P0174 on Audi

The PCV system can fail, causing rough idle, boost leaks, and increased oil consumption. The diaphragm in the valve cover can tear. This is a common issue on earlier 2.0T engines.

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.

Both the supercharged 3.0 TFSI (4L) and the EA839 3.0 TFSI (4M) are direct-injected, so no fuel ever washes the back of the intake valves. Oil vapor drawn in through the PCV system plus EGR soot bake onto the valve stems and seats, restricting airflow and preventing the valves from sealing. Deposits become noticeable around 60,000-80,000 miles - sooner on cars used for short trips - and show up as a rough cold idle, hesitation, reduced power under load, and eventually random or cylinder-specific misfires and lean codes. It is a maintenance item on these engines, not a defect, but it is the single most common driveability complaint owners bring to independent shops.

The supercharged 3.0 TFSI's crankcase ventilation module (PCV / air-oil separator) uses an internal diaphragm that hardens and tears. When it does, the engine pulls unmetered air through the crankcase, gives up its ability to control crankcase pressure, and starts drawing oil into the intake. Owners typically see it between roughly 60,000 and 80,000 miles. Symptoms overlap with several other faults, which is why it is so often misdiagnosed - the tell is a rough or hunting idle combined with rising oil consumption and lean/crankcase-ventilation fault codes. Excess crankcase pressure also pushes oil past the rear main and cam seals, so a neglected PCV can create leaks that look like a much bigger problem.

On the C7/C7.5 S7's 4.0T TFSI (EA824) V8, the PCV valve integrated into the oil separator is a high-failure item. The diaphragm fails and the crankcase ventilation system loses its ability to regulate pressure, leading to elevated oil consumption (often 1 quart per ~500-1000 miles), a vacuum whistle or knocking noise, oil pulled into the intake, and frequently a damaged rear main seal from over-pressurization. Owners and German motor-talk threads on the S7 4.0 TFSI specifically list the Ölabscheider/PCV and the breather hose as routine replacement items, typically appearing between 60,000-100,000 miles.

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.