From failure to fame: The evolution of Audi's 3.0 TDI engine

Throughout its history, the Volkswagen Group has had several missteps when it comes to engine design, but in my view, none were as significant as the 2.5 TDI V6. It marked the start of a series of failing engines - like the 2.0 TDI, 1.4 TFSI, or 1.8 TFSI. Asserting that the 2.5 TDI was the worst engine - in either the sheer number of faults or the cost to repair them - isn't without basis.

Let's start with two fundamental facts. Firstly, the 2.5 TDI was the brainchild of Audi. Secondly, the 2.5 TDI five-cylinder and the 2.5 TDI V6 share little more than their origins – both are Audi engines.

The V6 2.5 TDI engine landed on the market in 1997, launching with the Audi A6 C5, a brand new model generation. Its predecessor, marked with the C4, had a five-cylinder 2.5. Shortly after its debut, the engine was used in all other models with longitudinally installed engines, before the year 2000, like the Audi A4, Audi A8, Volkswagen Passat, and Skoda Superb.

When first released, the engine generated 150hp and 310Nm of torque. In 1999, the 180 horsepower variant was launched, followed by modernized 155 and 180hp units in 2001 (with the Audi A6 facelift). The following year, the power was increased from 155 to 163hp, with no significant technical changes. In 2003, however, another modernization occurred, which can be classed as the second generation of the 2.5 TDI engine.

The next development ushered in a new 3.0 TDI unit, based on the last generation of the 2.5 TDI engine. Launched in 2004, it continues to garner positive reviews. The 2.5 TDI was gradually phased out with each model's generation shift. The first-generation Škoda Superb was the last car to sport a 2.5 TDI under its bonnet, bringing the era to a definitive end in 2007.

From a mechanic's perspective, the 2.5 TDI engine is quite technically advanced compared to the 1.9 or 2.0 TDI. It boasts six cylinders, meaning two cylinder heads. The block is made of iron, the heads of aluminum, with two camshafts in each. The timing drive is powered by two gear belts.

The intake valve camshafts are driven by a lengthy gear belt, while the exhaust valve camshafts are connected by a gear drive to the first ones. An additional belt drives the injection pump, positioned in front between the heads. The oil pump is located in the sump, powered by a chain that also drives the balance shaft.

The Bosch VP44 injection pump supplies fuel to the injectors and is responsible for the entire injection system, unlike the contemporary 1.9 TDI. It wasn't until the end of the engine production that injectors were introduced. In its successor, the 3.0 TDI (and later, the 2.7 TDI), the Common Rail injection system was adopted immediately.

Any discussion about the 2.5 TDI engine and its problems must involve the engine codes. Generally, these engines fall into two generations: the first marked by a three-letter code beginning with 'A' (e.g., AKN), and the second with a code starting with 'B' (e.g., BAU). Theoretically, these codes also pertain to the model years, but unfortunately, it's not as straightforward as that. So, for clarity's sake, I'll discuss this section according to engine codes and model years. I'll also refer to the four most popular models with a 2.5 TDI.

Initially, engines with codes AFB and AKN were introduced, common to both Audi and Volkswagen. Of course, these are quite old engines today, often heavily used and likely repaired more than once. Typical problems for these engines include:

• Injector system malfunctions (pump control, pump wear, injector failures),
• Wear of timing system parts, including camshafts,
• Damage to hydraulic lifters,
• Lubrication system malfunctions (pump, valve cover/crankcase ventilation),
• Oil leaks.

These are the most serious and typical problems for this unit, but that doesn't mean other issues aren't present, typical of many diesel engines, like malfunctions of the EGR valve, turbocharger, or airflow sensor.

The biggest issue with AFB and AKN series engines on the market is the number of repairs they have already undergone, often done unprofessionally and using second-hand spare parts. The condition of these engines is generally poor, and they are nearing the end of their lifespan.

Buying a car with these units means either investing several thousands of dollars into their renovation, or using them until they break down for good. Don't expect to make a purchase, fix a thing or two, and have everything run smoothly. That might work with a 1.9 TDI, but not the 2.5 TDI.

For the 1999 model year (only in the Audi A6), a 180hp unit coded AKE was introduced. Alas, apart from increased power and noticeably better dynamics, this engine showed no great improvements over its predecessor. While issues with lubrication were addressed and the airflow sensor improved, it had no impact on the condition of the timing elements, which continued to deteriorate; the injection system also wore out quickly.

Typical malfunctions of the AKE engine are:

• Injector system malfunctions (pump control, pump wear, injector failures),
• Wear of timing system parts, including camshafts,
• Damage to hydraulic lifters.

In the 2001 model year vehicles (Audi A6 and Audi A4 B6), the AKE engine was modified, and a similar engine marked as AYM (Audi A4 B6, Audi A6, and Skoda Superb) producing 155hp was also released.

A significant change in both units was the improvement of the injection pump control system and the elimination of pump wear issues. Subsequently, the operation of the injectors also improved. Unfortunately, the wear of the timing system parts remained problematic.

Typical malfunctions of AKE (II) and AYM engines:

• Wear of timing system parts, including camshafts,
• Damage to hydraulic lifters.

Did these changes have a positive impact on the engine? Yes. But can they be considered good? Not by a long shot. To enjoy these engines after purchase, you need to check the condition of all the elements located in the cylinder head, ideally replace them, clean the lubrication system, replace the complete timing drive, and all accessory drive elements. However, you may not have enough funds for this, considering this might cost around $10,000.

Technically, the BFC-marked engine is merely a preview of the 2.5 TDI's second generation, but in practice, it's nothing more than a slightly modernized AYM. Its power was increased from 155hp to 163hp and was used from the 2002 model year in the Audi A6 and A4 B6. Technical improvements only rectified the control of the injection system, which theoretically should have prolonged the durability of pumps and injectors. But, from a present-day perspective, the engine's condition matters more than the engine code.

Typical malfunctions of BFC:

• Wear of timing system parts, including camshafts,
• Damage to hydraulic lifters.

To summarize, over time, problems with the lubrication and supply systems in the 2.5 TDI engines were rectified, but the weak timing components, such as the vulnerable camshafts, non-durable hydraulic lifters, and wearing valve pins, remained the Achilles' heel.

Starting from the 2003 model year, a new generation of engines with a 'B' prefixed code was used across all VW Group brands. The Audi A4 and A6 had the BCZ and BDG (163hp), as well as the BDH and BAU (180hp). The Passat didn't use the BCZ unit, and the Skoda Superb only used the BDG unit.

Importantly, the BDG and BDH engines meet the European emission standards (Euro 4), which weren't yet required at that time (introduced in 2005 for new models and 2006 for registered ones). However, it's worth noting that while the Audi A4 and A6, as well as the Passat until 2005, were phased out, the Skoda Superb remained in production.

The most pivotal technical change in all four engines was a new design of timing elements, effectively resolving the problems of wearing cams, valves and lifter malfunctions. Does that mean the 2.5 TDI engine problems have been completely resolved? Yes, but only to some extent.

The BAU and BCZ engines are quite exceptional. All defects were addressed, considerably prolonging the life of these units. From today's perspective, these engines are even better than the successor: the 3.0 TDI. How is that possible?

The age and mileage gap between the 2.5 TDI at its production's end and the 3.0 TDI at its beginning is minimal. Yet, in terms of timing drive or injection system, the 3.0 TDI engine costs much more to repair (chain and Common Rail injectors). Thus, if you were to hypothetically compare an Audi A6 (C5) 2.5 TDI with a BAU or BCZ engine and an Audi A6 (C6) 3.0 TDI with the same mileage, the 2.5 TDI would be a safer and undeniably more affordable long-term choice.

Of course, every rose has its thorns. Despite design improvements, the 2.5 TDI remains a typical diesel with issues common to all direct injection and turbocharged diesel engines.

Unfortunately, the same cannot be said for the Euro 4 standards-compliant units, such as the BDG and BDH. Changes in the fuel and injection systems to adapt to higher standards were regrettably unsuccessful. Typical issues for these engines include:

• Engine vibrations and power loss (improper operation of the fuel system),
• Wearing injectors (risk of engine seizure).

Although you can always replace worn-out injectors with newer ones and solve the problem, eliminating engine vibrations can be a never-ending endeavor. Many specialists undertake these repairs, but you should be aware that solving this issue is a long haul and can't be fixed with a quick fix. When you decide to tackle this issue, be prepared for a series of related repairs that lead to high costs.

So, is it worth buying a car with a 2.5 TDI engine?

As outlined earlier, the engine code is paramount, followed closely by its technical condition. BAU (Audi A4 B6, Audi A6 C5, and Volkswagen Passat B5) and BCZ (Audi A4 B6 and Audi A6 C5) engines are well-developed, long-lasting, and despite their age and mileage, they can still delight owners with their performance. However, they should not be thought of as economically efficient. Their fuel consumption is high, yet still less than petrol engines when driven mostly on highways.

Despite being safer than the 2.0 TDI at the start of production, they are not as cheap to service as the reliable 1.9 TDI. Some maintenance procedures are rather costly due to the complex construction and difficult access (e.g., timing drive replacement).

While significant improvements have been made to the AKE and AYM engines (BFC), it isn't worth buying them unless familiar with the car's history and ready to fix certain potential issues. Expect to invest several thousand dollars in repairs, especially when buying these engines with high mileage.