My sweet 03 X5 6cyl has been dealing with a vacuum leak and p0171 and p0174 codes for a while now. Twice so far, I've gotten to a point where the engine was misfiring. The first time it happened, the car fixed itself after sitting off for 10 minutes. After this incident, I changed my lower intake boot which I knew was cracked and taped by me, as well as my Disa valve. The vacuum leak error came back immediately. Two months later, which was this past weekend, in very cold weather the misfire returned. The car was undrivable on Saturday morning. Figuring I had nothing to lose and since I had the parts, I changed my valve cover gasket, which was shot, and all 6 ignition coils. Car then ran fine after this, no misfire, but my check engine light came back on after 10 miles.

So - I still have a vacuum leak. Car has 167k miles.

1) should I bother to replace the oil filler cap?

2) I will spray the MAF sensor with cleaner for a cheap try.

3) I've studied up on the DIY to pull the intake manifold and replace the CCV system with new parts. I think the CCV works, but maybe one of the lines is cracked.

4) I'll replace the other vacuum hoses.

5) I recently replaced my secondary air pump and I don't have reason to believe that check valve is an issue.

6) I read it could be my oxygen sensors, but those are expensive and probably ok...

Anything else I should be considering???

I don't have a smoke machine to test.

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2003 X5 3.0i Silver (build 07/03)
2006 X5 3.0i Blue
1977 Ferrari 308 GTB
1994 Ferrari 348TS Speciale #14/15
1989 Ferrari Testarossa