BMW's N20 engine: A legacy of innovation and pitfalls

From a market perspective, the N20 engine, introduced in 2011, was intended to replace naturally aspirated six-cylinder engines with moderate power levels, which could no longer comply with emission standards. Consequently, BMW's model range progressively included turbocharged gasoline units—four- and six-cylinder—equipped with direct fuel injection.

From 2011 to 2016, the N20 engine was installed exclusively in BMW models. The trade designation 18i represents a power output of 156 HP, 20i reaches 184 HP, and 28i was designed for the most powerful version, with a power output of 245 HP.

Below is a list of cars with the N20 engine and their designations:

• 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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It's also worth mentioning that the N20 engine has two capacities—1.6 litres and 2.0 litres—but the smaller one is rarely found in Europe. The N20 engine with a capacity of 1.6 litres should not be confused with the 1.6-litre engine installed in Mini brand cars during the same period. This is, in fact, a unit developed with PSA from the Prince family, which I have discussed extensively in the text below.

The N20 incorporated all the ecological "innovations" starting to be used in other gasoline engines at that time, but BMW achieved a new level of sophistication with the engine commercially named TwinPower Turbo.

It features double Vanos, meaning variable valve timing on the intake and exhaust sides, and a twin scroll turbocharger, which has dual channels, allowing it to spool up faster than a traditional one. There is a third-generation Valvetronic system, which allows continuous valve lift regulation, enabling the elimination of a traditional throttle, although the N20 engine has one used only during start-up and warming phases, or if Valvetronic fails.

The start-stop system is intended to enhance fuel savings, and for faster and more controlled temperature management, an electric water pump is used. The N20 is a lightweight design made of aluminum, featuring 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, preparing it for the Euro 6 standard. Among other changes, pistons and the fuel system were modified, though Bosch remained the supplier throughout the production period. The turbocharger was also changed, but only in its control mechanism, switching from vacuum to electric.

Initially, the main and significant concern for car users with this engine was the timing drive. Theoretically, it was a straightforward design with a chain, hydraulic tensioner, two variable phase gears, and several guides.

Unfortunately, after the first oil change, the chain often started making noise. In many cases, it didn't last more than 59,500 km. If the car was primarily driven on highways or serviced contrary to the manufacturer's recommendations (more frequent oil changes), the system lasted longer, but it rarely exceeded 100,000 km. In 2017, users in the USA even filed a class-action lawsuit against BMW.

The main issue was the durability of the parts, but the topic of fuel getting into the oil and uncontrolled pre-ignition, caused by fumes from the crankcase, was also significant.

If the engine operates under reasonably constant load (e.g., on the highway), the system manages somehow, but city driving or idling, as well as frequent starts and stops by the start-stop system, are destructive to the timing drive.

A stretched chain wears down the plastic guides (polycarbonate), and the debris gets into the oil, which not only further degrades it but also causes problems for the tensioners, Valvetronic system, or oil pump module. As if that weren't enough, the original guides were so fragile that they cracked, sometimes immediately blocking the oil flow, not to mention sudden chain loosening.

Over time, specifically in 2015, the manufacturer noticed the problem and improved the quality of the timing drive parts, so cars from this year are considered less problematic in this area. Unfortunately, the durability of the entire mechanism is still poor, as it doesn't result solely from the strength of the parts but also from the engine operating with frequent pre-ignitions. Therefore, it's recommended to replace the chain and all related parts at a maximum every 100,000 km.

After issues with the timing chain, a second problem arose, which was the oil pump, specifically its drive. What engineers devised in the lower part of the engine can be described in various ways—from a technical masterpiece to madness.

Beneath the engine block, there is a mechanism with a balance shaft for the engine, hence the unusual shape of the oil pan, made from plastic. The balance shaft module is connected to the oil pump, which receives drive through a gear just from this shaft. The entire module is driven by a small chain, which gets its drive from the crankshaft.

However, it was not the complex pump and balancing shaft module that turned out to be weak, but its simple drive. Both the tiny wheel at the crankshaft and the thin chain barely lasted 100,000 km. This caused the module to stop, thus the oil pump too, leading to engine seizure.

As if that weren't enough, there is also a hydraulic tensioner in this simple drive, which, operating on old oil, does not properly tension the chain and causes the wear of plastic guides.

The original set of the module drive costs about $211, and a substitute can be bought for half the price. It is necessary to replace this along with the entire timing drive set, which costs between $373 and $623, depending on the manufacturer, for a substitute. The original set is not much more expensive, provided you do not order variable valve timing gears, which have prohibitive prices.

Variable valve timing gears are also problematic. Over time, rattling appears and there is a problem reaching full power. Generally, they should be changed with the chain, but not everyone decides this. While the prices of original parts are frightening, substitutes can be purchased for under $100.

In principle, not only the timing drive of this engine should be replaced every 100,000 km. Everything suggests that at such a mileage, many areas require revision; adhering to this rule means the car will operate long and trouble-free. I will explain which areas are involved.

Engine N20 seizing is not always rapid; often, it's slow. One cause is excessive petrol getting into oil in large quantities. Not only because of poor lubrication and thus excessive loads, which the injectors attempt to compensate with excessive fuel, but also due to a basic issue—faulty sealing of the high-pressure pump. Gasoline leaks directly into the valve cover.

Another critical point is the pump drive in the form of a pusher that connects it to the camshaft's cam. If you hear tapping from this area, it's this pusher that needs replacement. It is recommended to do this, along with the aforementioned sealing, every 100,000 km. Fortunately, the injectors are durable, and do not cause problems.

Additionally, note that if there is excess fuel in the oil, the oil level sensor fails to detect it, often stopping indicating the level altogether. Unfortunately, the designers have abandoned the traditional dipstick in favour of electronics. Thus, users do not receive any information that something is wrong or assume the only problem is the sensor malfunction.

Excess gasoline in the oil results in flammable fumes escaping through the breather into the cylinders, where they burn uncontrollably, causing strong vibrations and carbon buildup. These vibrations certainly contribute to faster timing chain destruction, thus completing the cycle.

Oil leaks are frequent, including from the oil pan, heat exchanger seals, and the valve cover, which often cracks after 100,000 km, more often after about 150,000 km. The cost of a new original is approximately $474, while aftermarket ones can be bought for around $150.

This component is surprisingly complex, extensive, and vital for the operation of interconnected components, so it is worth investing in and buying the original. Add to this a set of gaskets for about $62 plus labour, bringing the total bill to about $623. Now it's easier to understand why these engines are often literally covered in oil.

In extreme cases, oil leaks onto the exhaust manifold, causing unpleasant smells or even smoke, and onto the alternator belt, which, if broken, winds around the pulley. Not just the pulley, but also the entire block sealing ends up needing replacement.

Leaks also occur in the vacuum pump, and oil gets into the vacuum system, but this does not affect braking system operation; instead, it affects the turbocharger. The vacuum pump should be inspected for leaks every approximately 100,000 km. Replacement with an original is very expensive, as parts alone cost about $673-$748.

A very common issue with the N20 engine and its turbocharger is a characteristic ringing, often due to play in the turbo control lever (at the wastegate valve), regardless of vacuum or electric control. Neglecting this area results in slight turbocharger operation issues. Specialists can fix it, though there is no factory repair technology; the entire actuator (approximately $373) can be replaced.

If the engine begins to vibrate strongly, the culprit is usually the Valvetronic system, but it is not faulty by itself. The actual cause is a weak battery, which, due to the electronic Valvetronic control, requires full efficiency to set all mechanisms relative to the crankshaft position before starting the engine. If it is weak, this does not happen, resulting in vibrations. If vibrations occur during the ride, it's recommended to stop, turn off the engine, and restart after several seconds. If the vibrations cease, it's clear that the battery needs charging or replacement.

The reliability of the entire Valvetronic system is estimated to last around 300,000 km. The first to wear out is the actuator with a worm gear, costing about $498 (original). Fortunately, replacements can be found for about $174. Over time, the eccentric shaft wears out, costing over $997. However, such a situation affects poorly serviced cars with mileage over 350,000-400,000 km.

After purchasing a car with higher mileage, it's wise to proactively replace the electric water pump, especially for frequent long highway drives. Although the failure is not common, it manifests as intensive fan operation. If it occurs, you should virtually stop driving. Fortunately, substitutes aren't excessively pricey (~$299), while an original will cost three times more.

Engines post-tuning are very risky, particularly when simple chiptuning seeks to obtain the power of a 28i from a 20i. These versions differ, among other aspects, in piston shape, hence compression ratio. Such tuning not only exacerbates the gasoline-in-oil issue but also results in severe failures, such as seizing or cracking of the head. It likewise increases the risk of injector and timing drive failures.

In summary, the N20 engine is not particularly successful, with its biggest issues being the timing drive and oil pump. It is believed that from the 2015 production year, these problems no longer occur, yet it is still recommended, out of caution, to replace the timing drive and review described areas every 100,000 km. Frequent oil changes are also advisable, as workshops with extensive experience confirm that with meticulous service, many customers have driven trouble-free for over 200,000 and even about 300,000 km.

So is it worth buying a car with this engine? It offers good power, dynamics, and reasonable fuel consumption, so if you purchase a well-maintained car and understand that the engine requires ongoing attention, which isn’t cheap, there’s no substantial deterrent. This also applies to older model years where the timing drive was replaced with an improved version, although replacing it soon after purchase along with reviewing all critical points is a prudent recommendation. Unfortunately, this won’t be inexpensive.