient- I had to look inside) As you can see, the back plate is severly grooved. I find it hard to believe this could cause enuf cavitation to trigger the alarm and affect water pressure but I'm no marine mechanic. Honestly, I just want it fixed so we can use the boat a few more times before its too cold. :eek:
 
Hi there Alicia, I figured maybe you'd just like to know that Hardin Marine now makes a stainless steel impeller housing that is universal for all the mercruiser engines at a lower cost then buying one from mercruiser. Also it's supposed to last longer than the brass style housing that you have now. You are at the point to where you definitely should have a new impeller housing. You should consider looking into buying this new style pump instead of the brass style housing that you have now. Typically most mercruiser service shops are now advising their customers that around every three years or so depending on use you should plan on buying a new impeller housing. So hopefully this new one from hardin marine will last longer plus it is cheaper to rebuild when needed.

Have a look at the two link below, Also google hardin marine stainless water pump for more information.
http://www.hardin-marine.com/c-132-mercury-sea-pumps.aspx

http://www.offshoreonly.com/forums/general-q/254650-496-raw-water-pump-problems.html

Also just as an FYI, If someone left a sensor unplugged, it will usually sound the warning horn before you start the engine. Is the water sensor mounted on the impeller housing? What is the engine serial number?

Good luck.
 
Thanks for the info. As far as the sensor- all are plugged in (that I'm aware of) and to my knowledge, I don't think there is a sensor on the pump itself. My ser # for the engine is OW317212.

Thanks
 
So what are your thoughts/opinions on which type of re build kit? I recieved some info on a stainless steel option- you like? Heard good/bad?
 
Do what feels best for your wallet,

Either way my opinion is to replace the entire pump housing with a new one and one reccomendation would be to buy TWO if you plan on keeping the boat for a while. Best to have one on hand when this happens again (which it will at some point). Always better to trplace with new than to rebiuld when affordable.

If you buy two, if it comes with a impeller already installed remove it, bag it and cable tie it to the housing for future use........


Stainless steel housings wont stop the damage that occurred to yours it will just slow it down. Stainlees steel used for that application is not much tougher than the brass/bronze used in the original pump. Dont get me wrong, it is a better choice but the same will eventually happen.
 
Ya I'm looking at $350 UGH! So none of it feels best for the wallet : ( It is what it is........ at least the problem is most likely solved.

Thanks again
Alicia
 
Welcome to "high performance" boating.....

I bet you spend more in gas in a day than what this parts costs.............lol

A 496 ci most likely uses what maybe 3-4 or should I say 6-10 gallons per hour............
 
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