Common causes of failure in TDIs, symptoms, and solutions (Articles)

Unfortunately, TDIs are not as robust as their predecessors.
The mechanics surrounding the engine, as well as the various electronic control systems including sensors, cables, connectors, hoses, solenoid valves, and actuators, are many times more complex, and this (despite years of experience and development) makes the entire engine more prone to failure than, for example, a diesel engine from the Golf I generation. With our HEX-V2 and HEX-NET, you'll be well-equipped for troubleshooting.

From this perspective, complete failures (breakdowns) in TDIs are relatively rare; however, the engines often cause problems with power loss, jerking, starting issues, and other issues.

Naturally, the possible error symptoms are as complex as the "Mechatronics" system of the TDI engine itself.
New subsystems often exhibit error patterns that are traditionally unknown, and the same or very similar symptoms can have different causes. A single error can manifest in different symptoms for different individuals, not to mention the even more perplexing error patterns that result from multiple simultaneous defects.

A large portion of novice mechanics, and unfortunately also some brand-name workshops(!), rely exclusively on the fault memory in the face of this confusing high-tech, "because the fault memory stores all errors, as the name suggests!"

In this process, it is often overlooked that the electronics, depending on the programming, sometimes operates with completely different definitions of errors than the normal use of language.
Example: Does the acceleration stop at 30 km/h before reaching the normal maximum speed? For the driver, it's clearly a mistake: "Lack of power!" But the electronics don't care about the maximum speed because they can't tell whether there's a strong headwind or whether it's going uphill.

However, a diagnostic scan of the vehicle's control units is a great help, as vehicles with serious or numerous errors are immediately identified, allowing you to focus on vehicles that are not problematic. Here, the diagnostic software VCDS offers a quick overview of potential problems for each vehicle, such as with an Autoscan (examples at the end of the article).

Thus, hopes for a foolproof diagnosis based on the error memory are regularly dashed in practice when the memory either displays a glaring emptiness "despite clear errors" or contains seemingly nonsensical or enigmatic entries – which workshops often secretly delete to avoid having to provide explanations.
The customer will then likely hear something like this: "If no error is stored, then everything is actually fine (a helpless-diplomatic way of saying you are only imagining the error.). "We could try replacing xyz as a precaution, but there's no guarantee that it will solve the problem."

The next, and significantly more informative, level of diagnosis involves reading and analyzing measurement data during operation (especially while the fault is manifesting), which requires...
a) Understanding the functions of the individual subsystems and the overall system.
b) some familiarity with operating digital diagnostic equipment.
c) some intuition.
d) purposeful reflection.
and seems to be hopelessly overwhelming many workshops.
In the example of the missing maximum speed, for instance, the fuel injection quantity limits could be analyzed, and depending on the results, further investigation could be conducted, initially focusing on areas such as fuel supply, throttle settings, turbocharger pressure regulation, injection timing, and mass airflow sensor.

Over time, experience has led to the identification of common symptoms and their typical causes, which can be used to initially guide troubleshooting in directions that are often promising.

The following "rules of thumb" do not offer any guarantee that the observed symptoms are actually caused by the specific reason described .

It is assumed that the engine is in generally good condition (with proper compression, filters changed at the recommended intervals, unobstructed air intake passages, fuel lines not pinched, etc.) and that there is sufficient diesel fuel in the tank.


Symptoms: Lack of power, often starting in the higher RPM range, occasionally "acceleration dips" only in certain RPM ranges, or a wavy acceleration pattern. Slight to moderate increase in consumption is possible.
No other noticeable issues, and usually no error codes are stored.
Common causes: Mass airflow sensor (MAF) is dirty or defective, connector is loose/disconnected.
Tests: Connector seat at the mass airflow sensor, measurement of air mass at full load, or... Logging of quantity limits using VCDS, VAS505x, 1551, or similar tools.
Troubleshooting via: verschandeln the LMM (using brake cleaner spray or similar), or... Replacement of the mass airflow sensor.
Warning, pitfall: Too low intake manifold pressure always pulls down the air mass reading, even though the mass airflow sensor (MAF) can be perfectly fine. So, always check the boost pressure (or check it first). VCDS provides the target values for this in the corresponding measurement block during test drives, along with the actual values.
Reaching the target air mass value in the EGR control module at full throttle does not prove that the mass airflow sensor value is sufficient to allow for the full fuel injection quantity! To verify this, the quantity limits must be read out: in this process, the air mass or enrichment limit must not indicate the lowest value (at full throttle and full boost pressure, and without chip tuning)!
"In chip-tuned engines, the specific adjustments depend on the tuning philosophy (or,..." The tuner's capabilities should ensure that the soot limit is always active at full throttle, even if there is no error detected!
More information:

-> Air mass at full load.


Symptoms: Intermittent or constant lack of power in some or all RPM ranges, occasionally "acceleration dips" in certain RPM ranges.
Full power is sometimes only available for a short time after the engine is started or when the accelerator is released.
Occasionally, error messages such as "Intake manifold pressure control differential" or "Forced induction pressure control exceeded limit" may appear.
Boost pressure indicator (if available) shows unusual/incorrect values.
Common causes: Turbocharger boost control or... Turbocharger failure.
If the boost pressure is too low, also consider the possibility of leaks in the intake manifold (-> boost pressure too low in the lower RPM range)!
Exams:
-> Read the boost pressure using the diagnostic system (if no LDA is present).
-> Check the VTG charging system with the basic settings in VCDS in module 11 (engine idle).
Is the boost pressure difference between VTG max and min at approximately 1400 rpm (apply a little throttle if the engine speed doesn't automatically increase) at least 100 mbar, preferably 150 mbar (for the 3L Lupo, VW only specifies a minimum boost pressure of 30 mbar)? If no: Does the VTG rod move quickly approximately every 10 seconds, without catching, from one stop to the next (total travel approximately 1 cm)?
-> If so: Remove the intake pipe to the turbocharger and check the compressor wheel for smooth rotation and proper bearing alignment. When tilting and rotating, the turbine should not rub against anything.
-> If the hub of the VTG rod is jammed and/or too small:
-> Check and/or improve the charging mechanism and membrane dosage for ease of use and functionality.
-> Check the tax hoses of the charging system for secure seating, kinks, soft spots (!!), and leaks, including. Pressure hose (if present) between the intake manifold and the engine control unit / turbocharger pressure sensor
-> Check the available vacuum in the pneumatic system at idle, e.g., using a pressure sensor with a vacuum range. If the turbocharger control unit (VTG) does not receive at least 600 mbar of vacuum at the charge air control valve (depending on the tolerances of the valve and the adjustment of the threaded rod), it may not be able to reach its maximum. Attack!
"Try replacing the turbocharger pressure control solenoid valve. With VTG turbochargers, it's often possible to swap it with the vehicle's own EGR solenoid valve, but this should not be considered a permanent solution due to the subtle differences between the valves, which can affect the control characteristics."

Troubleshooting via: Replacing defective and suspicious hose sections or the turbocharger pressure solenoid valve, ensuring proper function of the VTG mechanism, installing a larger VTG charge air cooler, and if necessary, replacing the entire charge air cooler.

To troubleshoot issues related to excessive pressure in a VTG turbocharger, you can disconnect the vacuum hose from the control valve and, during a full-throttle acceleration to at least 4000 rpm, observe the boost pressure. A certain amount of pressure buildup is normal, but the normal maximum pressure must not be exceeded!
If so, the possible causes are:
1) Membrane dose clogged due to internal defect (partially) -> replace the dose.
2) The diaphragm valve mechanism may stick (temporarily) due to oil residue -> Flush the can with diesel or gasoline.
3) Movement of the steering vanes in the loader is partially blocked due to Russian sediment buildup -> clear the blockage.
3) The turbocharger's wastegate mechanism is stuck due to overheating (caused by excessive tuning) -> save up for a new turbocharger.
5) Wastegate rod adjusted too short -> extend in increments of half turns.
If the boost pressure remains significantly below the normal maximum value, the fault lies within the vacuum system (hoses / vacuum pump / solenoid valve / wiring / engine control unit (ECU)).
More information:




:arrow:Symptom:arrow:: Low boost pressure in the lower RPM range.
Note: Below approximately 1300 rpm, the target lambda value (which can be logged using VCDS) is practically never reached in any (unmodified) engine when running at full throttle! Errors are best identified by comparing them with other vehicles of the same model.
:arrow:Possible causes:
-> The mass airflow sensor (MAF) reports insufficient air mass (e.g., due to a defect, electrical problems, or a leak between the MAF sensor and the compressor), which, via the soot limiter, reduces the injection quantity and thus the turbocharger drive, delaying the increase in boost pressure at higher engine speeds.
-> Leaks between the compressor and the engine (often accompanied by whistling/hissing sounds when accelerating, and oil traces on the intake air passages; larger leaks can also cause increased soot production).
-> Difficult-to-turn loading shaft / Loading defect.
-> VTG will not be positioned for maximum. Increased boost pressure, e.g., due to...
stuck VTG,
incorrectly adjusted VTG locking mechanism.
defective solenoid valve,
defective, detached, leaky, or incorrectly connected vacuum hoses.
defective vacuum pump,
Vacuum leaks caused by faulty "consumers": VTG (Variable Turbine Geometry), AGR (Exhaust Gas Recirculation) valve, AGR cooler, brake booster.
-> Wastegate not closing completely.
-> narrowed airways (e.g.,) AGR sludge in the intake manifold.
-> insufficient exhaust pipe diameter (e.g., Cat defective, pipes crushed.
Exams: see under Causes.
-> Check LMM values using a test drive and a target value table.
-> See above, under "Basic Settings" in the MWB 11 manual.
-> See above, under "vacuum in the pneumatic system."
Remedy: depending on the cause.
More information:





Symptoms: Wavelike acceleration, usually only in certain RPM ranges, often accompanied by a loss of power.
Common causes: Faulty mass airflow sensor, stuck boost pressure solenoid valve, or jammed turbocharger mechanism.
Exams: LMM, Turbocharging system.
Troubleshooting: depending on the results of the tests.
More information:



Symptoms: Occasional to frequent "hard" acceleration jerks, often only in certain RPM ranges, sometimes with a loss of power. Sometimes a note is included about this. Needle movement sensor (NMS) in the error memory.
Common causes: NBF failure.
Tests: Have an assistant bring the engine to approximately 3000 rpm while it's idling, and then tap the NBF injector with a tool: if the engine stutters in response, the NBF is defective.
Disconnect the NBF connector and perform a test drive: the absence of jerking confirms the suspicion of an NBF issue; a general loss of power (limp mode) is normal in this case. Subsequently, clear the error memory.
Monitor the injection pressure adjustment voltage and observe for any fluctuations using an analog meter during the test drive.
Troubleshooting by: Replacing the NBF nozzle.
More information:


Symptoms: Occasional to frequent "jerky" behavior, often only in specific RPM ranges. The engine may stall and then often can only be restarted after a waiting period. The error memory occasionally contains entries about implausible voltages or meaningless entries. The preheating indicator may glow less brightly than usual, or may not light up at all.
Typical issue: The engine won't start unless the preheating indicator light is on or barely lit.
Common causes: Defective EDC main relay "109" (internal cold solder joints, contact resistance of the load contacts).
Tests: Open and inspect relay 109, perform a test run with the relay confirmed to be fully functional.
Troubleshooting via: Replacing the relay, opening and resoldering the old relay; if the relay is noticeably hot: verschandeln the load contacts (low chance of permanent improvement, so it's better to replace the relay).


Symptoms: Shaking/vibration of the engine and/or a rougher engine running, mainly when accelerating in the lower RPM range, sometimes also at idle. Possibly also clicking noises synchronized with the engine speed.
Symptoms often become less severe or disappear altogether as the speed increases.
Common causes: Defective dual-mass flywheel (DMF). It can easily be confused with other types of vibration, especially with PD motors!
Tests: Push the vehicle forward and backward on a level surface with the highest gear engaged.
During this process, a intact clutch disc is twisted, and you feel like you're pushing against a spring, before the engine (which is poorly dosable and jerky) revs up; load change noises are only faintly audible, mainly coming from the transmission (due to the shifting of the tooth flank clearance).
Depending on the location and size of the observation port, the internal twisting of the clutch may also be visible through the port.
Defective ZMS (ZMS = Zweimassenschwungrad) can exhibit various symptoms, such as harsh noises during internal rotation, hard impacts at the ends of the play, or immediate engine rotation, where a (relatively small) effect of the intermediate spring is only present in the engine mount.
Given the extent of the repair work required (disassembling the engine and transmission), it is advisable to perform a comparative test with a functioning vehicle if there is any doubt.
Troubleshooting by: Replacing the ZMS.


Symptoms: Idle surging in PD engines.
Common causes: PD elements (PDE) with higher manufacturing tolerances relative to each other can cause the idle speed control to oscillate. The process can be further exacerbated by a worn or defective dual-mass flywheel (DMF).
Similar symptoms can be caused by worn or defective pressure drop elements (PDEs), which may sometimes be indicated by the warning light.
Tests: For example, observe the idle speed regulation using VCDS and check for significantly varying values for each individual cylinder.
Unfortunately, it has not yet been possible to specifically correlate certain symptoms with their causes, except when individual PDEs are reported as defective by the EDC.
Troubleshooting via: Replacing the defective PDE, or alternatively, Installation of a complete set of carefully selected PDE components with minimal variations between them.
Replacement of the ZMS.
Replacing the MSG with a device featuring a more recent software version (currently only available for the AXR engine).


Symptoms: Idle surging in PD engines, occasionally accompanied by misfires during driving and/or lack of power.
Common causes: Diesel filter clogged.
Tests: Replace the diesel filter with a new one for testing purposes.
Troubleshooting by: Installing a new / verschandeln diesel filter.


Symptoms: Jerking/stuttering in PD motors under higher load/full load.
Common causes: The tandem pump is not generating enough pressure, or... Throughput.
Due to insufficient circulation, the diesel fuel in the crankcase can become so hot that steam bubbles form, which can disrupt the function of the positive displacement element (PDE).
Exams: Measure the pump's outlet pressure using a pressure gauge (VAG tool), for which you need to unscrew the closing screw of the measuring port (located on the left side, below, above the return port when viewing the pump from the front). Target pressure for the air spring system (ASS) at 1500 rpm: at least 3.5 bar.
If the target pressure is not reached, pinch the return line before the diesel filter. Once the target pressure is reached, the O-rings of the common rail injectors (between the high-pressure and return lines) are likely damaged (this is the 2nd possible cause). (Ring from above) at least partially defective -> replace.
If the desired pressure is not reached during the inspection of the tandem pump, even with a blocked return line, then replace the pump.
If a pressure gauge is not available, VCDS can provide information by reading measuring block 13 and 23 (idle speed control and switching times of the common rail pump):
If the values of individual cylinders increasingly diverge with rising temperature and engine load, while remaining normal when the engine is cold or slightly warm, this could indicate the presence of steam bubbles.
If problems are exacerbated when a pre-feed pump is shut down or deactivated (if one is present), this further suggests the same issue.
Troubleshooting by: Replacement or. Repair of the tandem pump.
Attention: Narrow sections in the diesel circuit (supply and return lines), including partially clogged filters, can have similar consequences or cause the same symptoms!
More information:


Symptoms: Jerking/stuttering in PD motors. Error entries due to exceeding rule limits of Power Delivery Elements (PDEs) or other electrical problems in the PDE circuit.
Common causes: Transition resistances between the connector on the cylinder head and the relevant components. PDE, possibly caused by factory-made cable crimps with insufficient pressure.
Exams: Experienced mechanics replace the reported defective PDE with a non-obtrusive PDE. If the error persists, the PDE (Power Distribution Element) is likely defective; otherwise, it's usually a problem with the wiring harness.
Alternative: Remove the ZK cover, disconnect the PDE connector, and measure the individual wires with an ohmmeter (ensure a secure connection, and it's best to use appropriate metal pieces to insert or slide onto the contacts). If the resistance changes when the cable is moved, then the cause has been found.
Troubleshooting : Replacing the PDE cable harness. This is also often recommended as a "suspect repair" because the material costs and effort involved are minimal compared to replacing the entire PDE.


Symptoms: The PD motor only starts after several attempts, but then runs completely normally.
Common causes: Defective or incorrectly installed Northwest position sensor (Hall sensor), or wiring faults.
Tests: Hall sensors, cables, and connectors.
Troubleshooting : depending on the defect.


Symptoms: Idle surging in PD engines.
Common causes: The engine is sensitive to the EGR system.
Exams: Temporarily suspend AGR operation for testing purposes.
Let the engine run with the EGR/mass airflow sensor block in its default setting and observe whether the misfire worsens when the EGR is open.
Troubleshooting/reduction of errors by: Adapting the EGR/mass airflow sensor block to the highest mass airflow value (= lowest EGR rate).


Symptoms: Jerking at engine speeds between 1200rpm and 1800rpm and at medium engine load, accompanied by "clattering" or "rumbling" noises (keywords: clucking, turkey), which may occur in engines with a DPF.
Common causes: Flow separation at the turbocharger's compressor (pump) due to high boost pressure at low air mass flow during DPF regeneration. When the airflow is disrupted, the fuel injection quantity limiter intervenes based on boost pressure/air mass, which causes a drop in engine power.
Tests: Acoustic test (typical noise), Environmental conditions (low ambient air pressure, relatively high boost pressure, DPF regeneration mode active).
Troubleshooting/reducing errors by: Increasing the air mass flow (the DPF should be cleaned if it is full, see . Possibly, a software update for the engine control unit is available. The "error" is relatively minor and usually just a cosmetic issue that appears under unfavorable environmental conditions, such as low air pressure and a full DPF.


Symptoms: Poor starting behavior.
Common causes include a loose timing belt pulley on the crankshaft (be aware, this can lead to major engine damage!), as well as various other issues.
Tests: depending on the individual symptoms (e.g., whether the problem occurs with a cold or warm start, etc.) and suspicion.
Troubleshooting: depending on the cause.
More information:


Symptoms: Black smoke at high engine load and/or noise, or... Hissing sound from the engine compartment, partial loss of power, and increased fuel consumption.
Common causes: Leakage between the cargo outlet and the engine.
Examinations: Visual inspection for detached or burst/torn air hoses, and damaged liquid-cooled components.
Sure, here is the translation of the text from German to English:

"Additional tips:"

1. Since escaping charge air always contains some oil from the turbocharger, leaks can usually be identified by oil stains, or by tracing oil droplets upwards to their source.

Areas that are difficult to inspect can be checked by sealing off the charge air duct between the turbocharger and the intake manifold (or parts of it) with tight, pressure-resistant plugs on both sides, with one of the plugs being fitted with a bicycle valve. The charge air duct is then pressurized through the valve. If the leak is small enough to allow pressure to be built up by a pump, it will usually be revealed by a hissing or whistling sound.

3. The aggressive method involves a test similar to a dynamometer test: By abruptly applying full throttle from idle while the vehicle is stationary, the turbocharger's boost pressure in the higher RPM range increases for a fraction of a second to approximately the full load value achieved during normal driving (of course, with a warm engine!). Most of these leaks will be revealed by the hissing sounds they produce, although these sounds can be difficult to pinpoint amidst the engine noise.
Hearing protection can sometimes be helpful, as the ear (similar to the passenger compartment) can better distinguish sounds in a generally quieter sound environment.

Troubleshooting by: Replacing defective parts, replacing hose clamps with jubilee clips.


Symptoms: Increased exhaust smoke at higher engine loads, partial loss of power, and increased fuel consumption.
Common causes: EGR valve does not close properly.
Exams: Visual and functional inspection of the AGR valve and the vacuum control system.
Troubleshooting by: verschandeln and lubricate the EGR valve, repair the vacuum control system.
More information:


Symptoms: Jerky movement in certain RPM ranges without any other noticeable issues.
Common causes: only VP 37: worn injection pump, the metering unit is prone to jamming due to internal pump wear, or may be misadjusted due to tuning attempts.
Tests: Read the idle injection quantity using VCDS; it should not be below 3 mg.
Troubleshooting via: Adjusting the injection quantity, correcting the valve position, cleaning the internal pump, and if necessary, installing a pump in near-new condition.
More information:


Symptoms: The engine starts normally, but then stalls after approximately 1 second.
Common causes: The immobilizer (WFS) is blocking the MSG.
Tests: Read error codes, check cables and connectors around the WFS, if necessary. Soldering points in the WFS module; if there is suspicion of damage to the ignition key (integrated transponder!), try a different key.
Troubleshooting by: Using functional keys, repairing/replacing defective components.


Symptoms: The PD motor loses power during full-throttle acceleration in 3rd gear. The engine's power delivery drops noticeably around 3500 rpm (despite the mass airflow sensor being presumably functional and the boost pressure being correct, etc.).
Common causes: Incorrectly adjusted camshaft (from the factory or after work on the cylinder head / timing belt replacement).
Exams: Control of the NW setting.
Troubleshooting by: Correcting the NW setting.
More information:


Symptoms: Loss of power, mainly in the higher RPM range, despite a presumably functioning mass airflow sensor (MAF) and correct boost pressure; sometimes, a delayed boost pressure build-up in the lower RPM range.
Increased fuel consumption is possible, but usually there is no increased soot emission.
When reading the air mass values at full load, there may be an apparently inexplicable drop in the upper RPM range.
The problem arises or gradually increases over tens of thousands of kilometers.
Common causes: The intake manifold is constricted due to sludge deposits from oil (from the crankcase ventilation system) and soot (from the exhaust gas recirculation system).
Examshttps://community.dieselschrauber.org/en/viewtopic.php?t=3347,: Visual inspection of the intake manifold.
https://community.dieselschrauber.org/en/viewtopic.php?t=3128Troubleshooting byhttps://community.dieselschrauber.org/en/viewtopic.php?t=3101,: verschandeln the intake manifold and, if necessary, the intake channels in the cylinder head.


https://community.dieselschrauber.org/en/viewtopic.php?t=10810,Symptomshttps://community.dieselschrauber.org/viewtopic.php?t=3004,: Loss of power, mainly in the higher RPM range, despite a presumably functioning mass airflow sensor (MAF) and correct boost pressure; sometimes, a delayed boost pressure build-up in the lower RPM range.
Increased fuel consumption is possible, but typically there is no increased soot emissions.
When reading the air mass values at full load, there may be an apparently inexplicable drop in the upper RPM range.
Knocking or rattling sounds (resonances) may be audible from the exhaust.
The problem tends to arise suddenly.
https://community.dieselschrauber.org/en/viewtopic.php?t=2992,Common causeshttps://community.dieselschrauber.org/en/viewtopic.php?t=3011: The exhaust pipe's cross-section is narrowed due to fragments from a broken catalytic converter or due to being constricted.
https://community.dieselschrauber.org/en/viewtopic.php?t=3101,Examshttps://community.dieselschrauber.org/viewtopic.php?t=10810,: Tapping test (tapping?) on the exhaust near the catalytic converter.
https://community.dieselschrauber.org/en/viewtopic.php?t=3004,Troubleshooting byhttps://community.dieselschrauber.org/en/viewtopic.php?t=2992,: Replacing defective exhaust parts.


https://community.dieselschrauber.org/en/viewtopic.php?t=3011Symptomshttps://community.dieselschrauber.org/viewtopic.php?t=6529: Constant DPF regeneration attempts, high fuel consumption, the DPF is constantly clogged and does not regenerate.
https://community.dieselschrauber.org/en/viewtopic.php?t=3195Common causeshttps://community.dieselschrauber.org/en/viewtopic.php?t=161: EGR problems, excessive oil consumption, leaks in the charge air system, DPF overloaded with ash, temperature sensors on the DPF or engine showing incorrect values.
https://community.dieselschrauber.org/en/viewtopic.php?t=24243).Examshttps://community.dieselschrauber.org/en/viewtopic.php?t=3094,: See above.
https://community.dieselschrauber.org/en/viewtopic.php?t=2440Troubleshooting byhttps://community.dieselschrauber.org/en/viewtopic.php?t=3101,: Repairing the identified cause.
https://community.dieselschrauber.org/en/viewtopic.php?t=3095More informationhttps://community.dieselschrauber.org/en/viewtopic.php?t=658,: https://community.dieselschrauber.org/en/viewtopic.php?t=8537

Okay, bitte gib mir den deutschen Text, den du übersetzt haben möchtest.https://community.dieselschrauber.org/en/viewtopic.php?t=5408https://community.dieselschrauber.org/viewtopic.php?t=29120