Check for intake leaks
From BMW E34 Encyclopedia
Air leaks cause pressure loss within the intake manifold/unmetered air to enter the system.
- Inspect rubber intake boot (take it off and inspect all seams within the flexible part of the boot for any holes/cracks).
- Make sure you have rubber spacers on ICV and intake boot they go under the plastic where claps connect.
- Inspect check valve within brake booster.
- Inspect PCV plate in the rear for missing fittings, obvious signs of air sucking your hand in where plate mates with intake manifold.
- Inspect area where throttle body mates with intake manifold.
John Doe propane test
- Purchase hand held propane, turn it on (do not light it!) and let propane gas around areas where you suspect an intake leak. Your idle will change if leak
- Use a professional smoke machine to pressurize and smoke the intake. This is the best and most straight forward way. Usual charge is $100/per test.