I did the smoke test and found no leaks. I confess I used the cigar method. I had done it once many years ago and found a leak with it back then, but I did not find a leak this time.

As I was doing the test, and capping the system at the top of the boot, just before the MAF, I decided to have a look at the air filter, and I found that I had installed a reusable K&N filter (back in 2019 according to my records). The bottom side of the filter was covered in gunk. It is late now, but tomorrow morning I will go get the K&N cleaning kit and see if that's what's causing the issue.

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2001 BMW X5 3.0i, Automatic, Gasoline