BMW's N20 engine: Reliability concerns or overblown criticism?

Launched in 2011, the N20 engine was intended to replace naturally aspirated six-cylinder engines that couldn't meet new emission standards. BMW's lineup began featuring turbocharged four- and six-cylinder gasoline engines with direct fuel injection.

The N20 engine, which was exclusive to the brand and used in numerous BMW models from 2011 to 2016, has a trade designation of 18i, which stands for 156 HP 20i, which produces 184 HP; and 28i, which is the most powerful, with 245 HP.

Here's a list of cars equipped with the N20 engine and their nomenclature:

• Z4 sDrive18i / 20i / 28i (E89),
• X1 xDrive / sDrive20i / 28i (E84),
• X3 xDrive20i / 28i (F25),
• X3 sDrive18i (F25),
• X4 xDrive28i (F26),
• X5 xDrive40e (F15),
• 125i (F20),
• 220i / 228i (F22),
• 320i / 328i (F30),
• 320i / 328i GT (F34),
• 420i / 428i (F32),
• 520i / 520Li / 525Li / 528i (F10).

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The N20 motor has two capacities: 1.6 liters and 2.0 liters, but the smaller one is rarely found in Europe. It's important not to confuse the N20's 1.6-liter engine with that of the Mini brand, developed with PSA and part of the Prince family, which I'll discuss later.

The N20 featured all the ecological "innovations" becoming standard in gasoline engines at the time, but BMW set a new bar with their TwinPower Turbo engine.

This engine includes double Vanos, meaning variable valve timing on both intake and exhaust sides, and a twin scroll turbocharger that accelerates faster due to its dual channels. The third-generation Valvetronic system allows continuous valve lift regulation, eliminating a traditional throttle, though the N20 has one for specific purposes like startup.

The start-stop system helps save fuel, and an electric water pump ensures optimal temperature maintenance. The lightweight aluminum N20 design includes two camshafts in the head, a chain-driven timing system, and an additional chain driving the oil pump.

The N20 engine underwent technical changes in mid-2013, in preparation for the Euro 6 standard. Modifications included pistons and the fuel system, although Bosch remained the supplier. The control of the turbocharger also changed from vacuum to electric.

Initially, the primary concern was the timing drive. Theoretically simple, with a chain, hydraulic tensioner, two variable phase gears, and several guides, it often made noise after the first oil change. Many couldn't exceed 37,000 miles. Highway driving or more frequent oil changes, contrary to manufacturer recommendations, extended its life to rarely over 62,000 miles. A 2017 class-action lawsuit in the USA highlighted the issue.

The main issue was part durability, compounded by fuel contamination in oil and uncontrolled pre-ignition, caused by fumes from the crankcase.

At steady highway speeds, the system copes, but city driving, idling, and frequent stops stress the timing drive. A stretched chain degrades the plastic guides, with debris causing issues in the tensioners, Valvetronic system, and oil pump module. Additionally, fragile guides could crack, blocking oil flow and causing sudden chain loosening.

In 2015, BMW addressed this with improved parts quality. Though considered less problematic, the entire mechanism's durability remains an issue due to frequent pre-ignitions. Thus, it's recommended to replace the chain and related parts every 62,000 miles.

Following timing chain issues, the oil pump drive's complexity caused problems. While the pump and balancing shaft module are intricate, the simple drive is weak. The small crankshaft wheel and thin chain often wore out by 62,000 miles, stopping the module and the oil pump, seizing the engine. A hydraulic tensioner in the drive also contributed by failing to tension the chain properly, wearing the plastic guides.

The original module set costs 850 PLN (~$211) while substitutes are half. Replacement should coincide with the full timing drive set, costing 1500 to 2500 PLN (~$373 to ~$623), depending on the manufacturer. The original set costs a little more if variable valve timing gears are excluded due to their high prices.

Valve timing gears also cause issues like rattling and power loss. They're generally replaced with the chain, though some avoid this due to the intimidating original parts prices. Substitutes, however, are under $100.

It's not just the timing drive that needs replacing every 62,000 miles. Following this rule potentially extends trouble-free operation, and I'll cover which components should be checked.

N20 engine seizing isn't always rapid; it's often gradual. Gasoline in the oil, due to poor lubrication and injector compensation by excess fuel, is a root cause. Leaky high-pressure pump seals can leak gasoline into the valve cover, exacerbating the issue.

The pump drive's camshaft pusher can degrade, causing tapping. This pusher, along with sealing, should be replaced every 62,000 miles. Luckily, the injectors are durable and pose no significant problems.

Notably, excess fuel in the oil often goes undetected due to a faulty oil level sensor. Without traditional dipsticks, users may not realize a problem, mistaking sensor malfunctions for the issue.

Excess gasoline produces flammable fumes that burn uncontrollably in the cylinders, leading to strong vibrations and carbon buildup. These vibrations contribute to faster timing chain wear, forming a cyclical problem.

Oil leaks are common, especially from the oil pan, heat exchanger seals, and the valve cover, which frequently cracks after 62,000 miles and more often after about 93,000 miles. An original replacement costs around 1900 PLN (~$474), while aftermarket options are about 600 PLN (~$150).

This component is surprisingly advanced, integral to the operation of interconnected parts, making the original worth the extra cost. A gasket set costs about 250 PLN (~$62), plus labor, totaling 2500 PLN (~$623), explaining why these engines are often oil-soaked.

Excessive leakage can lead to oil onto the exhaust manifold, causing smoke, or onto the alternator belt, which once broken, wraps around the pulley, requiring full block sealing and pulley replacement.

Vacuum pump leaks are also an issue, with oil getting into the vacuum system. This doesn't affect braking but impacts the turbocharger. The vacuum pump requires inspection every 62,000 miles, and replacing it with an original is costly, with parts alone at 2700–3000 PLN (~$673-$748).

A common N20 engine and turbocharger problem is a characteristic ringing, often from play in the turbo control lever at the wastegate valve, whether vacuum or electric. Ignoring this leads to turbocharger issues. Specialists can fix it, though no factory repair exists; the entire actuator (~1500 PLN ~ $373) may need replacement.

If the engine vibrates strongly, the Valvetronic system is usually to blame, although often it's not defective itself, but a weak battery. Electronic Valvetronic control requires a fully efficient battery to set mechanisms relative to the crankshaft before starting. If weak, vibrations result. If vibrations occur while driving, stop the engine and restart. If they cease, the battery likely needs charging or replacement.

The Valvetronic system's reliability is around 186,000 miles. The actuator with worm gear wears first, costing about 2000 PLN (~$498) for the original. Fortunately, alternatives cost about 700 PLN (~$174). Over time, the eccentric shaft wears out, costing over 4000 PLN (~$997), typically affecting poorly maintained cars over 217,000-248,000 miles.

After buying a high-mileage car, it's smart to proactively replace the electric water pump, especially for highway driving. Although not a common failure, symptoms include intensive fan operation, and driving should cease immediately if detected. Substitutes are ~$299, while originals are thrice the price.

Tuning these engines is very risky, especially through simple chiptuning to boost a 20i to a 28i. These versions differ in piston shape and compression ratio. Such tuning worsens gasoline-oil contamination and brings severe failures like seizing or head cracks, as well as increased injector and timing drive problems.

In summary, the N20 engine isn't very successful, facing major issues with timing drives and oil pumps. It’s believed that after 2015, these problems diminished, but regular maintenance, checking key areas every 62,000 miles, and frequent oil changes remain crucial. Experienced workshops affirm that with diligent care, some customers have driven trouble-free beyond 124,000 and up to 186,000 miles.

So, is it worth buying a car with this engine? If you find a well-maintained car and recognize the engine's need for ongoing maintenance, nothing prevents you from proceeding. This applies even to older models if the timing drive was upgraded. Still, consider replacing it promptly after purchase, along with addressing critical aspects. Unfortunately, this won’t be inexpensive.