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Various systems and methods are described for controlling fuel injection of a dual fuel engine which includes first and second fuel rails and first and second fuel pumps. In one example, while pumping is suspended in the second fuel rail, the first fuel is injected to all but one cylinder of the eng

Various systems and methods are described for controlling fuel injection of a dual fuel engine which includes first and second fuel rails and first and second fuel pumps. In one example, while pumping is suspended in the second fuel rail, the first fuel is injected to all but one cylinder of the engine and the second fuel is injected to the one cylinder in a predetermined sequence. As such, the fuel injector injecting to the one cylinder is isolated and its performance may be assessed without significantly affecting engine performance.

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1. A method for controlling fuel injection of a multi-substance injection engine which includes first and second rails and first and second pumps, comprising: suspending pumping of a second substance into the second rail while injecting a first substance to all but a single cylinder of the engine; a

1. A method for controlling fuel injection of a multi-substance injection engine which includes first and second rails and first and second pumps, comprising: suspending pumping of a second substance into the second rail while injecting a first substance to all but a single cylinder of the engine; andwhile pumping is suspended in the second rail, injecting the second substance into the single cylinder and correlating pressure decrease in the second rail to injector degradation. 2. The method of claim 1, wherein injector degradation is indicated when the pressure decrease is less than a predetermined value. 3. The method of claim 1, wherein injector degradation is indicated when the pressure decrease is greater than a predetermined value. 4. The method of claim 1, further comprising: resuming injection of the first substance to the single cylinder and suspending injection of the second substance; andwhile injection of the second substance is suspended, resuming pumping of the second substance to the second rail and correlating pressure increase in the second rail to operation of the second pump. 5. The method of claim 4, wherein degradation of the second pump is indicated when the pressure increase deviates from a predetermined value. 6. The method of claim 1, wherein each fuel injector of each cylinder of a plurality of engine cylinders is sequentially isolated to generate respective correlations for each injector. 7. The method of claim 1, wherein the second substance is injected to the single cylinder through a predetermined number of injections. 8. A method for controlling fuel injection of a dual fuel engine which includes first and second fuel rails and first and second fuel pumps, comprising: suspending pumping of a second fuel into the second fuel rail while injecting a first fuel to all but a first group of cylinders of the engine;while pumping is suspended in the second fuel rail, injecting the second fuel into the first group of cylinders, and correlating pressure decrease in the second fuel rail to injector degradation; andadjusting fuel injection of the second fuel in the first group of cylinders based on the correlation once pumping of the second fuel is resumed by the second pump. 9. The method of claim 8, wherein injector degradation is indicated when the pressure decrease deviates from a predetermined value, and wherein the first group of cylinders is a single cylinder. 10. The method of claim 8, further comprising: resuming injection of the first fuel to the first group of cylinders and suspending injection of the second fuel;while injection of the second fuel is suspended, resuming pumping of the second fuel to the second fuel rail and correlating pressure increase in the second fuel rail to operation of the second fuel pump; andadjusting operation of the second fuel pump based on the correlation of pressure increase and pump operation. 11. The method of claim 10, wherein pump degradation is indicated when the pressure deviates from a predetermined value. 12. The method of claim 8, wherein the first fuel is gasoline and the second fuel is ethanol. 13. A system for an engine in a vehicle, comprising: a plurality of cylinders, each cylinder having a first and second injector where the first injector is coupled to a first fuel rail and the second injector is coupled to a second fuel rail; anda control system comprising a computer readable storage medium, the medium comprising instructions for: during a first condition, injecting fuel to all cylinders via the first injectors; andduring a second condition: injecting fuel to all but one cylinder via the first injectors, and injecting fuel to the one cylinder via only the second injectors; andsuspending pumping of fuel into the second fuel rail while continuing pumping of fuel into the first fuel rail. 14. The system of claim 13, wherein the first injectors inject a first fuel and the second injectors inject a second fuel. 15. The system of claim 14, wherein the first condition includes operating conditions in which only one fuel is used and a diagnostic routine is not being carried out. 16. The system of claim 14, wherein the second condition includes operating conditions in which both fuels are used and a diagnostic routine is being carried out. 17. The system of claim 14, further comprising instructions for, during a third condition, injecting all cylinders via the first and second injectors. 18. The system of claim 17, wherein the third condition includes operating conditions in which both fuels are used. 19. The system of claim 13, further comprising, during the second condition and while pumping is suspended in the second fuel rail, correlating pressure decrease in the second fuel rail to injector operation. 20. The system of claim 19, wherein injector degradation is indicated when the pressure decrease deviates from a predetermined value.