Audi S4 Multiple Engine Issues Points to Fuel System

Sounds very familiar. Had a similar issue with my 2010 Audi S5 last year. I have some experience working on VAG engines, mainly basic maintenance and diagnostics. The symptoms you describe match what I experienced, the check engine light, rough idle, and performance issues were identical. After checking the fuel rail pressure sensor readings, my mechanic found a significant pressure drop after the recent fuel filter change. The problem turned out to be quite simple: The fuel pressure regulator wasnt properly seated after the filter replacement. A complete inspection and proper reinstallation of the components fixed it, costing 45 Euro for labor. Before jumping to conclusions though, could you share: Have you recently had any fuel system work done? What error codes is the check engine light showing? Does the rough idle get worse when the engine is warm? Did you notice any fuel smell around the engine bay? These details would help determine if your issue matches what I experienced or if its something else in the fuel delivery system. When dealing with fuel system issues, its important to get an accurate diagnosis since similar symptoms can stem from various causes, from simple fixes like mine to more complex fuel rail or sensor problems.

Just completed my latest service at 157487 KM. After running diagnostic tests, it showed error code P0089 related to fuel pressure issues. The rough idle definitely gets worse when warm, and I have noticed a faint fuel smell near the engine compartment, especially after parking. The pressure sensor readings show fluctuating values between 45-60 PSI at idle, which seems quite inconsistent. No recent fuel system work has been done aside from routine maintenance. The check engine light started flashing intermittently before staying permanently on. Based on the fuel pressure drop pattern and symptoms, it sounds like my issue might be more serious than a loose connection. The pressure regulator seems the likely culprit, though I will get the entire fuel rail inspected to be thorough.

Your symptoms definitely point to a more serious fuel system issue than what I encountered. In my S5, once we properly reseated the components, the pressure readings stabilized immediately. Your fluctuating fuel rail pressure readings, combined with that P0089 code, strongly suggest a failing fuel pressure regulator. The fuel smell and worsening rough idle when warm are classic indicators of a degrading regulator. When I researched further after my incident, I learned these symptoms often show up together when the regulator diaphragm starts failing. A proper fix will likely require replacing the fuel pressure regulator and possibly inspecting the fuel rail for any related damage. The repair cost will be higher than my simple fix, typically around 350 Euro for parts and labor if only the regulator needs replacement. Be cautious about continuing to drive with these symptoms. A malfunctioning fuel pressure regulator can lead to: Further engine performance degradation, Potential damage to other fuel system components, Increased risk of stalling in traffic, Poor fuel economy, Catalytic converter damage Get it checked soon to prevent cascading issues through the rest of the fuel system.

Thanks for the detailed information. After comparing my situation with yours, I think I need to bite the bullet and get this properly fixed. The symptoms match too closely to ignore, especially the pressure readings and that fuel smell. Going to schedule an appointment tomorrow to have the fuel pressure regulator replaced and get a thorough inspection of the fuel rail system. The potential for more serious issues down the line isnt worth the risk, particularly with those pressure fluctuations and engine misfires getting worse. The repair costs sound reasonable considering what could happen if left unchecked. Better to address the fuel system problems now than deal with a failed catalytic converter or more extensive engine damage later. Will update once I get it sorted. Thanks for helping confirm my suspicions about the regulator being the likely culprit. Knowing someone else had similar symptoms, even if yours turned out to be a simpler fix, gives me more confidence in pursuing this repair path.