350 Overheating

dono16

Petty Officer 3rd Class
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Nov 1, 2007
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Have a 1984 350/260 v8 merc, Alpha 1 , power steering, Twins, ..... A little history first, bought boat end of last season, starboard motor consistently ran just slightly hotter than the port, kept an eye on it, didn't think much of it nothing changed till this year, Just replaced impellers, bellows boots, water hoses on drives, ran motor on muffs water was pumping out everywhere except the prop just a tiny tiny stream. and motor immediately began to overheat. Unfortunately had to put boat in water so now somewhat limited to what I can fix. Last night changed thermostat, didn't work, I took the top of stat off and check it was open,, I felt the hoses to see any differences on the motors and the two smaller hoses that go from the top of the thermostat housing to the risers were extremely hot, compared to the cold ones on the good motor. I don't know how the cooling system really works on this likie what hoses are going where, what should be hot or cold or doing what....? My guess is to start taking things apart look for blockage, obstructions,,,, Water pump on motor??? can I test that? What's my next step???? Thanks in Advance..
 

dubs283

Vice Admiral
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Jul 27, 2005
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5,058
Re: 350 Overheating

and motor immediately began to overheat. Unfortunately had to put boat in water so now somewhat limited to what I can fix.

so you splashed a boat that has an overheat issue?? - i think i found your problem, see the signature under don's posts, it will make sense then

the only thing you can do with the boat in the water is check for blockages after the thru transom water supply hose to the t-stat housing - verify that water is making it to the t-stat housing

if no blockages are found and water isn't present you have to pull the drive and check the transom water hose and the drive water pump components and passages

my WAG right now is you forgot the key on the shaft for the impeller
 

tpenfield

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Jul 18, 2011
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17,625
Re: 350 Overheating

Did you do the impeller and hose work yourself, or was it by marina/mechanic ?

my suggestions would be to get the maintenance manual for the engine and review the cooling system flow diagram. You can also find illustrations online via googling.

I'm not sure if you have an oil/power steering cooler on the engine, but you could back flush the intake portion of the system from the input hose on the thermostat housing.
 

dono16

Petty Officer 3rd Class
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Nov 1, 2007
Messages
76
Re: 350 Overheating

Yea, well it was that or I had to wait another month to get in...... I still have 1 good motor to get me around on... so the motor was pumping water on land with the muffs.,,, I guess I could crack the bolts on the t stat housing see if water shoots out.... then ill know if its before or after the stat..?
 

dono16

Petty Officer 3rd Class
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Nov 1, 2007
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Re: 350 Overheating

[All the work was done by a mechanic.
 

500dollar744ti

Senior Chief Petty Officer
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Jul 23, 2012
Messages
691
Re: 350 Overheating

I just went through this, it ended up being clogged manifolds and risers. It was mainly the port side riser but I guessed the other side wouldn't be far behind. I'm not a boat expert but I'm an experienced auto mechanic and instinct told me it would be best to replace both risers, manifolds, couplers and exhaust shutters while in there. I got all the stuff to do it, cursed the rusted bolts all day while I drilled them out and removed everything. I also replaced my valve covers which were starting to rust through. After I was all done I felt much more confident in my boat which needs to be utterly reliable as I'm not good with an oar.

Provided you did the impeller right, the base is good along with the gaskets and water is getting to the engine, I vote clogged manifolds or risers.
 

grewvin1

Petty Officer 2nd Class
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Apr 2, 2012
Messages
189
Re: 350 Overheating

You might check earlier posts on doing the water volume check from outdrive putting a hose on the tube at the transom and measure the water pumping into a bucket.
 

NHGuy

Captain
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May 21, 2009
Messages
3,631
Re: 350 Overheating

What you do get a length of hose for the thermostat inlet that's an extra 3' in length. unclamp it from the inlet but hold it against the inlet til the motor can be started and stabilized at 1000 rpm. Then move the hose to a bucket for exactly 15 seconds @1000 rpm. When the 15 seconds are up direct the water overboard til the engine can be shut off. Carefully measure the water, if you have a 1.50 ratio drive you should get 4 quarts per 15 seconds.
 

dono16

Petty Officer 3rd Class
Joined
Nov 1, 2007
Messages
76
Re: 350 Overheating

ok, did the test like NHGuy said,, everything is working ok up to that point had just about 4 quarts, solid water no air in the hose.. So my plan now is to start looking for obstructions in hoses risers manifolds, any other ideas , thanks.
 
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