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Thermostat bypass loop necessary?

7

70wellcraft

Member
Hi,
I am rebuilding my original 89 5.7 mercruiser 260 hp with new vortec heads and intake. The original did not have a thermostat bypass loop that has a hose from the intake water to the waterpump inlet plug. I have read that the reason for this is to ensure no hot spots and boiling occurs while the engine is heating up prior the t-stat opening and allowing flow though the block. My question is is this really necessary on a boat engine with a 140 t-stat? Automotive engines run 160 to 200 and usually around 180-190 so I can see why it may be more necessary here.
Thanks
 
You can drill a hole in the thermostat to achieve the same effect. You can put a bypass and at the same time install a water heater or a space heater just put a loop from the circ pump and the intake manifold.
 
Would that be defeating the purpose of the t-stat by allowing cooler water to enter the engine past the t-stat prior to heating to the t-stat open temperature?
My real question: Is it common practice to have a bypass loop? Is it really needed?
 
70wellcraft said:
1.... Would that be defeating the purpose of the t-stat by allowing cooler water to enter the engine past the t-stat prior to heating to the t-stat open temperature?
2.... My real question: Is it common practice to have a bypass loop? Is it really needed?
Click to expand...

1..... A true marine thermostat will have an air bleed hole in it. Also, the coolant (i.e., seawater or E/G coolant) has removed engine heat prior to its flow towards the thermostat.

2.... I don’t believe that it is necessary.


.
 
I cant remember hearing of an engine with a heater loop having the same issues with a raw water cooled engine. I could see not properly flushing a salt water system properly being a issue in the heater loop especially. I do wonder why marine engines do not have the bypass prior to the thermostat it just makes sense to circulate the coolant in the block and heads to eliminate hot spots but that's just me i guess.
 
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kimcrwbr1 said:
I cant remember hearing of an engine with a heater loop having the same issues with a raw water cooled engine.

Kim, if I understood you, I agree.
With exception to the much longer circuit length, a coolant loop for a cabin heater and/or a water heater is very similar to the by-pass mentioned.
Supply = an intake manifold port prior to the thermostat.
Return = a port in the suction side of the circulating pump.


I could see not properly flushing a salt water system properly being a issue in the heater loop especially.
For a raw water cooled engine....yes, especially if in ocean water. The cabin heater and water heater now become part of the winterizing effort.


I do wonder why marine engines do not have the bypass prior to the thermostat.....
The old 460 Ford did use the by-pass. Many other engines do not.

.... it just makes sense to circulate the coolant in the block and heads to eliminate hot spots but that's just me i guess.
With the car/truck engine, an owner is more likely to take off before allowing the engine to warm up, whereas with the marine engine, I would hope that most owners give the engine some warm-up time.
Also, liquids are very efficient at heat transfer.
However, air pockets in the coolant could certainly cause an issue!
Click to expand...
 
The bypass hole in a T'stat is on the order of 1/8", not likely to defeat the purpose of a t'stat... The circulating pump on the front of the engine circulates water thru the head and block. The t'stat housing is not directly "in line" with this flow but rather "off to the side" allowing hot water out and cold water to take its place.
 
Last edited:
sandkicker said:
The bypass hole in a T'stat is on the order of 1/8", not likely to defeat the purpose of a t'stat...

The circulating pump on the front of the engine circulates water thru the head and block. The t'stat housing is not directly "in line" with this flow but rather "off to the side" allowing hot water out and cold water to take its place.
Click to expand...

Yes.... Bob is correct!

Think of the engine circulating pump (the belt driven pump on the engine itself) as though it's charging the cylinder block/cylinder heads with coolant.
Think of the thermostat as though it's a temp controlled valve that allows coolant to "leave" the cylinder block/cylinder heads on an "as required" basis.
("as required" meaning as coolant temperature dictates)

Coolant = seawater (ocean, river or lake water) and/or ethylene glycol.... depending on whether open or closed system.


.

 
I was just thinking if the impeller of the circ pump has rough spots or buildup if it can cause the impeller to cavitate similar to a damaged prop. That could account for air in the block just a thought?
 
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