M 54 - one 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 valve becomes 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
upgraded parts and have a much more reliable DISA for 1/3 the cost.