For Efi cars a forward facing bimetal coil senses the airflow coming from the radiator. Heat causes the coil to turn a shaft which operates an internal control valve regulating the volume of oil which is able to flow between the grooves. Thus hotter air allows more fluid to enter the grooves resulting in a stronger bond between wheel and housing giving higher fan speed and increased cooling.

The first of two common fail conditions is when the fan does not speed up due to insufficient bond between the wheel and the housing resulting in overheating especially in slow traffic, urban driving, towing or running air-con. Lightly loaded at normal driving speed there is sufficient air passing through the radiator to cool the engine. There are three reasons for such failure. Faulty bimetal coil, oil control shaft/valve siezed, or loss of oil. A simple check is to press a rolled newspaper against the fan blades when a hot engine is idling. If the fan can be slowed or stopped it is probably faulty.

A second fail condition when the fan becomes permanently locked is potentially very serious. Because the grooves are thin they can break due to age, internal corrosion, vibration, etc, with debris jamming between wheel and housing resulting in an over speed fan. The air roars loudly and continously, more so at high engine speeds and such a condition may result in a thrown fan blade, resulting in severe imbalance and further breakage. If this happens broken parts are projected forwards into the radiator. Never drive a car with a locked-up viscous fan.

There is a third (possible) fail condition whereby the coupling is quite stiff when cold and upon start-up the fan roars at high speed for a short while, say from thirty seconds to two minutes, before it slows to a normal freewheeling speed. This is due to the internal fluid leeching into the grooves, usually over-night, through poor internal seals and locking up the fan immediately. However the fluid soon reverts to its normal routine and the clutch lets go of the fan until such time as normal heating of the mechanism engages the fan again when hot, as previously described. Perhaps not a true fail condition, but certainly a pre-cursor to total failure when the fan will eventually fail to engage, sometime soon.

A new viscous coupling costs loads of money and finding a good used item depends upon assurance from the seller or trying it out in situ before agreeing to buy. It is possible to adapt a unit commonly used by BMW to fit the Efi fan blades by redrilling the fixing holes taking care to avoid vibration. Alternatively a thermostatically controlled electric fan is cheaper, can be independently switched and absorbs no power when off. However, under-bonnet temperature may vary wildly which is bad for Efi systems and, unless fully cowled, will be have less overall efficiency.

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