2000 mercruiser 5.7 efi, cooling system problem

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poppaj4

Recruit
Joined
Aug 14, 2012
Messages
1
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.
?????????
 
NHGuy

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Re: 2000 mercruiser 5.7 efi, cooling system problem

So did anything else change recently? You could start by testing the drive unit cooling water output. With the boat in the water unhook the input water hose at the thermostat, direct the water into a container. Start the motor for 30 seconds and tell us how much water you get.
How does your motor run? Well, poorly?
 
P

PigInAPoke

Cadet
Joined
Apr 27, 2012
Messages
24
Re: 2000 mercruiser 5.7 efi, cooling system problem

poppaj4 said:
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.
?????????
Click to expand...

Your thermostat doesn't care about flow. The only thing it cares about is the temperature of the water that is touching it.

My guess is that your power steering cooler is clogged up with some old chunks of disintegrated impeller. One of the smaller hoses from the top of the thermostat housing will be heading towards the transom where the cooler is. The cooler probably looks like a guiness can with the water hose on either side and some metal power steering fluid lines connecting to the side. Disconnect the hose from the thermostat housing and where the hose from the other side of the cooler connects back to the engine. Hook up a hose to the non thermostat side and let water flow backward through the cooler. You should see some small bits of black impeller coming out.

Dave
 
S

SC_Regal

Petty Officer 3rd Class
Joined
May 16, 2007
Messages
78
Re: 2000 mercruiser 5.7 efi, cooling system problem

Do you run in salt water?

To me sounds like your exhaust manifolds/elbows are getting corroded. 5.0/5.7 from that era are not the best design. If you boat in salt you need to figure on replacing every 4-5 years. It will initially only have issues at high RPM or heavy load conditions or prolonged operation. Similar issues will also occur if your flappers break off and get lodged in the exhaust. I have a 2000 5.0 and have already had both issues.

Mike
 
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