Nailing is caused by too much fuel igniting all at once. This can be caused by many things, such as poor injector spray pattern, low compression, carbon in the prechambers, or low-cetane fuel. Cold temperatures will exacerbate the problem because a cold diesel has more delayed combustion compared to a warm diesel. I think only in rare cases is it the injector springs causing the problem. I've tested injectors that I believed were over 20 years old with all original parts, but the pop pressure was only slightly lower than where it should be and it was still within spec. Hope this helps.

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2004 VW Jetta TDI (manual)

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