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The overrun fuel cut off valve has been a troublesome component on the SU HIF44E since it was introduced on the first Maestro and Montego models until it was discontinued on the introduction of ERIC electronically regulated ignition and carburetion in 1989.
The valve was fitted as a fuel economy device; under overrun conditions the valve would cut off the fuel delivered to the engine at half-second intervals. This was said to save 2-3mpg on a simulated urban cycle.
The valve uses the most basic principle of carburetion to cut off the fuel, that being the air pressure differential between the venturi and the float chamber. On the model you can see that when the valve is closed the drilling Z to the venturi is blanked off, drilling Y to the float chamber is connected by a passage in the valve to the float chamber vent pipe X. In this closed position with the engine running atmospheric pressure (blue) passes into the float chamber and pushes fuel out of the main jet because the air pressure on top of the fuel is greater than the air pressure in the venturi (yellow) i.e. normal fuel delivery. When the valve is open the drilling Z to the venturi is connected by a passage in the valve to the drilling Y to the float chamber, and the float chamber vent pipe X is blanked off. In this position with the engine running venturi pressure and float chamber pressure are equal so no fuel is pushed out of the main jet.
When the valve is faulty it has a tendency to stick open any where between slightly off of its’ seat and the fully open position. This can result in all three ports XYZ being connected (green). In this condition fuel delivery is reduced at the main jet and also the constant flow of air through the valve results in dirt and debris entering the valve through the float chamber vent pipe X compounding the reliability problem.
The valve was fitted as a fuel economy device; under overrun conditions the valve would cut off the fuel delivered to the engine at half-second intervals. This was said to save 2-3mpg on a simulated urban cycle.
The valve uses the most basic principle of carburetion to cut off the fuel, that being the air pressure differential between the venturi and the float chamber. On the model you can see that when the valve is closed the drilling Z to the venturi is blanked off, drilling Y to the float chamber is connected by a passage in the valve to the float chamber vent pipe X. In this closed position with the engine running atmospheric pressure (blue) passes into the float chamber and pushes fuel out of the main jet because the air pressure on top of the fuel is greater than the air pressure in the venturi (yellow) i.e. normal fuel delivery. When the valve is open the drilling Z to the venturi is connected by a passage in the valve to the drilling Y to the float chamber, and the float chamber vent pipe X is blanked off. In this position with the engine running venturi pressure and float chamber pressure are equal so no fuel is pushed out of the main jet.
When the valve is faulty it has a tendency to stick open any where between slightly off of its’ seat and the fully open position. This can result in all three ports XYZ being connected (green). In this condition fuel delivery is reduced at the main jet and also the constant flow of air through the valve results in dirt and debris entering the valve through the float chamber vent pipe X compounding the reliability problem.
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