I had similar problems with my 90 525i. The best thing you can do is hook up a pressure gage to the fuel line BEFORE it goes to your pressure regulator. (Ties into fuel rail) The Bentley maintenance book tells you what you should have for a reading. This will tell you if your filter is the problem or not. Another quick check you could do is pull the 1/4" vacuum hose going from the intake manifold to the regulator. If there is any trace of fuel in the line, the regulator's rubber diaphragm has ruptured causing not enough fuel pressure at the rail/injectors and leaking fuel into the air intake duct. This problem is also characterized by hard starting if the car has sat for more than a few minutes.
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