Rough Idle & Power Loss Points to Audi Vacuum Leak

I experienced something very similar with my 2002 Audi A3 1.8T. With moderate experience working on cars, I initially thought it was just a dirty air valve causing the idle problems. The symptoms matched exactly what you describe, rough idle, drop in power, and the check engine light. After checking basic items myself, I discovered multiple vacuum hoses had deteriorated, particularly the connection from the pump to the main valve. These vacuum leaks were causing the erratic behavior. The repair involved: Complete diagnostic scan, Removal and inspection of affected hoses, Replacement of damaged vacuum hoses, Testing system integrity, Clearing check engine codes Total cost was 175 Euro at my local workshop, including parts and labor. The job took about 2 hours. Before suggesting more specific advice, could you share: Engine size and type, Mileage, Any other symptoms like unusual sounds, When the problems first started, Whether the check engine light is solid or flashing This would help determine if your issue is identical to what I encountered.

Thanks for sharing your experience. To add some details about my car: last service was performed at 74734 KM, where they did a complete inspection and general maintenance. After reading your reply, I checked the throttle body area and noticed some oil residue around the vacuum hoses, which might indicate a vacuum leak. The check engine light stays solid, not flashing, and the rough idle seems worse when the engine is cold. Would cleaning the throttle body and inspecting the manifold pressure sensor be a good first step before replacing parts? Just trying to avoid unnecessary repairs if a simpler fix might work.

Thanks for those additional details. On my A3, I initially tried cleaning the throttle body and checking the manifold pressure sensor connections before jumping to replacements. While cleaning helped briefly, the underlying vacuum leak was still causing issues. The oil residue you mentioned around the vacuum hoses is a telltale sign, I had the exact same thing. When the air intake system cant maintain proper manifold pressure due to these leaks, it creates the cold idle problem youre experiencing. A thorough cleaning might help temporarily, but based on my experience with the same issue, youll likely need to address those vacuum leaks. The solid check engine light actually points more toward a vacuum system problem than a throttle body issue. Before replacing anything, try this quick check: with the engine idling, spray some carb cleaner around those oily vacuum connections. If the idle speed changes when spraying specific areas, youve found your leak spots. This is exactly how I pinpointed the problem areas on my car. Any unusual whistling sounds when accelerating? This was another symptom I noticed just before getting the repairs done.

Thanks for the detailed troubleshooting advice. I tried the carb cleaner test you suggested and noticed the engine speed definitely changed when spraying near the vacuum lines closest to the throttle body. No whistling sounds yet, but the rough idle is definitely worse than before. Given that the symptoms match your experience so closely, especially with the oil residue and the manifold pressure issues affecting idle, I think I'll get those vacuum lines replaced rather than just attempting a cleaning. The 175 Euro repair cost you mentioned seems reasonable compared to the risk of causing more problems by waiting. I'll schedule an appointment with my regular shop and have them do a full vacuum system check. Better to address these air intake issues now before they create bigger problems. Will make sure they check all the connections while they have it apart. Do you think I should mention anything specific to the mechanic about checking the manifold pressure sensor too, since were already dealing with vacuum leak symptoms?