The best way is to hook up the injector to a tester and observe the spray pattern. I guess you don't have a tester. What makes you think you have a bad injector? Typically you would hear pinging while cruising if you had a bad injector.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL