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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