The 'E' Type Forum

Can someone please explain what does the Servo motor(SM) and the temperature compensator valve(TCV) do in the air cleaners of my V12 Roadster which is a 1973 model.

The Car is a right hand drive model and does not have the gulp valve or air pump etc as in the US spec cars.

Are the TCV and SM only for emission control and can you block them off. If they are essential for the correct running of the engine, how do you test them to confirm that they are working correctly.

The TCV controls vacuum supply to the flap valves in the inlet ducts which I think is what you mean by the servo bit. I'm hazy since I sold my S3 but it's all explained in the manual. If you've got a V12 and no manual then you are a braver man than me.

Basically it's an air-blend device to mix warm air with cold and speed warming-up on what isa fairly cold-blooded engine due to its mass. In theory the flap stays permanently open once the engine is up to temp and IIRC fail safe (such as when a diaphragm perforates or a vac the tube falls off) is in the open position allowing full cold air feed.

Meanwhile the little AC temp valves in the air cleaner only pass manifold vacuum when the engine is cold, which is when they pull the flaps against built-in psrings so that the engine sucks some air from over the exhaust manifolds. Once the engine is warm the bimetallic strip in the AC valve closes the vacuum supply and the flpas return to their defualy cold-air position although the warm aire supply is never actually shut off completely.

The idea is to ensure a steady supply of air at something like 40C. At least that was the figure for the XJ with similar air blending and therefore probably the same number for the V12).

We are all taught to think cold air is best for power, which it is, but a car maker has to think of emissions and fuel consumption, both of which are reduced by slightly warmer air needing less fuel to give a correct mixture most of the time. The Stranglebergs are designed and jetted to run leaner on this warmer air and if you supply cold air only by blanking off the warm supply, you ought to rejet marginally as part of a full rolling road work-up. Most people don't bother because they don't ever run their cars that hard.

Pete