Thanks gents, I really appreciate your help.

I hadn't thought that I have the wrong cap. That makes a lot of sense and I'll do some digging.

Regarding the lines, I've got all the brake booster "sucking jet pump" lines accounted for and ready to install tomorrow, and I've deleted the whole secondary air pump system. None of these lines exited the engine bay at any rate.

What is odd is that the line in question comes out of the left side of the engine bay behind the fuel line/heater valve vicintity, then heads over to the left side of the intake manifold, hits a 90 degree plastic "T" and looks like it ran to the front of the motor, judging by the chunks of 20 year old tubing laying on the manifold and in the valley.

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Current
Original owner 2002 E53 X5 4.4i to 4.6i swap
2026 G80 M3 6 spd
2025 G06 X5 50e
Former
1972 Audi Fox
1986 Saab 900S
1996 BMW Z3
1998 BMW E36 M3 Sedan
2004 BMW E46 M3
2006 Audi A3 Quatro
1993 Mopar 318 Jeep Grand Cherokee
2015 V6 Jeep Grand Cherokee