4 Of The Most Reliable Inline-6 Diesel Engines

According to the BBC, the U.S. market for diesel passenger cars "has been historically very small." This is because diesel engines are more expensive and known to produce higher nitrogen oxide emissions, on top of having suffered from poor reliability early on. Still, diesel engines are better for towing and hauling compared to gas, and that is why many pickup trucks and industrial equipment are powered by diesel. 

Diesel, not gasoline, is what the American economy relies on. This was the conclusion of a report by the Engine Technology Forum, which found that diesel moves over 80% of U.S. cargo, powers three-quarters of construction, mining, and farm equipment, and drives critical sectors like defense and emergency services. But what about the diesel cars that are used as daily drivers?

Within the mainstream diesel engine family, the six-cylinder diesel might be the sweet spot when it comes to size. In particular, the inline-six is well-balanced and does not suffer from excessive vibrations, eliminating the need for a balancing shaft. Four of the most reliable inline-six diesel engines out there are the Cummins 6BT, the Toyota 1HZ, the Mercedes‑Benz OM606, and the BMW M57 — and there are plenty of reasons to celebrate them.

The Cummins Million Miles Club is an official Cummins program intended to celebrate engines that have reached one million miles. Among the longest‑mileage entries in this club are vehicles powered by the 5.9‑liter Cummins diesel — also known as the 6BT. The top spot is taken by a 1996 Dodge Ram with over 3,016,789 miles on its 5.9 Cummins, and the next two highest vehicles come very close with the same engine under their hoods.

We have also included this engine in our own reliable diesel engines that just won't quit, largely because it's so simple despite being overbuilt. The 5.9 relies on a block and head made out of iron, forged steel crankshaft and rods, and a durable Bosch P7100 fuel pump. It has none of the modern DPF filters and emissions systems, and that's also why it's so reliable. The 6BT can also be paired with the NV4500 transmission. According to MotorTrend, "the NV4500 is considered to be one of the toughest and most reliable five-speed transmissions ever built."

No engine is perfect, and the 5.9 Cummins can experience fuel pump, exhaust manifold, turbo, and ECM issues. Still, most problems are easy to fix, and with no dreaded DPF or complex emissions systems, the engine remains simple, reliable, and rebuildable throughout its long life. To top it all off, the 5.9 also reacts well to tuning. Safely pushing it to 8000 horsepower on a stock block is possible with proper tuning management.

Toyota has earned its reputation for making some of the most reliable engines thanks to heavy-hitters such as the 1HZ, the 2ZZ-GE, and the 2JZ-GTE. The 1HZ was primarily for 1990s Land Cruiser 70, 80, and 105 Series. As with the 5.9 Cummins, the 1HZ was designed for simplicity, and that earned it an almost cult-like status among Land Cruiser enthusiasts due to how reliable it is. Thanks to a mechanical indirect-injection fuel system with minimal electronics and no forced induction, there aren't many things that can go wrong with it.

Like the 6BT, it also uses a cast‑iron block and head. Paired with a long‑stroke design and sturdy internals, the 1HZ can tackle pretty much everything. Although just about 130 horsepower from a 4.2-liter six-cylinder may seem insufficient, it enabled the 1HZ to never truly reach its limit. Land Cruisers are used all across the world, from remote African deserts to extreme environments in Asia, and they are a symbol of reliability and dependability — with the 1HZ engine providing a large part of that reputation.

Such revered global reliability is also why Land Cruisers have such good resale value. However, the 1HZ can suffer from issues related to its cooling system and cylinder heads. Still, its owners have reported that the engine can also cross the million mile mark. If you take proper care and make sure to maintain it properly, your own 1HZ might be able to reach that mark, too.

Although buying a cheap old Mercedes AMG can be financially unwise, buying an old-school diesel Mercedes from the 1990s can be the exact opposite. As far as Benz diesels are concerned, the OM606 3.0-liter inline six-cylinder is widely regarded as one of the most reliable. It uses a Bosch mechanical fuel injection system, and its timing chain requires doesn't need much maintenance at all. You could find this engine in the E300 W210 Mercedes E-Class, which was a very popular choice for taxi drivers in Europe. That alone is a major indicator of how reliable the OM606 can be.

Thanks to a strong iron block, a forged crankshaft, and chain-driven camshafts, these engines are able to endure hundreds of thousands of miles. Moreover, the OM606's lubrication system and a sturdy cooling system also allow it to endure demanding conditions. According to Engine Finders, a U.K.-based engine sourcing specialist, the OM606 is one of the most reliable Mercedes engines ever, showing alongside the five-cylinder OM617, the M112 V6, and the M119 V8.

However, as with all engines on this list, the OM606 also has its drawbacks. The most common problems include oil leaks, injector issues, and issues with its timing chain's longevity. With that in mind, though, if you do your due diligence and take care of this engine, it's still going to last a long time. Some owners even reported that it could withstand more than 500,000 miles.

The 1998-2013 BMW M57 3.0-liter engine is the predecessor to the complicated BMW quad-turbo diesel B57S. The M57 has long been praised by BMW enthusiasts as a fantastic diesel workhorse, and it has also won numerous international awards thanks to its robustness and performance. However, it's worth keeping in mind that the M57 isn't necessarily a simple engine. This means that its superior reliability is highly dependent on how you maintain it.

According to Visio Racer, "The M57 is one of the most reliable diesels and especially in Europe there are many versions to choose from on the used car market." BMW has used forged steel connecting rods, a forged crankshaft, a chain-driven overhead cam design, and 12 millimeter head bolts that ensure the engine's ability to handle turbocharger boost without failing. Paired with a dependable Bosch fuel pump, the M57 earns a well-deserved spot on this list.

A well-taken care of M57 can go more than 500,000 miles. Still, it can suffer from multiple issues if you neglect it. These include problems associated with the injector, the Exhaust Gas Recirculation valve, and the turbocharger. Other notable problems include intake swirl flaps breaking off and efficiency issues with the high-pressure fuel pump.