BMW E34 vacuum leak and intake boot
Why does the E34 intake boot cause running problems?
The intake boot is the rubber tube between the mass air flow sensor and the throttle body. When it cracks, unmetered air enters the engine and the mixture leans out. Symptoms are an uneven idle and hesitation. The crack is often visible only when the tube is bent or squeezed.
Where else can a vacuum leak be found?
Check the vacuum hoses, the crankcase breather valve rubber plugs and diaphragm, the intake manifold gaskets and the rubber plugs at the end of the intake manifold. A leak after the mass air flow sensor is a problem, but a leak before the sensor does not affect running. The fuel pressure regulator hose works on vacuum, and it can leak too.
How is a vacuum leak found?
Spray cleaner or starting spray on the intake manifold joints and hoses with the engine running. If the revs change, that spot is leaking. The method does not find every leak, so suspect hoses are worth removing and flexing. Pressing alone is usually not enough.