Straight answer: on a modern turbocharged BMW, Audi or Mercedes, plan on spark plugs roughly every 60,000 to 90,000 km, and sooner if the car is tuned. Older naturally aspirated engines can stretch closer to 100,000 km or more. The number owners hear most is the "100,000 km" figure, but that was written for a different generation of engine, and it is optimistic for the turbo petrol engines these brands sell now.
Here is why the interval is shorter than you would expect, and what happens if you leave it too long.
What do spark plugs do?
A spark plug ignites the air and fuel mixture in each cylinder. That spark is what makes the engine run, so as the plug wears, the engine runs worse: harder starting, a rougher idle, weaker acceleration and more fuel used. On a turbo engine the plug has to fire under much higher cylinder pressure than an old naturally aspirated one, which is exactly why it wears faster.
How often should spark plugs be changed?
The manufacturer's figure is a starting point, and it varies by engine, but here is what we see hold up in practice on German cars in Melbourne.
Turbo petrol engines (most current BMW, Audi and Mercedes) do best on roughly 60,000 to 90,000 km. High cylinder pressure and direct injection are harder on plugs, and many owners and specialists change them at the earlier end as cheap insurance. BMW itself tightened its guidance over the years, moving turbo engines to shorter intervals than the old standard.
Naturally aspirated engines can genuinely last longer, often up to around 100,000 to 160,000 km on the long-life iridium or platinum plugs these cars use.
Tuned engines are a different conversation. A Stage 1 or Stage 2 tune raises boost and cylinder pressure, which taxes the plugs and coils harder. On a tuned turbo, changing plugs closer to every 30,000 to 40,000 km is normal, and a slightly colder-heat-range plug is often the right call.
Two things shorten the interval regardless of the number: mostly short city trips, and hard or high-load driving. If your car lives in Melbourne stop-start traffic, treat the earlier end of the range as your target.
The warning signs
You do not always get a warning before a plug causes trouble, but when you do, it usually shows up as one of these:
- A rough or lumpy idle, or a noticeable shudder
- Hesitation or a flat spot under acceleration, especially under load
- Harder starting than usual
- A drop in fuel economy
- A misfire code or the check-engine light, and on a bad one, a jolt into limp mode
On a turbo German engine, misfires do not stay subtle. They bring fault codes and, left alone, they put the car into a protective mode. If you feel a misfire, get it looked at rather than driving through it.
The part people forget: coils
Here is the bit that catches owners out. Worn plugs do not just fail on their own. As the electrode wears and the spark gap grows, the ignition coil has to work harder to fire across it, and that extra strain wears the coil out too. Leave plugs in far too long and you often end up replacing coils as well, which turns a modest job into a more expensive one.
There is a second hidden issue on some of these engines. The plugs sit in deep wells in the cylinder head, and a leaking valve cover gasket can let oil seep down into those wells. Oil sitting around a plug and coil causes misfires and, over time, can kill the coil. It is a good reason to have the plug wells checked when the plugs come out, and to fix a valve cover leak before it does damage.
What it costs in Melbourne
Spark plug replacement on a German car typically runs around $300 to $500, depending on the engine and how much has to come off to reach the plugs. On some V8s and larger engines, access is more involved and the labour is higher. Because the plugs live under the coils, and a worn plug can take a coil with it, it is often sensible to inspect the coils at the same time rather than pay the labour twice.
Use the correct plug for your engine. BMW, Audi and Mercedes specify particular plugs (usually Bosch or NGK) with a set heat range and gap for a reason, and a cheap generic plug is a false economy on a turbo engine.
Not sure where your car is at?
If you have bought the car second-hand and the history is patchy, plugs are one of the first things worth checking, because there is no way to know how long the current set has been in. We work on BMW, Audi and Mercedes every day, we stock the correct plugs, and we will tell you honestly whether yours need doing now or have life left in them.
See our BMW, Audi and Mercedes-Benz servicing, browse all our services, or get in touch.
Frequently asked questions
How often should spark plugs be changed?
On a modern turbocharged BMW, Audi or Mercedes, roughly every 60,000 to 90,000 km, and closer to every 30,000 to 40,000 km if the car is tuned. Older naturally aspirated engines can last up to around 100,000 to 160,000 km on long-life plugs.
What do spark plugs do?
They provide the spark that ignites the air and fuel mixture in each cylinder. Without a strong spark the engine misfires, loses power, uses more fuel, and in bad cases will not run properly.
How long do spark plugs last?
It depends on the engine. Turbo German engines are harder on plugs and typically need them at 60,000 to 90,000 km. Naturally aspirated engines last longer. Short-trip city driving and tuning both shorten the life.
How much does it cost to replace spark plugs?
On a German car in Melbourne, usually around $300 to $500 depending on the engine and how hard the plugs are to reach. Larger engines with tricky access cost more, and it is often worth checking the ignition coils at the same time.
Can worn spark plugs damage other parts?
Yes. As a plug wears, the coil works harder to fire across the wider gap, so old plugs can wear out coils too. On some engines, oil leaking into the plug wells from a failing valve cover gasket can also foul plugs and coils.
Should I replace all the spark plugs at once?
Yes. They wear at a similar rate, so replacing them as a set avoids paying the labour twice and keeps the engine running evenly. Replacing a single plug is usually a short-term patch, not a fix.
