Vacuum problems can cause a rough idle condition. You may just need to replace the injector seal at the suspect cylinder, and not the whole injector.

If you are going to throw parts at the problem, do the cheap fixes first!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car