# mercruiser 5.7 efi cooling problem



## poppaj4 (Aug 15, 2012)

hi
the boat started going from normal 3/4 on the temp to redline and alarm going off. turn off the motor, turn it back on, its fine, so the egine isnt overheated. run it fast idle, 3-4000, no problem, but it in gear, go 10-15 mph, no problem, kick it up , does it again.
had the lower unit taken off, serviced, new impellor and housing, they say they checked the plastic tube that takes the water up, not sure the name of that, the shop said it wasnt cracked, that was a thought, that the exhaust may be getting into the cooling system and decreasing the flow enough that it doesnt get to the thermastat enough and the sensor goes off?????
took it out for a spin, did it again,
replaced the thermastat, it still does it.
the 4 hoses going to the thermasat housing, the one on the front left feels hot, the others didnt, not sure, which ones should be hot or not. That is all i know, any advice, anyone experience this ever
called merc, they had no idea, called 4 shops, they said start with what i did and go from there. at $80 hr, that will be expensive.
?????????


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## outbrdwrench (Apr 8, 2009)

sounds like the manifolds &risers might be clogged. what year is the motor & how old are the manifolds & risers?


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## wld1985 (Oct 2, 2007)

I thought those had regurlar water pumps like cars,and not in the foot? Maybe Im wrong,been awhile since i've owned an inboard motor..The water pump on the motor could be bad..


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## TheCaptKen (Mar 14, 2011)

If both the gauge is showing red and the alarm is sounding, you are getting hot!!
There can be a number of things causing this. Did this just start or did you have a temp problem that led to the replacement of the water pump? How old is this unit?


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## SHO-NUFF (May 30, 2011)

outbrdwrench said:


> sounds like the manifolds &risers might be clogged. what year is the motor & how old are the manifolds & risers?


As said.

Manifolds and Risers on Raw Water cooled I/O's last about 3 years if you are lucky. The cast iron corrodes and restricts water flow causing engine to overheat. 
Pull a riser off. 4 bolts hold it to the manifold and a clamp on the exhaust hose and it should come free. Inspect the water passages on the manifold side and the exhaust hose side. I think you will find the answer to the overheating problem.


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## poppaj4 (Aug 15, 2012)

Thanks
i had read that somewhere else and spoke to the mechanics, and they say we can take that apart, but the motor is not staying hot, i can run for hours at 15 mph, 2500 rpms, the term opens and closes at the 3/4 reading on the guage,
you kick up the speed, it red lines almost immediately, if i wasnt getting water flow to cool the engine, wouldnt it stay hot, and not have the guage go back to normal as soon as i shut down and turn right back on?


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## Patriot (Jun 4, 2009)

Sounds like you have an either a bad water temp sensor or a loose connection. Engine vib can cause the wire connection to send stray voltage or ground and cause funky reading on guage. Could also be the guage is going bad.

To rule out engine overtemp. take one of those hand held temp readers and shoot it at the engine in several places at different RPMS. Take readings on block, heads and manifolds. Write them down and compare readings at different rpms.

If there is a huge up spike in temps like you described, then your indicating system is correct. If not, then your indicating system has issues like I said above.


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## outbrdwrench (Apr 8, 2009)

poppaj4 said:


> Thanks
> i had read that somewhere else and spoke to the mechanics, and they say we can take that apart, but the motor is not staying hot, i can run for hours at 15 mph, 2500 rpms, the term opens and closes at the 3/4 reading on the guage,
> you kick up the speed, it red lines almost immediately, if i wasnt getting water flow to cool the engine, wouldnt it stay hot, and not have the guage go back to normal as soon as i shut down and turn right back on?


at lower rpms the nearly clogged water passages in your risers & manifolds, can pass enough water. at higer rpms they cannot. remember it is just as important that the motor can get rid of its cooling water . if the water stays in the motor too long. it is just as bad as it not getting enough. just pull a riser and see for yourself.


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