Thermostat Bypass Pipe

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garrycol
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Thermostat Bypass Pipe

Post by garrycol »

On the early Land Rover 3.5 V8s (Strombergs) there is a bypass pipe/hole in the bottom of the thermostat housing that has a s rubber pipe that connects to the back of the timing cover - it allows coolant to pass between the inlet manifold and water pump while the thermostat is still closed. It allows the circulation of a small amount of coolant from the water pump into the engine and back through the inlet manifold, thermostat housing to the water pump so helping warm the engine.

I have a 4.6 in my vehicle with the 4.6 timing housing and water pump but at this stage still have the original 3.5 carbs and manifold fitted. The issue I have is that the 4.6 does not have the pipe at the back of the water pump to allow the thermostat bypass pipe to be fitted so I just have it blanked.

My simple question is this an issue?

Older engines never had a bypass and the coolant in the engine just heated up until the thermostat opened and all was good.

Thanks

garry



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ChrisJC
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Re: Thermostat Bypass Pipe

Post by ChrisJC »

No, I believe what happens is that some heaters are always 'hot' and use ducting and flaps to change air temperature. In this case, there is no need for a separate thermostat bypass because the heater circuit does that job.

If your heater shuts off the water flow, then you do need the bypass.

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Re: Thermostat Bypass Pipe

Post by garrycol »

Thanks Chris - no flaps or vents. Old style heater with a tap control to turn it on and off. The heater plumbing is separate to the thermostat bypass - there is a heater outlet pipe at the rear of inlet manifold as per all older Landrover V8s.

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Garry

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Re: Thermostat Bypass Pipe

Post by ChrisJC »

If I recall correctly, the heater outlet at the back of the manifold and the thermostat bypass outlet at the front are both equivalent.

As long as one of them has a permanent return path back to the water pump inlet (radiator bottom hose), then all will be OK.

If you have neither open, then the water will not be able to circulate until the thermostat opens, which is bad news as it means the thermostat will probably never open as it won't get any hot water coming past it to warm it up!

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Re: Thermostat Bypass Pipe

Post by garrycol »

Thanks Chris - you input is appreciated.

I am going to run some heater hose from this outlet in the thermostat housing to the lower radiator hose so that irrespective if I have the heater on or off there will be a flow past the thermostat even when it is closed.

Cheers

Garry

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Re: Thermostat Bypass Pipe

Post by DaveEFI »

The heater on my SD1 EFI is of the air blending type - but shuts a valve in the heater coolant circuit when the AC is in use.
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Re: Thermostat Bypass Pipe

Post by minorv8 »

I have my heater valve closed in the summer and do not have a bypass hose. I have drilled a couple of 6 mm holes in the thermostat allowing some water to bypass. Works nice and there is less hose connections.

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Re: Thermostat Bypass Pipe

Post by ChrisJC »

minorv8 wrote:
Fri May 08, 2020 10:58 am
I have my heater valve closed in the summer and do not have a bypass hose. I have drilled a couple of 6 mm holes in the thermostat allowing some water to bypass. Works nice and there is less hose connections.
So that is not quite the same. It will keep water circulating when the thermostat is closed, but of course that circulation is going through the radiator.

This means the warm-up time will be longer than if the bypass coolant wasn't being cooled.

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Re: Thermostat Bypass Pipe

Post by minorv8 »

Mine is strictly a summer car so warm-up time is irrelevant :D

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Re: Thermostat Bypass Pipe

Post by SuperV8 »

garrycol wrote:
Wed Apr 08, 2020 10:31 am
Thanks Chris - you input is appreciated.

I am going to run some heater hose from this outlet in the thermostat housing to the lower radiator hose so that irrespective if I have the heater on or off there will be a flow past the thermostat even when it is closed.

Cheers

Garry
The 4.6 cooling system relocates the thermostat to a separate housing on the rad bottom hose and has additional ports to make this bypass connection easier.
One of the benefits of this remote housing is it also has internal plates that gradually close off the bypass flow circuit when opening the main rad circuit so when required you get 100% coolant flow through the rad rather than some flow missing it and through the bypass circuit.
You can get different versions with different number of ports and different temperature thermostats. I'm using on of a freelander - works good.

example:
https://www.google.com/url?sa=i&url=htt ... AdAAAAABAG

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Re: Thermostat Bypass Pipe

Post by ChrisJC »

The only downside I have come across with having the 'stat in the bottom hose is because those stats don't appear to have a bleed hole, filling the cooling system is very difficult. Filling the header tank does not fill the radiator at all.

I always remove the top hose, and fill both directions until overflow. Then top up the header tank. Then run it, and keep it topped up!

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Re: Thermostat Bypass Pipe

Post by SuperV8 »

Interesting. I didn't notice any issues last time I filled my cooling system - but i'm using the simpler Freelander remote stat so maybe that one has an internal vent?
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Re: Thermostat Bypass Pipe

Post by unstable load »

ChrisJC wrote:
Tue Jul 07, 2020 9:25 am
The only downside I have come across with having the 'stat in the bottom hose is because those stats don't appear to have a bleed hole, filling the cooling system is very difficult. Filling the header tank does not fill the radiator at all.

I always remove the top hose, and fill both directions until overflow. Then top up the header tank. Then run it, and keep it topped up!

Chris.
There should be a small hose coming out the top of the rad into the header tank to allow the rad to vent air out while filling it and to give a small bypass into the header when running to keep the top tank air free. It is connected to a small barbed fitting in the neck of the filler of the tank with a small (2mm-ish) hole.
At least, my D2 and P38s have it, don't know about others.
Cheers,
John

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Re: Thermostat Bypass Pipe

Post by ChrisJC »

unstable load wrote:
Sun Jul 19, 2020 7:50 am
There should be a small hose coming out the top of the rad into the header tank to allow the rad to vent air out while filling it and to give a small bypass into the header when running to keep the top tank air free. It is connected to a small barbed fitting in the neck of the filler of the tank with a small (2mm-ish) hole.
At least, my D2 and P38s have it, don't know about others.
[/quote]

Yes, I do have that pipe. The problem as I understand it is that if you fill the header tank, the water will run down to behind the thermostat, and then fill the block and heater as they have connections from behind the 'stat too. But the 'stat will be closed, so water will not run into the bottom of the radiator. The only way to get water into the radiator is if the engine is filled right to the top, where it can flow through the top hose into the radiator. I am not sure, but this means the header tank is also full right to the neck, i.e. well over where you are supposed to fill it to.

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Re: Thermostat Bypass Pipe

Post by unstable load »

ChrisJC wrote:
Sun Jul 19, 2020 8:31 am
Yes, I do have that pipe. The problem as I understand it is that if you fill the header tank, the water will run down to behind the thermostat, and then fill the block and heater as they have connections from behind the 'stat too. But the 'stat will be closed, so water will not run into the bottom of the radiator. The only way to get water into the radiator is if the engine is filled right to the top, where it can flow through the top hose into the radiator. I am not sure, but this means the header tank is also full right to the neck, i.e. well over where you are supposed to fill it to.

Chris.
I have never had an issue with what you describe. When I did my disco cylinder head, I filled it from the header tank and it gurgled and burped for a while, but eventually, it was full. I opened the little valve on the top hose when I started it and when it got to temperature, there was a slug of air that came out and it was fine after that.
In my Disco, the top hose is the highest point in the system, and the small radiator line was trickling into the header tank quite merrily by then.
Cheers,
John

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