Paul, unless you are intimately familiar with a 1991 300E from working on one in the past do yourself a favor and have a PPI done by a MB-trained tech. Otherwise, you are liable to spend considerable money making the car right, especially if the AC system needs a new evaporator.

I am a DIY'er and have worked on our MB automobiles for the past 12 years; however, I would never consider buying a pre-owned MB automobile without having a PPI done. I can't afford to spend thousands of dollars for neglected preventive maintenance by the previous owner(s).

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Fred Hoelzle