I know it's not a M119, but I've been under the understanding that they designed both engines similarly.

My M117 has the aux air runners cast into the intake side of the heads, with a passage joining them through the intake manifold. When they get fouled with carbon from the exhaust flowing back up into them, you end up having to tear the intake off, at minimum, to get to them and clean out the accumulated carbon deposits blocking the ports and channels that the aux air is routed through.

Again, it's not a M117, but I cleaned mine out while I had the intake off, just to be on the safe side (even though I don't have aux air connected currently on this car). On this engine, it's not an impossible task, but you should definitely know which end of the wrench to hold before diving in in your own garage. Not being my daily driver, this is a "just for fun and learning" engine rebuild, and it's involving lots of learning!

Best of luck with it, and sorry to hear you're having trouble.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...