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Old 18-06-2009, 11:46   #16
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Just because the impeller is new. Does not mean it is NOT the problem. I recently had an over heating problem, and it was barnacles that passed through the pump, and lodged on the other side of it in the hose. Checking the flow of water out the exhaust at high rpm will help in where to look.....i2f
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Old 18-06-2009, 11:51   #17
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Originally Posted by senormechanico View Post
Check the prop for barnacles. Too much fouling will overload the engine.
Actually any fouling will impact your fuel consumption and speed, but it can also cause overheating when it reaches a certain point.

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Thanks very much -- bottom and prop were just well scrubbed and clean as a whistle. I'll keep that in mind in future though.
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Old 18-06-2009, 11:51   #18
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Just because the impeller is new. Does not mean it is NOT the problem. I recently had an over heating problem, and it was barnacles that passed through the pump, and lodged on the other side of it in the hose. Checking the flow of water out the exhaust at high rpm will help in where to look.....i2f
Thanks -- I'll do that. Cheers.
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Old 18-06-2009, 11:56   #19
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good point on the prop/barnacles. IF your exhaust elbow is clogged up with rust or carbon, the exhaust is restricted. This effects the engine more when it is trying to push more exhaust out. It is amazing how exhaust size does effect diesel performance. When manufacturers build production boats and select an engine supplier, the engine supplier will come out to test the first engine installation and will monitor exhaust temperature, pressures etc. If they are not right, the engine manufacturer will require a different exhaust size prior to warranteeing any engines..... This is more common with power boats, but my Kubota wholesale supplier even requested it when I built my first 12V 6 HP generator years ago!
I'm thinking an exhaust temp gauge would be a good thing to install in any case. Maybe that's a little anal, but we use them on two-stroke very high specific output snowmobiles, where coolant temperature doesn't tell you anything about what how the engine is doing (because it varies so much according to how much snow is being thrown on the heat exchangers, and because -- like a diesel I guess -- heat can spike up in the blink of an eye if something is wrong with mixture or loads, too fast to fully appreciate from coolant temp in any case). So I'm used to watching exhaust temps, and that could be a second overheating alarm too.
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Old 18-06-2009, 12:04   #20
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Thermostat

Coolant to water ratio (1:1 - more coolant than that IS BAD - water conducts heat MUCH better than ethylene glycol),

PULL your impeller again, there have been reports of bad, brand new impellers - the hub breaks away from the fins

Exhaust elbow

Packing gland too tight (prop spins by hand?)

As noted, check your water flow, all the way to the exhaust
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Old 18-06-2009, 12:05   #21
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Check the valves and rockers also it could be that the rockerspring on intake or exhaust valves may be defective if it is a newer engine,becuase if the valve isn't opening enough it will work at lower RPM's but it will run slightly hot nor will it be efficient, if the valve isn't opening enough or is getting stuck then the air won't be mixed right and will cause all the exhaust heat and fumes to stay in the cylinder and not allowing the air and fuel to mix right and there to be alot of extra fuel that will burn on the power stroke when the air intake valve opens instead of the compression stroke when the fresh fuel is injected thus causing the engine to overheat and robbing you of your fuel economy and power. By the way the book by Calder is a very informative publication and I think a copy of the book should be given to everyone who buys a boat from a dealer.
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Old 18-06-2009, 13:49   #22
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I just posted a similar question with an overheating problem. Went through the system end to end. the culprit? Bad thermostat. It is fine until the engine gets up to load temp and won't open all the way.

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Old 18-06-2009, 15:19   #23
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Does it matter whether the engine panels are on? Make sure that the hoses are not getting "pushed" in when the covers are on due to being too long etc. This would allow water though the sytem and out the the back, but not enough once the engine rpms increase. Would still appear to have flow during this. Had a friend who spend a lot on money troubleshooting a similar problem when all that was needed was to shorten the hoses.
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Old 18-06-2009, 17:59   #24
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I had a over heat problem, checked everything, some twice! It ended up being the impeller, which looked perfect but had actually spun on the brass center section. Did not find it until I started the engine with the impeller cover off. And it did not spin as fast as it should have
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Old 18-06-2009, 19:03   #25
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I had a similar problem this year , after taking everything apart and finding nothing wrong it turned out to be a slipping worn belt. Lost a lot of knuckle meat for nothing.
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Old 18-06-2009, 19:19   #26
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At 3000 hours I'd say a your mixing elbow is clogged if it has not been replaced. This reduces you raw water flow. At increased RPM the constriction prevents the volume of raw water from flowing over the heat exchanger hence lowering colling capacity.

Remove your mixing elbow (or straight pipe) and check for carbon buildup. Temp fix is to chisel it out as much as you can, but best to replace it. The old 83 3GMF in my previous boat (H34) suffered from similar symptoms as you described.
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Old 18-06-2009, 19:27   #27
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you can also check the elbow by removing the exhaust hose from it and looking in with a flashlight to see if it is clear or clogged up. hose may be stuck so you will need to work at it.
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Old 18-06-2009, 20:13   #28
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What type of engine is it?
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Old 18-06-2009, 21:09   #29
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If you have ever had to replace an impeller in the past...did you get all the pieces out? I have seen pieces which lodge in the hoses or the first fitting after the pump.

How old is the cooling elbow? Yanmars for example are good for about three years or less in salt water.

Impeller hubs can fail....I have seen that in a lot of different engines.

Are you trying to push the vessel past theoretical hull speed?

There are as many different causes as there are opinions on this one.
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Old 19-06-2009, 05:50   #30
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What type of engine is it?
Perkins 4-90.
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