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I have a 1988 Mastercraft with 351 that will rise in temperature…

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I have a 1988 Mastercraft...
I have a 1988 Mastercraft with 351 that will rise in temperature to 190-200 in about 10 minutes of operation. If I slow down and idle, it cools back to 160 and stay there. I have replaced raw water pump, and exhaust/risers and thermostat. It just seems like not enough water is going through. Circulating pump bad? Engine cooling clogged? Head gasket?

Any thoughts would be much appreciated!

Thanks.
Submitted: 12 years ago.Category: Boat
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Answered in 10 minutes by:
5/26/2010
Marine Mechanic: James, Marine Mechanic replied 12 years ago
James
James, Marine Mechanic
Category: Boat
Satisfied Customers: 4,111
Experience: 12 years Certified marine mechanic, performance boat builder
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When the raw water pump was installed, where there any missing pieces? A missing blade caught in the oil cooler can cause restriction of the cooling system.

A worn impeller housing will also cause the effects you describe.

Have you grounded the boat, possibly sucking debris into the water intake? Sand or other sediment in the engine block will adversely effect the engines cooling. Pull the block drains on either side of the engine and check to see if water freely pours out. If it doesn't, use a stiff wire to dig the debris out and get water flowing form the drains, then flush the remaining debris out with water.

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Customer reply replied 12 years ago
I did open up the drain and stuck about a six inch piece of coat hanger in each an moved in all around. I would say quite a bit of rust did come out. But the engine still heats up. Do I need a longer piece of wire? FYI, this is a boat on fresh water in Wisconsin with about 900 hours of usage on it.

Forrest

Marine Mechanic: James, Marine Mechanic replied 12 years ago

If you have water coming out the drains, there isn't enough clog to cause the motor to over heat.

When you replaced the impeller, was there any pieces of the old impeller missing? Your symptoms sound a lot like a blockage in the oil cooler. Did you pull the water inlet hose off of that and look for debris? That needs to be done as well. The same with checking the condition of the impeller. It could be defective, or have been damaged during installation. If you ran the motor without water even briefly, (30 seconds) the impeller is damaged.

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Customer reply replied 12 years ago
The impeller was in good shape, I have pulled hoses, and i don't notice any debris. Actually, last week I installed a new raw water pump with new impeller. This did not have an affect on the cooling issue.
Marine Mechanic: Tan, MerCruiser Certified Technician replied 12 years ago
Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11,507
Experience: 40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Hi, What type of manifolds do you have log style the riser on the rear of manifold or center the riser in the center of manifold? When you had the risers off did you check the manifold for blockage? How old are the manifold? Thanks Tan
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Customer reply replied 12 years ago
I have the center type. both manifolds and risers are new as of last summer when this cooling issue started.

Forrest
Marine Mechanic: Tan, MerCruiser Certified Technician replied 12 years ago

What type of raw water inlet do you have and is there a strainer on it? Check all of your hoses on the suction side of the raw water pump. If there is any place that the pump can suck air instead of water. If your pump can suck air this will cause the water to boil. When you are running feel the pump when the engine starts to get hot if pump is hot you are sucking air. Thanks Tan

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Customer reply replied 12 years ago
The inlet is straight through the hull (no strainer), there is a transmission oil cooler in line. I have checked the inlet hoses and they appeared unclogged, I will check again for any air leakage, but I doubt there is.
Marine Mechanic: Tan, MerCruiser Certified Technician replied 12 years ago
Did your engine have different manifolds and risers? Did this problem start when you changed manifold and risers? Thanks Tan
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Customer reply replied 12 years ago
No, the problem started before I replaced the manifolds and risers. I had purchased the boat and it did have a cracked exhaust manifold that the previous owner had "JB Welded." So, I thought this may be one possibility with the cooling issue after I check the normal suspects. The gasket were blown between the old risers and manifolds.
Marine Mechanic: Tan, MerCruiser Certified Technician replied 12 years ago
What gaskets do you have between the riser and manifold? Should be all open ports.Did you clean all the ports in the thermostat housing? Tan
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Customer reply replied 12 years ago
Yes, the gaskets are all open ports. Ports in the thermostat housing seem fine, a little rust buildup, but nothing serious.
Marine Mechanic: Tan, MerCruiser Certified Technician replied 12 years ago
Is there a scoop on the water inlet on the bottom of the boat? What kind of water flow do you have out the exhaust? Do you have a 140* thermostat?
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Customer reply replied 12 years ago
yes, there is a scoop on the bottom of the boat. I do have a 140 thermostat, which was replaced also. The waterflow, I believe, is not as much as it once was. I seem to remember a "flow" of water under power, and a "spitting" at idle. Now I observe more of a "Spray" of water under power and at idle, no "spitting" is observed at idle.
Marine Mechanic: Tan, MerCruiser Certified Technician replied 12 years ago

Check your spark plugs see what color they are possibility head gasket leaking exhaust gases into cooling system. From overheating possibly or air introduced inti the cooling system. If 1 of your plugs is cleaner than the others head gasket.

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Customer reply replied 12 years ago
Should I do a compression test also?

Marine Mechanic: Tan, MerCruiser Certified Technician replied 12 years ago
Yes absolutely.
Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11,507
Experience: 40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
Verified
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Customer reply replied 12 years ago
With through the whole system starting at the hull this past weekend. Turned out there was pine needles in the transmission cooler. Which was not there when I starting addressing this issue last summer! So, probably a combination of things that I was dealing with in addressing this issue. It ran all weekend at 170 degrees, with a 160 degree thermostat.

Thanks for your help with this.

Forrest
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