P0174 on Audi

The Q3 across both generations (2015-2023) experiences frequent coolant leaks from plastic thermostat housings and water pump failures. Audi uses plastic impellers in the water pump that crack or disintegrate over time (80,000-120,000 miles), and plastic thermostat housings that crack from heat cycling. Coolant leaks present as a sweet smell under the hood, low coolant warnings, or pink/green puddles under the car. If left unaddressed, leaks cause overheating, warped cylinder heads, and blown head gaskets ($3,000-$5,000 repair). The water pump is timing belt/chain driven, so replacement during timing service saves labor. AudiWorld forums report water pump failures between 60,000-100,000 miles.

Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits on intake valves, the intake manifold, fuel injectors and combustion chambers. Possible results include rough running after cold start, extended cranking, hesitation, rough idle, reduced performance, poor fuel economy or an MIL with misfire, lean-system or air/fuel-ratio-imbalance faults. This does not establish a universal Q5 intake-valve defect, a mandatory walnut-blasting interval or a fixed failure mileage.

The 2.9T twin-turbo V6 in RS4 Avant and RS5 (2018-2023) suffers from carbon buildup on intake valves due to direct injection. Similar to other Audi DI engines, oil vapors from PCV system bake onto valve backs. The twin-turbo setup and high performance accelerate carbon accumulation. Over 60,000 miles, carbon causes rough idle, misfires, and power loss. RS246.com forums recommend walnut blasting every 60,000 miles. The V6 configuration requires removing both intake manifolds, costing $1,000-$1,800. Audi's official recommendation is to drive hard regularly to burn off carbon—not baby the RS.

The Audi S5 (2010-2017) with the 3.0T supercharged V6 has a chronic PCV valve (also called air-oil separator or crankcase vent valve) failure. The PCV valve diaphragm degrades over time, causing oil leaks, check engine lights, rough idle, and a loud squealing/whistling noise from the engine. The 3.0T PCV is located at the rear of the engine near the firewall, making it moderately difficult to access. Many B8/B8.5 S5 owners replace the PCV valve and water pump simultaneously to save on future labor costs, since both are common failure items. Audizine, ShopDAP, and Motor Werke report PCV failure as a near-certainty before 100,000 miles. The repair is moderate in cost but ignoring it leads to vacuum leaks and accelerated oil consumption.

Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits in injectors, combustion chambers, the intake manifold and intake valves, with rough running, hesitation, reduced performance, poor fuel economy or related ECM faults. This does not establish a universal SQ5 intake-valve defect, a mandatory walnut-blasting interval or a fixed mileage. Audi separately warns that unapproved fuel additives are not permitted on 2018-2025 EA839 V6 TFSI SQ5 models because a negative effect on engine life cannot be ruled out.

The Audi TT across all generations (Mk2 2008-2015, Mk3 2016-2023) uses a plastic impeller water pump with a notorious 60,000-mile lifespan. The plastic impeller cracks or disintegrates from heat cycling, causing coolant leaks and overheating. Unlike other Audi models, the TT water pump is timing belt-driven (early 2.0 TFSI) or electric (later models), making it critical to replace during timing belt service. Symptoms include coolant puddles, sweet smell, overheating, and temperature gauge fluctuations. Ignoring a failed water pump causes severe overheating, warped cylinder heads, and blown head gaskets. TTForum.co.uk reports water pump as one of the most common TT failures, with many owners replacing preemptively at 60k miles.