Here's one for the archives...

I bought a used E34 540i several years ago. From the time when I bought the car it would throw check engine codes if I was coasting downhill for an appreciable time, such as when descending a mountain pass. The standard stomp test revealed codes related to the lambda control (1221 and 1212).

Since the engine always ran fine, I did not worry about it much to begin with. But as the years passed the check engine light was more and more frequent. Recently, the same codes would be thrown if I started the car and allowed it to idle for a few minutes. After a time delay (3 minutes I think) the ECU tries to go into "closed loop", which is where it takes feedback from the O2 sensor(s) to adjust the intake mixture. Once the engine came up to full temp the codes at idle would go away, though the coasting downhill codes were many.

Conventional wisdom said vacuum leak somewhere on the intake manifold. Poking around, I could hear some air movement sounds but could not pinpoint a leak. Because the M60 motors are known to start leaking at the intake manifold seals, I bought all the required parts (online) and planned to do an intake manifold reseal.

The job was not a particularly easy one, but not that hard either. Mostly just time consuming... The worst part was cleaning all the gook from the manifold and the intake ports without getting crap inside the engine. Needless to say, 6 hours, $100 in parts (gaskets and a PCV plate) and a whole bunch of nitrile gloves and paper towels later, the problem is now resolved.

A few interesting side notes: Now, with my completely resealed, code-free intake, I still hear a hissing noise all around the intake manifold with the soundproof and cowling off. I think it is/was just normal injector and intake noise and is part of the reason the BMW engineers put the soundproofing on there to begin with. So, I probably could not hear the leak to begin with.

When reassembling the throttle and pcv plates to the manifold (with new seals), do yourself a favor and go buy 13 new allen head cap-screws (metric 6mm) and heave those stinking, soft, BMW torq-head screws as far as you can.

A few folks said they had difficulty getting the oil pipe back onto the PCV plate nipple with the manifold assempled and in place. I didn't find it that hard. The key is to slide the pipe all the way forward on the slip fitting before lowering the manifold in place. Then by pushing on the front end of the pipe with a long flat-blade while guiding the back end of the tube you can get her right on there. OTOH, gthe clip thing in place was more challenging for me...

Thanks to forum participants here, I was able to collect the info I needed to do this job with zero surprises. In fact, it was info on these boards that gave me the diagnosis and encouragement to do the intake reseal in the first place. Especially the excellent write up that I found from stargazer_61:

Thank you!!

Next project, tackling the "lifetime fill" transmission... before it is too late.