I'm impressed at how well injectors are designed and they seem to never wear out. Just keep them clean and they last forever.

That link is the cheapest I've seen for injector service. Some people, including myself, have been able to clean them at home. This involves an ultrasonic bath cleaner, appropriate solvent, a way to reverse flush the injector, and a power source to open the injector pintle. Caution, do not use 12v as that may fry the injector. They don't operate at 12v. The pintle opens at aroud 1.5V.

Though the DIY method may clean the injectors, I'm not sophisticated enough to flow test them. That's where this mail order cleaning service is better.

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95 E320 Cabriolet, 169K