Look for simple solution first

My 2003 Jetta had a new injection pump installed and a new timing belt. My friend who did the work took apart the old injection pump and it was rebuilt prior to my buying the car and was rebuilt incorrectly causing it to eventually fail. I got the car running again and then began to have decrease power driving on grease and black smoke from the tailpipe. We looked at all the complicated things like the timing, fuel temp and fuel flow and couldn't discover the problem. Eventually it dawned on me that the car was acting like it was vapor locked and I did install check valves in the system as the valves that control the system leak small amounts of fuel and I was worried about cross contamination. I removed the check valve on the supply line and checked my connections and the car runs great! It is easy to assume a problem is due to something complicated like timing or a bad pump, when in reality it was a $30 part that wasn't critical for the operation of the car.