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