New Impeller and still overheat

[Loefflerhans]

Found this board, and it looks great!<br />Hopefully somebody has a clue what is going on with my 2000 Mercury 50HP 4-stroke Bigfoot, ser# OT008291.<br />A week ago the buzzer (overheat) came on at 2/3 throttle. Let it cool off and was able to get home. Water was coming out the tell tale as normal , investigation showed the thermostat in poor condition, actually stuck in the open position. Got a new one and motor worked perfect for about 3 hrs, now the buzzer comes on about 5 min after startup in idle and gets hot! <br />Still strong waterflow out the tell tale (getting hot quickly, so the temp. sensor is correct). Anyhow changed the impeller and gaskets, no change: motor gets hot very quickly, strong waterflow out the back. Removing the thermostat does not make any change.<br /><br />Any suggestion what to look for next?<br />Hans

 

[wilde1j]

Re: New Impeller and still overheat

Have you checked the poppet valve? You probably need a shop manual as well.

 

[2springers26]

Re: New Impeller and still overheat

test your new t stat.. to see if it working properly..... You need a t stat Or it will Over heat... Need to sow the water down...

 

[2springers26]

Re: New Impeller and still overheat

to early in the am I'll try again.... test your t stat see if it is opening at the correct temp... you Need a t stat To slow the water down or you will over heat...

 

[Loefflerhans]

Re: New Impeller and still overheat

There is no poppet valve on a 50HP 4-stroke Bigfoot.<br />yes, thermostat is opening correctly, tried 2 new ones and the one in it opens at about 55 C (and is rated 60 degrees C)<br />Got the service manual but I am stuck now and out of ideas - anybody with suggestions?<br />Hans

 

[quantumleap]

Re: New Impeller and still overheat

Was the impeller in one piece when you took it out? You could have a clogged water passage and still get a strong tell tale stream.

 

[Loefflerhans]

Re: New Impeller and still overheat

Originally posted by Quantumleap:<br /> Was the impeller in one piece when you took it out? You could have a clogged water passage and still get a strong tell tale stream.

 

[Loefflerhans]

Re: New Impeller and still overheat

Yes, the Impeller was not only in one piece - it looked great. No obvious signs of wear or deterioration.<br />In addition when I connect a waterhose to flush and let it run, it does not overheat. <br />Took the foot off again and flushed the motor from below where the pump connects: very strong stream out the telltale. then I removed the t-stat (put cover back on): water rushes right thru, so I do not believe something is clogged inside.<br />Hans

 

[eurolarva]

Re: New Impeller and still overheat

When you replaced the impeller how did the water pump look. Do you use this motor in salt water? If so your water pump may be scored and not pushing enough water. Seem kind of strange though with water coming out the telltale.<br />Here is the stuff you may want to check<br /><br />-clogged intake<br />-removal of previous lost impeller blades<br /><br />If it still overheats check<br />-water tube grommets<br />-head gaskets<br />-exhaust cover gaskets<br />-power head base gasket<br />-any pressure relief valves in the head<br />-ignition timing<br />-carburetion

 

[peterpagnott]

Re: New Impeller and still overheat

I'm having a similar problem, My dealer told me to remove the lower unit, take out the T stat and back flush. there may be some blockage?

 

[Loefflerhans]

Re: New Impeller and still overheat

eurolarva: Yes motor is used in saltwater, but coming home I go upstream a freshwater canal for about 3 miles. Anyhow, the pump -especially inside- loks like brandnew, shiny no scores or whatever. this was the fiirst time of foot removel, first impeller change and as mentioned before the old impeller looks pretty good - no piece missing. I was hoping the old impeller just got weak where the blades bend, causing low pressure. But no bad wear signs at all!<br />The other things: Water tube grommet (I guess you mean that rubber piece connecting to the pump when installing the foot) looks good too.<br />-head gaskets<br />-exhaust cover gaskets<br />-power head base gasket: how can you check those without taking the whole motor apart? are there any visual indications of bad gaskets?<br />Motor is running perfect, even in idle very smooth up to the point where its overheating - thereafter i dont know.<br />Can I make the assumption that gaskets, timing and so on are o.K. because the motor is not getting hot / overheating with the waterhose connected?<br /><br />all mynn too:<br />That is exactly what I did - check my previous post. With the foot removed and flushing, water rushes right thru. Doing it the opposite way from where the pump normally connects, I have a very strong stream coming out the pee vent with a lot of pressure, not too much down at the foot when the thermostat is installed. After taking the t-stat out (cover back on) lots of water down at the foot and kind of a week stream out the telltale. Of course kind of hard to hold the waterhose against the tubing down there.

 

[Loefflerhans]

Re: New Impeller and still overheat

Any further Inputs?<br />I am stuck at my lift and running out of ideas. <br />Hans

 

[eurolarva]

Re: New Impeller and still overheat

Sorry I am at a loss. Hope someone here has a suggestion for you. Good Luck

 

[2springers26]

Re: New Impeller and still overheat

a few things.... did you install the small clear washers on each side of the impeller??? I will cause pumping issues... did you apply silicone on the dam for sealing the water pump area from the exhaust area.. will let exhaust iN the pump area if not... those are only things i can come up with if its ok w/o a case on it...

 

[FranksMerc]

Re: New Impeller and still overheat

Have you definately ruled out the overheat switch and or module?

 

[Loefflerhans]

Re: New Impeller and still overheat

No, there are no clear washers - haven't seen any when dismanteling and the manual doesn't show them either. Same for "silicone on the dam for sealing the water pump area from the exhaust area", but that is an interesting point: my manual nor any other expert never mentioned anything about it, however there was some residual silicone stuff when taking it apart. you have any further details what kind of silicone?<br /><br />One thing I noticed now: when running the engine without the thermostat and leaving the cover of the t-stat off, there is hardly any water coming out off there - compared to when a flush with the waterhose - lots of water coming out off there.<br />I know that the cylinder block is becoming hot really quick (cold engine it takes about 3 minutes in idle) , then the thermostat opens - cylinderhead immediately becomes very warm to hot while the buzzer comes on.<br />Have a constant pee stream the whole time, could have been stronger in former times.<br />Hans

 

[2springers26]

Re: New Impeller and still overheat

any silocone bubber( rtv) will work.... I've had this happen.. Most people don't reaseal this area.... how hot did the motor get??? To HOT.. as in could have warped the mating surface of the power head / exhaust plate area.. & water is leaking by.. had this happen to a customer last year.. alarm went off.. he kept going!.. It would get Hot @ idle.. But be ok when ran over 1500 rpms... To see how hot it got.. look @ the plastic parts( holder for temp sensor) mounted to the block.. are they melted??? that will be you clue

 

[Loefflerhans]

Re: New Impeller and still overheat

 

[Loefflerhans]

Re: New Impeller and still overheat

I never kept going, as soon as the alarm comes on I stopped the motor. Well, did the silicone thing - I am getting really good removing the foot now- and still no chanche. Sparkplugs look good and there is no water in the oil.<br /><br />Good news are there are no plastic part warped or melted.<br /><br />Question: with the thermostat off and cover off, should there be a lot of water coning out or is just a wek flow o.k.? With that port open and connecting the waterose to the telltale there is lots of water coming out.<br />Hans

 

[Loefflerhans]

Re: New Impeller and still overheat

Problem solved: as a last ditch resort I started taking motor apart.<br />Powerhead and adapter plate are off now, wating for new gaskets. Water tube from pump to the adapterplate is corroded where it goes into the powerhead restricting waterflow big time, actually sqeezing the rubber grommet. No corrosion anywhere else, I believe because of aluminium (adapterplate) and the tube from the pump interacted with saltwater leading to corrosion, In my opinion, that looks like a design faw - anybody had that problem too?