VW t4 2.4 diesel van runs great for 15 miles or there abouts.. then runs very sluggish as if running out of power gets slower and slower . If you stop and take fuel cap off. No hissing when you do this. Then start up. It drives spot on. Checked fuel tank breather. Fuel tank taken out cleaned and checked. Pipes from tank to filter are fine. New fuel filter. Removed rubber seal from fuel cap. Still runs rubbish until the fuel cap is removed.. there is no injector management or accelerator sensor anyone please help