Guide Jun 22, 2026 4 min read By Marcus Elite

Audi R8, RS4 & S5 4.2 V8 Cold-Start Misfire: The FSI Injector Failure Owners Mistake for Coils

You fire up your R8, RS4, or S5 on a cold morning and the 4.2 V8 stumbles — a rough, uneven idle, maybe a flashing engine light, and a hesitation that clears once everything warms through. By the time you've reached the end of your street it runs perfectly, which is exactly why this fault is so often misdiagnosed. On the 4.2-litre FSI V8, that cold-start misfire is frequently a failing fuel injector.

Because the symptom fades with heat, owners and shops reach for coil packs and spark plugs first, replace them, and watch the cold stumble return. This guide explains why the FSI injectors fail, how to tell them apart from an ignition fault, and how to source the genuine part.

What the FSI injector does

The 4.2 FSI uses direct injection, spraying fuel straight into each combustion chamber at high pressure rather than into the intake port. That delivers sharper throttle response and efficiency, but it also places the injector tip directly in the harsh, carbon-rich combustion environment. Each injector must atomise fuel precisely on every stroke; when its spray pattern degrades or it leaks, combustion in that cylinder suffers — and it shows up most at cold start, when the engine is least tolerant of a poor burn.

Why they fail

Direct injectors carbon up and wear at the tip, distorting the fine spray pattern they depend on. Internal sealing can also degrade, causing a cylinder to run rich, lean, or inconsistently. Cold start is the moment of truth: a marginal injector that can mask its weakness once warm can't deliver a clean cold burn, so the cylinder misfires until heat and closed-loop fuelling compensate. Age, mileage, and carbon accumulation all push a borderline injector over the edge.

The symptoms owners report

Rough, lumpy idle on cold start that smooths out once warm
Check engine light, sometimes flashing during the cold misfire
Cylinder-specific misfire codes stored in the ECU
Brief hesitation or stumble when accelerating cold
A faint fuel smell at start-up from incomplete combustion
Symptoms that all but vanish at operating temperature

The misdiagnosis to avoid

This is the classic "replaced the coils and plugs, still stumbles" story. Ignition parts are the reflex fix for a misfire, but when the misfire is tied to cold start and a specific cylinder, the injector is the more likely culprit. Confirming which cylinder, and whether the fault follows ignition or fuelling, saves you from buying the same ignition parts twice.

How to confirm it is the injector

Read the misfire codes. A scan tool identifies the exact misfiring cylinder — your starting point.
Test ignition first to exclude it. Swap the coil and plug from the affected cylinder to another; if the misfire stays on the original cylinder, ignition is cleared and the injector is implicated.
Look at fuel trims and timing of the fault. A misfire concentrated at cold start with abnormal cylinder behaviour points to fuelling.
Inspect/flow-test the injector. A specialist can confirm a degraded spray pattern or leak on the suspect injector.

Affected models and part number

Model	Engine	OEM Part
Audi R8 (V8)	4.2L FSI V8	079906036D / 079906036C
Audi RS4 (B7)	4.2L FSI V8	079906036D / 079906036C
Audi S5 (B8)	4.2L FSI V8	079906036D / 079906036C

Injector applications can vary by exact engine code and year, so confirm fitment against your VIN before ordering.

The genuine fix and what it costs

The repair is replacement of the failed injector with fresh seals, and on a high-mileage engine some owners replace the set to restore uniform fuelling across all eight cylinders. Because these are direct injectors working in a carbon-heavy environment, fitting genuine units with correct seals is important for a lasting fix. Our genuine Audi 4.2 FSI V8 injectors (079906036D / 079906036C) fit the R8, RS4, S5, and related applications and are in stock from our UAE and USA warehouses.

What happens if you ignore it

A cold-start misfire dumps unburnt fuel into the exhaust, which over time can overheat and damage the catalytic converters — a far costlier repair than an injector. A cylinder running poorly also stresses the engine and wastes fuel. Fixing the injector early protects the expensive emissions hardware downstream and keeps the V8 crisp.

A cold-start stumble on the 4.2 FSI V8 that clears when warm is the signature of a tired direct injector, not a coil. Confirm the cylinder, rule out ignition, fit the genuine injector, and the V8 idles clean from the first cold crank.