replying to John Burns, wektor wrote:
M 52 has one main problem
he DISA valve, otherwise known as the «Intake Manifold Adjuster Unit», is
located on the side of the intake manifold and is a common failure on the M54
and M52TU BMW engines. The failure is caused by the type of materials used in
the manufacture of two of the three key valve components. The three key
components of the DISA are the flat plastic flapper valve which opens and
closes depending on engine rpm and throttle position, the bell crank lever
that rotates it, and the vacuum pot that actuates the bell crank lever. The
flapper valve is made of 30% glass reinforced nylon and the bell crank lever
is made of standard 6/6 nylon. As shown in the pictures on the left, the bell
crank lever has a male, hex shaped protrusion, which snaps into a matching
female pocket on the flapper valve. These hex shaped sections are what allow
the rotation of the bell crank lever to rotate the flapper valve.

During normal engine operation, vibrations from pressure pulses inside the
intake manifold cause the bell crank lever and flapper valve to vibrate
against each other where the hex shaped portions contact each other. These
vibrations start a wear process which will cause the tiny glass fibers in the
flapper valve to become exposed. Once the fibers become exposed they start to
erode away the male hex portion of the weaker bell crank lever. As soon as the
erosion starts it’s all down hill from there until the hex is completely
worn away. The symptoms of a worn DISA valve include rough idle, lean engine
codes and lack of low to mid rpm power.
If you disassemble a worn out DISA valve and look at the bell crank lever
it’s common to not even recognize that it once had a hex shape on the
protrusion. Once the hex is shape is worn off the lever, the flapper valve no
longer opens and closes in a controlled manner by the rotation of the bell
crank lever. The brittle flapper valvebecomes free to slap around uncontrolled
within the DISA support framework. This uncontrolled movement can lead to
complete destruction of the flapper valve. The broken pieces of valve have no
where else to go except through the intake manifold runners and through the
motor, sometimes resulting in severe engine damage. Even a small piece of
plastic broken off the flapper valve can hold an intake valve open, resulting
in bent valves or much worse.

Since the only parts on the DISA valve that typically fail are the flapper
valve, bell crank lever and housing seal, why replace the entire unit. Replace
the failed internal parts with these http://vanos-bmw.com/disa-bmw-repair-kits
upgraded parts and have a much more reliable DISA for 1/3 the cost.

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