Rough-running engine and a flashing check engine light: DTC P0300-P0308 on the BMW 3 Series (E90/F30)

At a stoplight, the steering wheel of a BMW 3 Series shakes gently, like an old diesel. Pulling away, the car stumbles in the midrange. A few days later the check engine light comes on, and one afternoon on the highway it starts flashing. On E90 and F30 3 Series cars, this pattern often leads to one family of codes: misfires, P0300 to P0308.

The good news: misfires on BMW gasoline engines of this generation can usually be tracked down step by step. The bad news: putting off the repair while the light flashes can turn a simple coil problem into a damaged catalytic converter. Here is how to work through it.

P0300 means a random misfire, or misfires in several cylinders at once (Random/Multiple Cylinder Misfire Detected). P0301 to P0308 point to one cylinder. The last digit is the cylinder's number by its physical position in the engine, not by firing order, so P0304 means cylinder 4.

The engine computer does not hear combustion. It measures crankshaft speed. A cylinder that fails to fire makes the crankshaft slow briefly on its power stroke, and the camshaft position sensor tells the computer which cylinder should be firing. When those slowdowns pass a set threshold, the code is stored.

A steady check engine light means a misfire was detected. A flashing light means the misfire is bad enough to damage the catalytic converter: unburned gasoline is pushed into the exhaust and burns inside the converter.

E90 and F30 3 Series cars were built with several engines. Which engine your car has depends on the variant and model year, so confirm it from the engine code. This article covers the two gasoline engines for which we have sources. The N52, a naturally aspirated inline six, is used in the 325i, 328i and 330i, among others. The N20 (and its sibling, the N26), a turbocharged direct-injection four-cylinder, is used in the 2012–2016 F30 328i. Both engines have one coil and one spark plug per cylinder, so a misfire in one cylinder can often be traced to a single set of parts.

On the four-cylinder N20/N26, the misfire codes you will see run from P0300 to P0304.

1. Aging ignition coils. Thousands of firing cycles wear out coils and spark plugs and degrade their performance, and this wear is the most common cause of misfires. For the N52, FCP Euro recommends replacing the spark plugs every three years and the coils about every 100,000 miles as preventive maintenance. For the N20/N26, they suggest about 100,000 miles for both, and half that if the engine has been modified for more power.

2. Worn spark plugs. A worn spark plug weakens the spark so the fuel mixture does not burn completely. So ask about the spark plug service history before blaming a coil.

3. Oil in the spark plug holes (typical of the N52). The N52 valve cover has two seals: the gasket around the cover and the seals on the spark plug tubes. The rubber hardens and shrinks with heat cycles, usually starting to show at around 80,000–120,000 km. A leaking spark plug tube seal lets oil seep into the spark plug well, where it soaks the coil boot and the spark plug and causes a misfire. A leaking perimeter gasket usually shows up as a burning oil smell, because oil drips onto the exhaust manifold. The N52 valve cover is composite plastic and can crack or warp. If the cover itself is damaged, a new gasket alone will not stop the leak.

4. Unburned gasoline (N20). According to FCP Euro, a lost spark can leave raw gasoline in the cylinder. That gasoline can wash the thin oil film off the cylinder wall, so the piston rings can score the wall. A scored cylinder wall eventually makes the engine burn oil. That is why a misfire on the N20 should not be put off. A clogged injector is also a common cause of misfire on all engines.

5. Other causes. Vacuum leaks, low fuel pressure and mechanical problems such as low compression or a loose timing chain can also cause P0300, especially if the misfire moves from cylinder to cylinder.

Misfires rarely come alone. Besides shaking at idle, 3 Series owners usually report weak, stumbling acceleration, harder starting, worse fuel economy and sometimes popping from the exhaust. Where cars face an emissions test, a misfire will also fail it.

A few code patterns are worth knowing, so you do not head down the wrong path:

• Lean codes alongside. When a cylinder fails to fire, its unused oxygen goes out the exhaust. The oxygen sensor reads that as a lean mixture, although the real problem is failed combustion. Fix the misfire before chasing mixture codes.
• P0300 with no cylinder code. Occasional misfires that wander between cylinders are sometimes logged only as P0300. That points to engine-wide causes, such as evenly worn spark plugs, a vacuum leak or fuel supply, rather than one coil.
• The converter suffers. A severe misfire left alone can damage the catalytic converter, which can set other codes later.

A good habit before taking anything apart: check for a service bulletin or repair campaign for your engine, and use a scan tool to view the per-cylinder misfire counters. They show which cylinder misfires most, even before a permanent code is stored.

1. Watch the light. Flashing means ease off and find a safe place to stop. Steady means you have time to book an inspection.
2. Read and record the code. A P030x with a cylinder number is far easier to trace than P0300. Save the freeze frame data before anyone clears the code.
3. Check the service history. When were the spark plugs last changed? If they are overdue, replace them before blaming a coil.
4. Swap coils between cylinders. Move the coil from the misfiring cylinder to the next one, clear the code and drive. If the code follows, for example P0303 becomes P0302, that coil is bad.
5. Look into the spark plug well. With the coil out, check for oil in the spark plug hole and on the coil boot. Oil there means a leaking tube seal, and a new coil will soon fail again unless the leak is fixed.
6. Swap the spark plugs. If the misfire does not follow the coil, do the same with the spark plugs.
7. Injectors and compression (workshop job). If the misfire stays in the same cylinder after swapping coils and spark plugs, the injector circuit is tested next. If that checks out, test compression and look for vacuum leaks.

The repair follows the swap test. A coil that is proven bad is replaced. For the N52, FCP Euro recommends new spark plugs every three years and coils as preventive maintenance. So on cars with an unknown history, plugs and coils are often done together. Spark plugs go in with a special socket so the threads are not damaged. For the N20, FCP Euro lists a torque of 23 Nm.

If an N52 has oil in the spark plug wells, the job is a new valve cover gasket and spark plug tube seals, plus any coils and spark plugs that sat in oil. If the plastic cover is cracked or warped, the cover itself is replaced. The oil filter housing gasket nearby is often checked as well: it is a companion leak point and usually comes due at about the same mileage.

An injector shown to leak or clog is replaced. Mechanical problems such as low compression are a bigger job that needs its own diagnosis. After the fix, the code is cleared with a scan tool and the car is retested to confirm the misfire counter stays put.

Get to a workshop right away if the check engine light flashes, the engine shakes hard or power drops sharply. Do not take a long trip like this: a damaged converter means a much bigger job.

If the light is steady and the symptoms are mild, you can book an inspection soon. A workshop with BMW diagnostic equipment can read manufacturer codes, view per-cylinder misfire counters and test injectors, which a generic code reader cannot do.