FAQ
TL;DR: On Audi A6 C5 1.8T, a vacuum drop >300 mbar during the brake‑pump test flags a bad vacuum accumulator/check valves. "It eliminates sudden fluctuations in the system." Fix vacuum leaks, verify the servo, and adapt the throttle to stop wavy idle and RPM dips when braking. [Elektroda, mkubol, post #17295876]
Why it matters: This helps Audi 1.8T owners quickly diagnose brake‑related vacuum faults that cause stall‑risk and poor drivability.
Quick Facts
- DTC 17887/P1479/005241 = Brake Boost Vacuum System mechanical fault; inspect booster hose, reservoir, check valves, and lines. [“Ross-Tech Wiki: 17887/P1479/005241”]
- Brake‑pump test: at idle, repeated pedal presses shouldn’t drop system vacuum >300 mbar; if it does, suspect accumulator or hoses. [Elektroda, mkubol, post #17295876]
- After throttle replacement, run throttle body adaptation with a scan tool or idle may hunt. [Elektroda, mkubol, post #17127597]
- Start by checking for leaks at the intake before/after the turbo, and verify vacuum pump/servo integrity. [Elektroda, mkubol, post #17126252]
- Symptom noted to appear mainly in the first ~20 km; smooth braking lessens the RPM drop. [Elektroda, Daniel198632, post #17128331]
What causes wavy revs and RPM drop after braking on an Audi A6 C5 1.8T?
Unmetered air from vacuum leaks and brake‑booster vacuum fluctuations upset idle control. Inspect all vacuum hoses, seals, the vacuum tank (accumulator), check valves, and the solenoid valves driving boost and, if equipped, EGR. A leaking brake servo can mimic a large vacuum leak. "There was also a hardened servo seal several times." Complete a thorough vacuum system check before replacing more parts. [Elektroda, mkubol, post #17131811]
How do I perform the brake‑pump vacuum test?
- Warm the engine and let it idle.
- Tee a vacuum gauge into the system feeding the brake servo/accumulator.
- Pump the brake repeatedly and watch the gauge; a drop above 300 mbar indicates an accumulator or hose issue.
This test isolates sudden vacuum fluctuations that cause idle dips when braking to a stop. Repair leaks and replace failed check valves or the reservoir as needed. [Elektroda, mkubol, post #17295876]
Do I need throttle body adaptation after replacing the throttle?
Yes. After replacing the throttle body, run adaptation with a diagnostic tool. Without adaptation, idle control can hunt and misbehave. The procedure ensures the ECU learns the new throttle’s end‑stops and airflow characteristics. Complete adaptation before chasing other causes of wavy idle. [Elektroda, mkubol, post #17127597]
Can a faulty brake booster or its vacuum circuit cause RPM dips when I press the brake?
Yes. A leak in the brake boost vacuum circuit can trigger DTC 17887/P1479 and cause idle instability, stalls, and a hard brake pedal. Inspect the booster hose, reservoir, check valves, and plumbing for leaks or misrouting. Repairing the brake vacuum system often stabilizes idle immediately. [“Ross-Tech Wiki: 17887/P1479/005241”]
Where should I start checking for leaks on the 1.8T?
Check the intake tract for leaks before and after the turbo. Inspect every vacuum hose and connection, including those to the brake servo. Verify the vacuum source is stable. Small, invisible cracks are common and can cause big idle problems. Fix these first to avoid unnecessary parts swaps. [Elektroda, mkubol, post #17126252]
My issue happens mostly in the first ~20 km. What does that suggest?
Log when it occurs and replicate the condition during diagnosis. The OP observed the RPM drop mainly in the first ~20 km and noted smoother braking reduced the dip. Use that timing to run vacuum and brake‑pump tests early in the drive. It helps isolate temperature‑sensitive leaks. [Elektroda, Daniel198632, post #17128331]
How do I check the vacuum source or pump correctly?
Use a vacuum gauge. Measure at the vacuum source, then at the system with hoses connected. Strong vacuum at the source but weak in the system indicates leaks, kinked lines, or failing check valves. Weak vacuum at the source points to the pump or its drive. Test method matters for accurate results. [Elektroda, mkubol, post #17294510]
Is the Multitronic transmission to blame for the RPM drop?
Unlikely in this scenario. The gearbox was regenerated and the oil changed with no change in symptoms. That points away from the CVT and toward the engine’s vacuum/idle control. Focus on vacuum leaks, accumulator, check valves, and the brake booster circuit first. [Elektroda, Daniel198632, post #17293508]
What scan‑tool steps help diagnose this quickly?
Capture a dynamic log while coasting and braking to a stop. Note target idle, actual RPM, throttle angle, and brake switch status. Correlate the RPM dip to brake application and vacuum tests. A clear timeline speeds root‑cause isolation and avoids guesswork. [Elektroda, mkubol, post #17127597]
Can I temporarily blank the EGR for diagnosis on this engine?
If the engine is equipped with an EGR valve, you can temporarily block it for diagnostic purposes. Restore it after the test. This helps determine if delayed EGR closing or leakage contributes to idle instability on decel. "For diagnostic purposes it may be blinded." [Elektroda, mkubol, post #17131811]
Could the brake light switch (stop sensor) cause this behavior?
It is rarely the root cause of idle dips on braking. In this case, replacing the stop sensor did not change the symptom. Verify the switch signal with a scan tool, but prioritize vacuum system checks, reservoir integrity, and throttle adaptation. [Elektroda, Daniel198632, post #17126683]
What does a hard brake pedal tell me during this fault?
A hard pedal indicates lost brake‑servo assistance due to low vacuum. That points to leaks, a failing servo, or blocked hoses/check valves. If braking feels weaker while the idle wavers, focus on the brake vacuum circuit immediately. Safety comes first. [Elektroda, mkubol, post #17127597]
Any trusted shop near Poznań for this kind of diagnosis?
One recommendation from the thread is BRK‑Tuning in Inowrocław, if the distance works for you. A shop experienced with VAG vacuum and brake systems can save time by testing the reservoir, check valves, and servo correctly. Call ahead and describe the symptoms. [Elektroda, mkubol, post #17138535]
