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Repairs to Engines and Boats BOAT means "Break Out Another Thousand" to many of us. Discuss your engine and hull repairs here.

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Old 03-24-2013, 07:35 AM   #1
Super Squid
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Overheating

My buddy just bought a project boat. Everything is in working condition, except one problem with the engine. At idle, everything is fine. Once you increase power and get on plane, the engine overheats. It's an older Mercruiser inboard/outboard.

The impeller is in working condition as far as we know.

Our obvious conclusion is that there's a blockage somewhere within the cooling system. Our first steps are going to be to start taking hoses off and checking the flow, but I have a few questions first.

How do we know how much flow is enough. When we disconnect a hose and there's plenty of water coming out, how are we sure it's at the level it needs to be at?

He has a friend that briefly looked at it while I was out of town. His friend seems to think the problem lies in the manifolds. If we were to disconnect the lines going into the exhaust manifolds and check the flow, can we mess up the manifold?

Any other tips or tricks to try and narrow down this problem?

Thanks so much.
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Old 03-24-2013, 10:11 AM   #2
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Re: Overheating

It could be the pick up where it gets water to cool. I'm assuming it is raw water cooled. If it still has the pick up that goes through the outdrive and it is all dirty and has a blockage through that, that cold be where the problem comes from. I would be interested to know what it is coming from, also. Good luck.
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Old 03-24-2013, 02:57 PM   #3
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Re: Overheating

Thermostat
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Old 03-24-2013, 04:41 PM   #4
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Re: Overheating

Quote:
Originally Posted by Super Squid View Post
...The impeller is in working condition as far as we know.....
Then you dont know. Look there first, if you are not 100% sure of its age it needs replacing anyway. Thermostat is not a bad second check but if it does not overheat at lower speeds I really doubt its the Thermostat. But again, if you dont know the age of it, replace it now. Would not be surprised when you go to check it, there’s none there.

You have a flow issue, if its a V4, 6 or 8 also feel the risers and see if one side feels hotter than the other. If you dont know when the last time it was the risers were R&R'ed then they need to come off.

And not to be a kill joy but it could be a head gasket....
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Old 03-24-2013, 07:46 PM   #5
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Re: Overheating

After looking for obvious blockages, check the thermostat, it costs nearly nothing (not sure if a gasket needs replacing). You can put it in a pot and heat it until its opening temp, which will be less than boiling. If that works, look at the impeller.
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Old 03-25-2013, 04:20 PM   #6
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Re: Overheating

Your problem is the risers. Common problem with Mercruisers.
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Old 03-28-2013, 06:08 PM   #7
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Re: Overheating

Corroded or cracked risers can allow water to reach undesirable places. Take em off and take a look.
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Old 03-29-2013, 08:47 AM   #8
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Re: Overheating

Check the simple and likley stuff first.
Is the water intake or filter basket resticted by marine growth?
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Old 04-03-2013, 08:45 PM   #9
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Re: Overheating

The ring of the riser elbow, where the water comes out and goes down the big exhaust hose, has tiny holes where the used, hot water sprays into the exhaust to cool it, and muffle the sound some. That gets corroded. The holes get clogged with rust. Poke them out, and clean well. You can soak that in muratic acid, if you know what you are doing, and rinse well and put it all back together, but you can only do it once. Then it is time for a new riser elbow when it starts getting bad again. If the impeller and thermostat are good and nothing is bad, and when running it gets hot like you said and water hoses to elbows and other hoses too, are hard when running, (feel them when warm, but idling or off to feel how they should be, if flexible- find a softish hose) it is exhaust elbow ring clog. If soft when running hard, it is impeller, or some other problem. The impeller should be replaced yearly. It will get a set, the blades will be bent, and not push as much water. You can see your water temp slowly getting higher when this happens. New impeller is cheap. Best to really look at all parts of the cooling system, from the intake grate or lower unit, Thermostat, hoses, and any heat exchanger-of course these need maintanance, exhaust manifolds can get clogged if really old, or if ran in sand, and the parts I stated earlier. Or if inboard engine check belt to raw water pump, and circulating pump and sea strainer and scoop as Capt Gene said.

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Old 04-10-2013, 12:08 AM   #10
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Re: Overheating

While you're in there pulling parts and hoses. Pull the hose on downstream end of your oil cooler. Back flush through the cooler and out the upstream side into a bucket or the bilge and see what comes out. Could be bits of an old impellar clogging the pores. Do this annually anyways for PM.
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Old 04-16-2013, 07:09 AM   #11
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Re: Overheating

Yes, on boats with a heat exchanger you will often find all kinds of trash on the inlet end of the heat exchanger. It is good to pull the tube bundle out and dip in diluted muratic acid. Dip it until it stops fizzing. But like was said check the inlet side and hose coming from the raw water pump, parts of old impellers, zincs, seaweed, all will clog flow, as will the lime that slowly clogs it. It is possible to run diluted acid through the heat exchanger and then through the elbows and out, this is done by some companies, but if you dont know what you are doing, dont do it. When you pull the heat exchanger apart, it is wise to pull the block plugs and really flush out the heads and blocks. When it is all put back together it is wise to just put in water and cooling system cleaner and run till warm, then flush real good, rinse it all out again, then put in new 50 % glycol coolant.
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Old 04-16-2013, 12:29 PM   #12
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Re: Overheating

Start with the obvious parts of the cooling system prone to failure (i.e. impellers, thermostats, and raw water strainers). Then start to pull your hair out and take the cooling system apart piece by piece. I have had problems in the heat exchangers, a boat not having an anti siphon valve, plastic bag caught in a reducer into the engine, and the worst was a piece of plastic caught in the oil/hydraulic coler...
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Old 04-18-2013, 12:01 PM   #13
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Re: Overheating

I had an identical problem with SeaRay SXL. It had a 350ci w/ mercruiser Gen 1 outdrive.The first problem was hose routing. Make sure that you have the hoses correct on the t-stat housing, going to the risers. You can always use a racing t-stat in there also. Then we found that the inlet hose was getting kinked, the one going thru the transome. That was real bugger. THEN it was still heating up, we found the heads were cracked. Somewhere along the line, it was overheated enough to take out the heads. End of story............ we sold the boat!! (I have an outboard now on my Luguna)
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Old 06-20-2013, 01:35 PM   #14
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Re: Overheating

Just came across this thread. Did you ever solve your issue?
I'm still fighting the same battle two new waterpumps and a new engine. On plane, the flow pretty much stops. Mercruiser 888 Ford 302.
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Old 06-25-2013, 01:18 PM   #15
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Re: Overheating

Gator, did u replace the exhaust parts with that new engine? Also, is your water pump outlet drivers side or passenger? I have 351Ws, same small-block as the 302. I had a bitch of a time routing the hose from the outlet of the pump up to the heat exchanger. Of course if youre raw water cooled, this is a non-issue. Also if raw water cooled, make sure there is no restrictor plate between the riser and the manifold... only a gasket... otherwise your not cooling the risers.
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