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Thermostat  Rating:  Rating
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 Posted: 05-17-2007 06:06 pm
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Jay
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Last night I set out to change out the old thermostat with a new fail-safe new one.
Before I install the new one I wanted to ask for the members opinions of the differences that I noted between the two thermostats. I have attached a photo. The brass colored one is what I removed and the other one is the fail-safe design.
  1. The fail-safe design appears to employ small metal clips located inside the spring to grab the spring and lock the valve open. I understand that this type is not to be reused after the clips have engaged and locked the unit open.
  2. The thermostat that I removed is a two stage type with a little giggle relief valve.
  3. The opening of the old 2 stage thermostat is noticeably larger that the new single stage one. Assuming similar thermal expansion rates between the two, I would think that the old one would be less restrictive than the new one.
  4. There is a small sort of weep hole with a giggle valve in the old thermostat, but there is no hole in the new one. I was wondering if the lack of such a hole would make it difficult to bleed any trapped. Are there any other concerns with the lack of such a hole?
I would welcome any thoughts of the appropriate design for the 907.

Thank You.


Attachment: DSCF3814.JPG (Downloaded 161 times)

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 Posted: 05-17-2007 07:02 pm
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Jim Sohl
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Jay, you must use the dual type on all 9XX Lotus engines.  When cool, the opening at the top closes and the lower opens.  The result is for a cold engine, coolant leaving the head enters the thermostat housing between the lower and upper valves and flows through the lower opening directly back into the coolant pump with out passing through the radiator.  A much faster warm up results which is one of the many pass-fail regulations an automobile must pass when first submitted to DOT for type acceptance as an emission certified automobile for U.S. sale.  As the coolant warms up, the lower opening closes blocking the direct path to the pump and the upper passage opens creating a path through the radiator.  The single opening thermostat will only work on a cold engine.  Once the coolant becomes hot, the single valve thermostat will open as it should, but the lower passage directly to the pump remains 'open' because a single valve thermostat has no method of blocking the path directly to the pump.  The result is that with a single valve thermostat, a cold engine will warm up more or less normally.   However, when the coolant is hot enough to need cooling via the radiator, some coolant passes out the top to the radiator, but some goes directly back to the pump as though cold.  Unless the weather is quite cold, the marginal flow through the radiator will simply not be enough when mixed with the direct flow allowed by the use of the wrong thermostat.  End result?  Incurable overheating until a correct thermostat is installed.  This issue has been discussed many times before but is does not hurt to point out again that the best damn gold plated super duty single thermostat will simply not work through no fault of the (incorrect) thermostat.  The club store has these gadgets and they are not too expensive considering that you have little choice but to buy a thermostat made specifically for a 907.

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 Posted: 05-17-2007 07:32 pm
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Jay
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Thank you Jim.
I thought there may be more to the story. By all means I want as much cooling effect as I can get. I too would think that the direct to the pump bypass should be closed off when engine is up to temperature. I figure that if the engine is too cool from the increased flow, the thermostat will simply close to maintaine temperature. I also like the idea of a small opening to vent any trapped air.

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 Posted: 05-17-2007 08:29 pm
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Jay
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I found a write-up of the cooling system and thermostat for the 907. This information also seems to make a lot of sense to me.

http://www.gglotus.org/ggtech/907overheat/907overheating.htm

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 Posted: 05-18-2007 03:54 am
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73healey
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Well, I just changed to a fail safe thermostat two weeks ago. Thought it would be a good idea. After reading what was posted, should I go back to the low temp regular thermostat? I did not have any overheating problems in the past nor do I now.

Thanks,

Jim

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 Posted: 05-18-2007 02:26 pm
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Jensen Healey
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Yes. Now that you understand how the systyem works (or doesn't work with a standard thermostat) you can make up your own mind about actually getting out the wrenches. When it's over 100^F you'll be glad you did.

Kurt

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