When I started the car yesterday, the engine suddenly developed a vibration. This vibration is with the engine at all speeds. I thought I was running on five cylinders. I tried to diagnose the problem by disconnecting each injector to find the missing cylinder, but this isn't the case. The engine runs rougher each time I disconnect an injector. What could be causing this vibration?

No check engine light came on. I went to read the codes, and interestingly, the #2 LED light was on, and there is no response to testing. Perhaps related?

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95 E320 Cabriolet, 169K