Question about 1990 Volkswagen Cabriolet

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Fuel rail pressure drops from normal at 3500 rpm to zero at 4500 rpm. already replaced fuel filter, checked rail pressure regulator, replaced in-take pump, external pump replaced by previous owner and tested by me. the external pump makes a whining noise in the vehicle, but is silent during bench test. what is the box above the fuel tank flange passenger side, just forward of the right rear tire, the external pump takes its suction from this box. is the box related to the noise and/or pressure.

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some info; hope it is helpful

Posted on Sep 14, 2010

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How to test fuel pressure

I have been working on a simular problem on my daughter's vehicle. with the following additional symptoms. It only fails while hot. Runing fuel preassure checks, I noticed that the preassure rises from 30 psi while running to 40 psi after turning off the hot engine. This is caused by the expansion of the fuel in the rail. I believe that this hgh preassure is causing the regulator to stick closed preventing the fuel to get to the rail during a restart, (as the preassure drops to 0 while the fuel pump does whirrrr during the startup proceedure). It seems to free up after it has cooled then works normally. We ordered a new regulator but haven't received it yet to see if this fixes the problem. There is a possibility that the problem is a weak pump that cannot overcome the stuck regulator but since all the checks are in spec, I'm leaning toward the regulator at the moment.

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Two, the check valve at the pump is weak. To verify, turn the key to the run position, when peak pressure is reached, gently pinch the pressure line and watch the pressure gauge. If the pressure remains near the peak pressure the check valve is bad. Replace the fuel pump.

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Any one of these failures will cause the "vapor" issue you describe.

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