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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 08-02-2010
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What is a bypass-type circuit (coolant)?

I have been diagnosing an overheating problem on my Universal M4-30, and, in the process have been studying Peter Compton's Troubleshooting Marine Diesels. In his troubleshooting chart he mentions a bypass-type circuit for the FWC, but never explains it. I would like to understand how it differs from a non-bypass circuit, how I can recognize it if it exists on my engine, and whether this dictates a different type of thermostat.

I have googled without success, and even searched the other book in my not-so-extensive library on diesel engines.

Can anyone enlighten me?
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Old 08-03-2010
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I believe the by-pass circuit it mentions could be a line for internal boat heaters using the hot water from the motor to heat registers and returning to the motor FWC system.

Very common on powerboats up here in the PNW. For sailboats it's usually capped off or a hose loop installed. It most commonly comes out of the fresh water pump and goes back in the upper manifold, don't know about Universal.

Four main causes for over heating with a FWC are the thermostat, plugged tubes in the heat exchanger, an air pocket in the FWC or a water pump not suppling enough flow.
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Old 08-03-2010
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A bypass cooling loop allows constant pressure and constant flow on the water pumps; necessary so that you dont overload and break the vanes on such a pump. Universal Engines typically have bypass cooling systems.

The bypass loop is controlled by the thermostat and such a thermostat is normally OPEN, the bypass allowing the cooling water NOT to flow INTO the engine until the engine is near to completely warm and at operating temperature. Only as the engine nears its operating temperature does the thermostat CLOSE and thereby shunts water from the bypass loop INTO the engine.
The result is that the water pump is always pumping water at the same pressure and flow rate ... and doesnt get 'overloaded' as it does with systems that partly BLOCK the entire flow until the engine gets to full operating temperature.
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Old 08-03-2010
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Quote:
Originally Posted by RichH View Post
The bypass loop is controlled by the thermostat and such a thermostat is normally OPEN, the bypass allowing the cooling water NOT to flow INTO the engine until the engine is near to completely warm and at operating temperature. Only as the engine nears its operating temperature does the thermostat CLOSE and thereby shunts water from the bypass loop INTO the engine.
Does this mean that the thermostat is a different type from what would be used in a non-bypass configuration?

I guess the bypass loop in my engine is what is used to heat the water in the water heater.


Thanks for the replies.
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Old 08-03-2010
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Does this mean that the thermostat is a different type from what would be used in a non-bypass configuration?
*** Not necessarily, it depends WHERE the T'stat is located ... either IN the bypass loop OR in the engine.

I guess the bypass loop in my engine is what is used to heat the water in the water heater.
*** All depends if the engine is fresh water cooled or raw water (only) cooled. If fresh water cooled then the hot water circuit is usually on the 'fresh' side. If raw water cooled, then usually the hot water circuit is piped into the portion between the exhaust manifold cooling and the injection elbow.
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Quote:
Originally Posted by RichH View Post
Does this mean that the thermostat is a different type from what would be used in a non-bypass configuration?
*** Not necessarily, it depends WHERE the T'stat is located ... either IN the bypass loop OR in the engine.
Now I am really confused by your answer...

The thermostat is in a housing which is part of the engine manifold. The hot water heater is heated by engine coolant (not raw sea water) which comes from a fitting on the FWC pump and returns to a fitting very near to the FWC pump.

So, let me ask again: in this engine is the thermostat any different from a "normal" (automotive) thermostat?
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Old 08-03-2010
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Bypass

Hello,

in the attached picture, the bypass is part number 8. When the engine is cold, the thermostat is closed. The water pump circulates coolant throughout the block by pumping it through the bypass hose. When the thermostat opens, coolant gets pumped into the radiator (and then water heater and heat exchanger if so equipped).

If you are interested, you can take a look at how things are set up on my boat. This past season I have changed the raw water pump, engine water pump, water heater, and had the heat exchanger cleaned. At this time I am al too familiar with how the engine cooling works on the Universal M 25 engine.


Barry


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