Used car inspection tips for TDIs (Articles)

The used TDI car inspection should begin with a VCDS Autoscan. This process queries all control units installed in the vehicle and lists their error memory contents and coding information.
"The entries in the error memory allow for the rapid identification of potential problem areas, even if the vehicle does not exhibit any obvious defects. It is important not only to consider the quantity of errors displayed, but above all the quality. A listed "low voltage in control units X, Y, and Z" is generally less critical (especially if low voltage is present in multiple control units, which is often caused by a weak vehicle battery, but can also indicate problems with the idle current consumption) than issues with the turbocharger or exhaust aftertreatment system."

If the used car inspection performed via VCDS diagnostics does not reveal any serious defects, the next step is a mechanical and visual inspection of the vehicle:

Starting: Pre-glow until the indicator light illuminates, then immediately engage the starter. The engine should start within approximately 1 second, whether the battery is warm or cold, and should immediately settle into its idle speed and run smoothly.

Quiet valve ticking is often audible and is not a reason for complaint.

Slight differences in the operating noise (a muffled ticking sound) of the individual cylinders are common and usually harmless. However, a strong knocking or hammering sound from individual cylinders, even when the engine is warm, is always a warning sign, for example, of faulty fuel injectors. As a consequence, the pistons may even have seized!

All intact TDIs possess an almost unbelievable torque delivery compared to previous generations of diesel engines. The full torque is available at least between approximately 2,000 and 4,000 rpm and remains consistently maintained throughout the entire range when revving out the lower gears, without noticeably decreasing. Above approximately 2,000 RPM, when the accelerator is fully depressed, the full acceleration should kick in within a maximum of half a second.
In common rail engines, which have a significantly higher torque around 2000 rpm (compared to engines with a distributor injection pump), the range above 3,500 rpm often appears relatively weak, but this is usually not a real lack of power.
The subjectively better performance of VP engines at higher RPMs is due to the lower torque loss above 2000 rpm and, often, the lighter weight of the older VP automatic transmissions.
Conversely, the often excessive power of the newer PD engine generations can often only be tamed by the superior torque delivered in the mid-rpm range.
Current CR-TDI engines that meet the EU-5 emissions standard do not exhibit any significant weaknesses in terms of driving performance; acceleration begins just above idle speed, and typically offer a wide usable rev range extending beyond 3,500 rpm.

The performance of the turbocharger can be best tested in higher gears (4th or 5th) during full-throttle acceleration from approximately 1,100 rpm. Initially, very little boost will be felt, but at the latest from 1,500 rpm, a noticeable increase in power should be felt, allowing acceleration even on moderate inclines in the highest gear. If this power increase is absent or occurs significantly later, there is almost certainly a fault in the turbocharging system, and the cost of repair is difficult to estimate. Everything is possible, from reattaching a detached control hose to replacing a loader for several thousand euros.

Any jerking above approximately 1,300 rpm is completely foreign to healthy TDIs. If the engine still "stalls" or hesitates when the required power is demanded, there is always some kind of fault present, which many workshops often find difficult to resolve - which, in turn, leads to correspondingly high bills for repair attempts.
Even a smooth, but significantly varying power delivery (consistently observed across multiple attempts in the same gear at full throttle and similar RPMs) during a test drive is a sure sign of potentially "expensive" problems.

The top speed, as stated in the vehicle registration document, is usually reached quite quickly. If a flat, unobstructed track is available, where you can accelerate at full throttle for approximately 90 seconds, the speedometer (which is often ahead of actual speed) should indicate that the registered speed has been largely reached, possibly reduced by headwinds. If this is not possible with nearly standard aerodynamics (no large spoilers, extreme wide tires, etc.), there is likely a power loss, the causes of which can range from a clogged air filter or diesel filter (cheap) to a worn-out engine (replacement engines are very expensive).

A sooty vehicle rear should make any potential TDI buyer suspicious, because soot is not usually a problem with healthy TDIs, and at best, it appears as a slight puff of smoke when accelerating fully. The interior, with its soot-black exhaust pipes, is still normal.
Increased soot levels can also indicate electronic tuning, which is relatively easy and quite common in TDIs. Many of these vehicles are driven in a way that the owner emphasizes as "sporty," which inevitably leads to more advanced wear and tear based on the mileage compared to "normal" driving. Answers to questions about this are usually most honest when you give the impression that you are not averse to a tuned engine and are interested in experiences with acceleration comparisons against other "sports cars." If the owner starts boasting excessively, the car may sell quickly and for a good price, but the engine may not have the average remaining lifespan.
Special caution should be exercised with TDI engines that have a closed DPF (Diesel Particulate Filter), which are typically vehicle models from 2006 onwards. If you see the rear of a black vehicle, it's likely that the DPF (Diesel Particulate Filter) is defective, and there are probably other problems with the engine as well.

If the salesperson immediately starts driving the cold test car aggressively to show how well it performs, they probably do that often. Stay away from this one; any engine can achieve full throttle when cold.

A noticeable shaking of the engine when stopping is normal.

Since every TDI is fully electronically controlled, the cable connections in the engine compartment should give a sense of reliability. Light layers of dirt on otherwise undamaged cables and connectors are not critical, but severely bent cable bundles, exposed or corroded cable connections can easily cause a system to fail.
Unlike earlier "mechanical" diesels, which only required voltage at the shut-off valve to run, while the rest of the entire electrical system could fail.

A noticeable diesel odor in the engine compartment indicates leaks that, with a bit of luck, can be repaired cheaply. However, the fuel injection pump (VP-TDI), tandem pump (PD-TDI), or high-pressure pump (CR-TDI) may also be leaking, which usually means repair costs exceeding 1,000 EUR. In particular, frequent use of biodiesel can sometimes make pump seals brittle. Notably, the fuel injection pump manufacturer Bosch does NOT approve its products for biodiesel, while Volkswagen approves biodiesel starting in the year 1996 – and after the warranty period for leaking pumps, they usually refer customers to Bosch, where the customer then hears exactly what has been said! Since there is only a draft standard (DIN 51 606) for biodiesel, it is at least important to ensure that the gas station being used complies with it.

Oil and fuel consumption can hardly be determined "objectively" during an inspection/test drive, which means you have to rely on the owner's statements. Typical consumption rates for "normal" driving are about 5 to 7 liters of fuel per 100 km and up to 0.5 liters of oil per 1,000 km. Significantly higher readings from the owner indicate potential issues with the vehicle.

It is crucial for the TDI engine that the timing belt for fuel injection and valve timing, along with the tensioner pulley, is replaced at the specified intervals (depending on the version, every 60,000 or 90,000 km (for 5/6-cylinder VP-TDI/PD-TDI, the interval is longer or may vary), and 120,000 km - 210,000 km for CR-TDI engines). If the vehicle has exceeded these mileage intervals, proof of the timing belt and tensioner pulley replacement should be provided. Otherwise, tearing the belt will inevitably cause major and very expensive engine damage.

The glow plugs sometimes cause considerable trouble in older TDIs:
Not only are they (partially) defective and need to be replaced, but they are often more or less stuck in the cylinder head, and even considering the required loosening torque, it doesn't bode well.
Often, the hexagonal head of the spark plug shears off, leaving the threaded portion firmly stuck in the cylinder head, or the aluminum threads in the head become rounded, preventing the spark plug from being removed.
To properly repair the damage, the cylinder head must be regularly removed, which involves a significant amount (and expensive) labor.
To prevent this risk, when inspecting a used car, one should carefully examine the area around the glow plugs for dirt and corrosion, and if in doubt, insist that all glow plugs (not just the defective one) be removed and inspected (more information on this can be found at Starting problems on VP-TDI, Starting problems on PD-TDI, and Starting problems on CR-TDI).
If the provider does not respond, this can be a strong argument for lowering the purchase price or even excluding the vehicle from your shortlist.


Other points, such as the transmission, rust, tire wear pattern, straight-line driving, brakes, oil and water leaks, condition of maintenance, etc., should be checked as with any "normal" vehicle.


Comparative compression test on the go:
If there are doubts about the condition of the engine, a preliminary comparative compression test can provide initial indications (this also works with gasoline engines, but not with automatic transmissions).
Stop at a gently sloping, moderately steep downhill section with a level road surface. Turn off the engine, engage 2nd gear, release all pedals and the handbrake, and keep the steering straight. The warmed-up vehicle should roll a short distance downhill approximately every 3 to 5 seconds and then come to a stop again (i.e., hop downhill gradually) - if not, use 1st or 3rd gear. Try a different route, possibly looking for a steeper section.
(In the upper levels, the motor tends to run continuously, which makes it difficult and inaccurate to determine the timing between individual hops.)
The intervals between each hop should be approximately the same length. Repeated time deviations of more than approximately 30% in the rhythm corresponding to the number of cylinders indicate differences between the cylinders that can affect the compression even when the engine is running can.
However, the "speed" in this test is unrealistically low, and the achieved compression pressures are very low, which impairs the normal sealing functions between the piston rings and the cylinders.
Therefore, one cannot automatically conclude that a "bad" result in the Hoppel test indicates a similar result in a regular compression test (with starter motor speed). The latter is the "official" method, and therefore its reliability and validity are definitely higher than those of the Hoppel test.
Nevertheless, a "bounce test" with a concerning result suggests a standard compression test, as, for example, leaky valves are detected by both methods.
If the provider refuses to perform a standard compression test after a Hoppel test, you should carefully consider whether or not to buy the car.

If a suitable slope is not readily available, you can try the following:
Jack up the right wheel of the driven axle and carefully secure the vehicle to prevent it from rolling.
Warning: Danger of accidents! Definitely turn off the ignition!
Shift into the highest gear and rotate the engine, which should be at operating temperature, using a hand crank with as consistent force as possible, ensuring that the torque applied to the wheel bolts does not significantly exceed the specified tightening torque. Again, pay attention to the times during which compression is lost due to (usually minor) leaks in the piston rings and, if applicable, the valves. For all-wheel drive vehicles, this stationary test may not be possible depending on the design.

See also:
Key role of OBD diagnostic device/VCDS when buying a vehicle