2000 VW Jetta Diesel Problem
 

Have about 62,000 miles on it. When I start out, everything is okay until after about 50 miles, it starts to lose power up hills. Does this sound like a fuel pump problem? I have noticed this when I replaced my fuel filter. The filter I bought was not a VW Filter, and I have heard that the aftermarket fuel filters are known to cause problems. What do you guys and gals think? Thanks in advance. PS is the fuel pump in the fuel tank?