Audi 4.2 V8 FSI: Problems, Rear Timing Chains and What to Check

The 4.2 FSI is a genuinely good engine - a naturally aspirated, free-revving V8 that most owners are glad they bought - and the single most important thing to understand about it has nothing to do with reliability. It is chain driven, with no cambelt, and in the front-engined cars the chain drive sits at the flywheel end of the block against the gearbox, so any work on the chains, guides, tensioners or camshaft adjusters means the engine or the gearbox comes out. That one fact dominates the ownership case. It also makes identifying your exact engine the first job rather than an afterthought, because other Audi V8s wearing a 4.2 badge are built differently - and some of them genuinely do have a cambelt. After that, the recurring themes are carbon build-up on the intake valves, a cold-start rattle from the cam adjusters or chain drive, and oil consumption that varies a great deal from car to car.

Which cars got it, and how to tell it from the other 4.2 V8s

The 4.2 FSI grew out of the 40-valve port-injected 4.2 that came before it and shares a fair amount of hardware with it, so calling them unrelated engines overstates the case. What changed are exactly the things an owner cares about: direct injection instead of port injection, four valves per cylinder instead of five, and a camshaft drive relocated to the flywheel end of the block.

Cars that used the 4.2 FSI include the B7 RS4 (engine code BNS), the R8 4.2 (BYH), the B8 S5 coupe (CAUA), the B8 RS5 (CFSA), and the 4.2 FSI versions of the A6, A8 and Q7, plus the Volkswagen Touareg, which carry codes such as BAR and BVJ. Do not go by registration year alone. The C6 A6 and D3 A8 were sold with the older port-injected 40-valve 4.2 first and the FSI later, so two cars of the same age wearing the same badge can have different engines in them.

Valve count on its own will not identify the engine, and this catches people out. The pre-FSI 4.2 was built both as a 32-valve, four-valve-per-cylinder engine in the 1990s and as a 40-valve, five-valve-per-cylinder engine after that. A 32-valve 4.2 can therefore be an early port-injected engine or a modern FSI. The only reliable answer is the engine code, read off the block, taken from the service record, or pulled from a diagnostic scan of the engine ECU.

Aspiration is worth being precise about, because Audi's V8 story either side of this engine is full of forced induction. Every 4.2 FSI is naturally aspirated: no turbocharger, no supercharger. The forced-induction 4.2 V8 people usually have in mind is the earlier port-injected twin-turbo in the C5 RS6, engine code BCY. The supercharged 3.0 TFSI V6 that replaced the V8 in the facelift B8 S5 is a different engine entirely, as is the later 4.0 TFSI twin-turbo V8, and the 4.2 TDI is a diesel that shares nothing with any of them.

• Read the engine code, not the valve count. A 32-valve 4.2 might be an early port-injected engine or the FSI, and only the code separates them.
• Take the code from the block, the service documentation or a diagnostic read of the engine ECU rather than assuming it from the year.
• B8 S5 coupe only: early cars are the 4.2 V8, facelift cars are the supercharged 3.0 V6. The S5 Cabriolet and Sportback were never offered with the V8.
• The B7 RS4 and B8 RS5 were only ever this engine. The A6, A8, Q7 and Touareg were sold with more than one 4.2, so those need checking individually.

The timing arrangement: chains at the flywheel end, with caveats

The 4.2 FSI has no cambelt. Its camshafts are driven from the flywheel side of the block by a multi-stage roller chain drive. That is good news, but it is not the same as maintenance-free, and it is emphatically not a statement you can extend to every Audi V8 badged 4.2.

Some earlier 4.2 V8s do have a cambelt, and this matters because people cross-shop them. The earlier engine series - the 1990s 32-valve cars and the first five-valve 40-valve engines, including the C5 RS6's twin-turbo BCY - use a hybrid arrangement: a toothed belt from the crankshaft, with short chains linking the camshafts on each bank. On those engines the belt is a genuine interval-based service item on an interference design, and skipping it is how they get destroyed. The later 40-valve engines, the ones in cars like the B6 and B7 S4, the D3 A8 and the Phaeton, moved to a full chain drive at the front of the engine. So whether your 4.2 V8 needs a belt is a question about the engine code, not about the badge, the year or what someone told you about Audi V8s in general.

On the 4.2 FSI itself the chain drive sits at the flywheel end, at the back of the block where it meets the bellhousing. To reach the chains, guides, tensioners or cam adjusters, either the engine comes out or the gearbox comes off, depending on the car and the workshop's preference. Audi carried the same basic architecture over to the 5.2 V10 FSI, which was developed directly from this V8, so it is a design choice rather than a defect - but it is a design choice you pay for in labour if the timing gear ever needs attention.

One caveat on the R8. It is the same engine family, but Audi repackaged it for the mid-engine installation, and published descriptions of where the camshaft drive ends up on the R8 do not agree with one another. If you are buying or servicing an R8 4.2, get that answer from R8-specific workshop documentation or a marque specialist rather than assuming the front-engined layout carries across.

Treat all of these as interference engines. If a chain jumps a tooth, the valves and pistons meet and the repair stops being about chains. That is why a noise you might shrug off on another engine deserves proper investigation here.

The weak points, roughly in the order they bite

Taken as a whole the 4.2 FSI has a sound reputation, and it is not a chain-eater in the way some smaller VAG engines of the same era became notorious for being. What follows is a list of what to check, not a list of what will certainly happen.

• Carbon on the intake valves. Direct injection sprays fuel straight into the cylinder, so nothing washes the backs of the valves. Deposits build over years and miles and show as a lumpy cold idle, misfires, hesitation and a slow drift away from the engine's usual crispness. The intake ports on this engine carry a divided, tumble-inducing arrangement that the same deposits can foul. The fix is mechanical cleaning, typically walnut or media blasting with the intake off, and on a V8 that means both banks. Treat it as a maintenance item, not a failure.
• Cam adjuster and chain noise. A short rattle from the back of the engine on a cold start is the classic symptom. It can be a camshaft adjuster, a tensioner that has bled down overnight, or genuine chain and guide wear - telling them apart needs a scan and someone who knows the engine. A rattle that persists, lengthens over time, or arrives alongside a timing correlation code should stop you driving the car.
• Oil consumption. Some examples use barely any; others use a noticeable amount between services. Piston ring and valve stem seal wear and the crankcase breathing system are the usual contributors. Consumption by itself is not an emergency, but running low on an engine whose timing gear is behind the gearbox is how a modest problem becomes a very expensive one.
• Secondary air injection. The system that pumps air into the exhaust for a short period after a cold start clogs up, or its pump and valves fail. It usually shows as a warning light and an emissions test failure rather than anything you can feel.
• Coils, plugs and injectors. Eight of everything. A single misfiring cylinder on an engine this size can feel like almost nothing at first, a faint vibration at idle, long before it becomes obvious.
• The high-pressure fuel system. Direct injection means pumps, sensors and a rail running at pressures the old port-injected engine never saw. It is extra hardware that a pre-purchase inspection should look at, and fuel pressure faults typically show up as hesitation, hard starting or a limp-home mode rather than as a noise.

Oil: the thing that decides how this engine ages

Use the oil specification Audi lists for your engine code, change it on time, and be aware that many specialists prefer shorter intervals on these engines than a long-life schedule allows. No figures appear in this guide on purpose - grade, capacity and interval vary by engine code, market and service regime, so take them from the documentation for your specific car.

Check the level regularly rather than waiting for a warning. Some cars in this family have a conventional dipstick and some have only an electronic level sensor read from the driver's display, so find out which yours has before hunting under the bonnet for a dipstick that is not there. Electronic sensors can also fail, which is worth remembering if a reading suddenly looks unlikely.

What to check on a test drive or pre-purchase inspection

The cold start tells you more than the rest of the viewing put together, and it is the one thing a seller can most easily deny you. Arrange to be there before the engine has been run.

• Insist on a genuinely cold start and listen from the rear of the engine, near the bulkhead, for a rattle in the first few seconds.
• Watch the first minute of idle. Carbon and misfire problems are worst when cold and often clear up once warm.
• Check the oil level yourself on level ground and ask directly how much oil it uses between services.
• Have it scanned before the drive, and check whether the fault memory has recently been cleared. Readiness monitors that have not re-set can have innocent explanations, such as a battery disconnect, but at a viewing they are worth asking about rather than ignoring.
• Look for oil weeping at the back of the engine around the bellhousing. A leak there can mean the same access problem as a chain job.
• Drive it through the full rev range under load, listening for misfire and feeling for hesitation.
• Weigh service history and oil-change discipline more heavily than mileage on this engine.
• Confirm which 4.2 you are actually looking at before you agree a price, particularly on an A6, A8, Q7 or Touareg where both the older engine and the FSI were sold.
• Budget separately for model-specific items such as B7 RS4 Dynamic Ride Control, S5 clutch or gearbox work, or Q7 air suspension. These are car costs rather than engine costs, but they are real.

The fault codes it commonly throws

Two numbering systems are in play. Generic OBD-II P-codes are what a basic reader shows; VAG's own five-digit codes carry more detail, and much of what is genuinely useful here - freeze-frame data, per-bank measuring blocks, adaptation values and per-cylinder misfire counters - sits behind a VAG-specific tool. VAGPULSE, which is Windows software plus a cable, scans every module in the car, shows live measuring blocks and handles coding and adaptation, which is what you want when you are trying to separate a cam adjuster fault from real chain wear, or watch misfire counts build cylinder by cylinder at cold idle.

A stored code tells you where to look, not what to replace. On an engine whose timing gear sits behind the gearbox, that distinction is worth serious money.

• P0016, P0017, P0018, P0019 - crankshaft and camshaft position correlation, per bank and per camshaft. The timing family. Take these seriously on this engine.
• P0011 and P0021 - camshaft timing over-advanced or system performance, bank 1 and bank 2. Adjusters, adjuster solenoids or the oil supply to them.
• P0300 with P0301 to P0308 - random and per-cylinder misfires. Classic carbon, coil or plug territory.
• P0171 and P0174, or P2187 and P2189 at idle - system too lean, bank 1 and bank 2. Air leaks, crankcase breathing, fuelling.
• P0087 - fuel rail pressure too low, on the high-pressure side.
• P0491 and P0492 - secondary air injection insufficient flow, bank 1 and bank 2.
• P0521 and P0524 - oil pressure sensor performance and engine oil pressure too low. Treat as urgent, not advisory.

When to stop driving immediately

Most faults on this engine give you time to book it in. These do not.

• A red oil pressure warning, usually an oil can symbol with STOP: pull over safely and switch off. Do not drive it to a garage, and do not drive it home. Oil pressure loss ruins an engine in a very short distance, and the damage is often done before you notice any change in how the car sounds.
• A loud or persistent rattle from the back of the engine, particularly with a timing correlation code stored: stop and have it recovered. A chain that jumps on an interference engine bends valves, and it can happen without warning.
• A coolant or temperature warning: stop and let it cool rather than pressing on.
• A flashing engine management light: that means unburnt fuel is reaching the catalyst. Keep driving and you can add catalytic converters to the bill.
• An oil level below minimum: top up before you move, then find out where it went.

Common questions