Abstract: A secondary fuel premixing controller for introducing a secondary fuel into an air intake manifold of a primary fuel combustion engine. The controller includes a hollow housing having an air inlet, a main controller body, and an air/secondary fuel mixing outlet connected together in central alignment one to another. The air inlet introduces incoming air into the controller while the air/fuel mixing outlet discharges a blended mixture of secondary fuel and air into an air intake manifold of the running engine. The main controller body includes a secondary fuel manifold positioned centrally within an interior of the controller which supports for axial movement an air pressure valve positioned proximate to the air inlet and a secondary fuel/air blender connected thereto. The secondary fuel manifold also has a secondary fuel inlet formed therethrough connectable at an outer end thereof to a source of secondary fuel, an inner end positioned for sealing engagement with the secondary fuel/air blender.