Do a search, lots of info here. Main problems are rusty spring perches, fuel leaks, the K40 relay (which I think is what your mechanic mentioned) and glow plugs getting stuck.

Sounds like a great price. I'm curious as to why the IP was rebuilt. Performance-wise these are the best diesels of the 20th century (at least in this country).

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2004 VW Jetta TDI (manual)

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