One thing to be aware of with the gas 6-cylinder engines is the relatively weak head gasket. Find out if it's been done recently. If not, check for obvious signs of leakage. With the V8's I'd want to know if the timing chain has been replaced in the last 100K miles. If not, time to check for stretch.

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2004 VW Jetta TDI (manual)

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