Check under the black plastic cover that covers the injectors. You may have some leaks there from the injector hoses. I would also raise the vehicle, remove the belly cover and inspect for leaks. The engine may need a good injector cleaner if the injector tips are clogged with carbon. The 98 has a different intake manifold from mine, but inside mine there are several flaps that may stick and cause hesitation and the check engine light. It's good to clean and lube them every now and then.

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2004 VW Jetta TDI (manual)

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