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  #1  
Old 05-18-2008
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Who's the Overheating expert?

1987 H28.5 / Yanmar 2GM20F
Motored a solid 11 hours at 2800 rpms couple months ago without incident and put it in its slip where it remained till yesterday. Motored out of the harbor about 10 minutes and then sailed about 2-3 hours. Coming back into the harbor, started the engine and within about 2-3 minutes the high temp alarm sounded. Today, I found the coolant completely full, the raw water strainer slightly dirty, but not near enough to cause problem. The impeller and its bearing was replaced last week. I also scraped quite a nice family of barnicles off the prop and shaft. Then I started the little girl and.....for the life of me.....since the discharge is below the waterline, I can't tell if water is being discharged......or blowing air and the resulting splash from the big "bubble" makes it look as if water is coming out. I held my fingers over and in the outlet and just can't tell. Don't really know how much water is suppose to come out, but it doesn't feel like any. Any help appreciated!
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Old 05-18-2008
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You say the impeller in the raw water pump was replaced a week ago, was this because of the overheating or just regular maitenance? As fast as it overheated, I would suspect the impeller. Can you hold the exhaust hose while it's running to determine temp? If it's HOT there probably isn't any water going through it. Also check the intake, it could be clogged with barnacles.

Good luck, John
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Old 05-18-2008
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Close your overboard through hull and remove the hose to a bucket while someone else turns the motor on. You should get good flow of water in spurts. If you DO have flow...then focus first on the thermostat.
You may have RESTRICTED flow that only shows up when you put a load on the engine...does it heat up at the dock in neutral or only under power? Look for a blockage (plastic bag, kelp etc. ) in the intake through hull.
Then...in order...check the output after the impeller by briefly removing that hose (new impeller have been known to fail) ...check the exhaust elbow for restricted flow and finally pull and check the heat exchanger.

These are the main "cooling system" related steps to take but obviously overheating can also be caused by lack of lubrication (check oil) or actual internal engine damage but those tend to be accompanied by other symptoms and you haven't mentioned anything other than temperature.
Good luck.
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Old 05-19-2008
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My methodology

#1 Check and clean engine intake thru-hull/seacock.

#2 Check and clean hose from intake seacock to strainer and any elbows that may be in this path. Large (very large) zip ties work well for this.

#3 Remove strainer basket and snake both inlet and outlet ports.

#4 Remove hose from strainer to water pump and check/clean

#5 Remove and check or replace impeller and also check the old one for broken vanes. If vanes are broken you NEED to find the pieces.

#6 Test run engine make sure you have good water flow. If you are still over heating move on.

#7 Using an infrared thermometer (you can't effectively trouble shoot a cooling system without knowing actual temps!) measure the temperature at the sending unit, on the thermostat housing, and compare that reading with your temp gauge. Also, measure the temperature differential between the inlet and outlet sides of the HX and write down the differential. If the gauge and infrared are close and high remove and replace the thermostat. If you must, and are of the overly cheap mentality, remove and test the stat in a pot of water using the infrared to confirm opening temp and re-install.


#8 Make sure "sealed" side of the system is not air bound and use the air purge valve to check for any trapped air.

#9 If still overheating check the temperature of exhaust elbow (on the engine side) and about 6-8 inches from the exhaust elbow on outlet side for a large temp differential. If you have a large temp differential you have a blockage at the exhaust elbow. Fix this before moving on and re-check engine.

#10 If exhaust elbow checks out open up the heat exchanger and look for "gunk" if junk, corrosion, pieces or crap are present remove the HX and bring it to a radiator shop for an acid bath cleaning.

#9 Re-install HX and check engine.

#10 If still overheating check for flow and then if it seems restricted purge and clean the "sealed" anti-freeze system side. It's possible the journals and pathways are loaded with rust and corrosion and you have a partial blockage.


P.S. When the engine is finally fixed and running well use your new infrared thermometer to record the following numbers in a notebook. This will save LOTS of time when you have a cooling system issue in the future..

HX Temp differential between inlet and outlet sides.
T-Stat housing temp
Exhaust elbow - both sides
Exhaust hose temp
Various points of the cooling system

It's best to mark every one of your infrared thermometers "laser" measuring spots with a Sharpie marker so you can repeat these numbers and readings accurately by always measuring the same exact location.
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Last edited by Maine Sail; 05-19-2008 at 08:46 AM.
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Old 05-19-2008
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Red face

"You say the impeller in the raw water pump was replaced a week ago, was this because of the overheating or just regular maitenance? As fast as it overheated, I would suspect the impeller."

Trouble shooting requires a dialog.
Untill this question is answered I will just be pissing in the wind.
Sure I can shotgun ALL the things that can cause over heating, but then you would not have the answer you are looking for.
Personally I chose rule #1.
Sounds like the pump is not working.
Maybe the belt broke?
Kinna like calling the doctor and saying,"I have a temprature what is wrong with me?".
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Old 05-19-2008
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Haleki gave a good thorough step by step including recording temps on a known good system at various locations so that next time you'll have a starting point.
I'll add one small thing. Record both air and water temps as they matter also.
Lower temp air will dissipate engine heat faster, lower water temp will affect temp readings on the system up until the exchanger. It's all a matter of degrees.
You probably have an air lock (Hal's #8) to get overheated that fast.
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Old 05-19-2008
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Got It!!!

Pulled the hose from the thru hull and NOTHING CAME OUT! Then after running a long screwdriver thru several times, a big clump of stinking broken barnicles popped out and then.......a fountain of beautiful water! Back in business thanks to all of you!
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Old 05-19-2008
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Quote:
Originally Posted by halekai36 View Post
#1 Check and clean engine intake thru-hull/seacock.
Yep that's why it's number 1 on my list from above.... Glad it was easy!!
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Why do they design boats with underwater exhausts?
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