Common Causes of Problems on TDIs, Symptoms and Solutions (Articles)

TDIs are unfortunately not as robust as their predecessors.
The mechanics around the engine, as well as the various electronic control circuits along with sensors, cables, connectors, hoses, solenoid valves and actuators, are much more complex, making the entire engine more prone to failure than a passenger car diesel, such as the Golf I generation (despite years of experience and development). With our HEX-V2 and HEX-NET, you are perfectly equipped for troubleshooting.

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

Logically, the potential failure symptoms are as complex as the "Mechatronic" system TDI itself.
New sub-systems traditionally exhibit unfamiliar failure patterns, while similar or very similar symptoms can have different causes. A single failure can manifest in a wide range of symptoms, and even more complex failure patterns can arise from multiple simultaneous defects.

A large portion of amateur mechanics and, unfortunately, even some official workshops (!) rely solely on the error memory, "because that's what the name implies: it stores all errors!"

It is also overlooked that the electronics, depending on the programming, may operate with completely different error definitions than normal usage. For example, is 30 km/h the limit before reaching the normal maximum speed? For the driver, this is clearly an error: "Lack of power!" However, the electronics do not care about the maximum speed because they cannot detect whether there is strong headwind or whether it is going uphill.

Nevertheless, a diagnostic scan of the control units installed in the vehicle is a great help, as vehicles with serious or numerous errors can be immediately identified, and attention can be focused on vehicles that are not problematic. The VCDS diagnostic software provides a quick overview of what to expect for each vehicle, for example, with a [b]vehicle scan (examples at the end of the article).

So werden Hoffnungen on a foolproof diagnosis via fault memory in practice regularly dashed when the memory "despite clear errors" shines with gaping emptiness or contains seemingly nonsensical or mysterious entries - which are often secretly deleted by workshops to avoid getting into trouble.

The customer then hears something like this: "If no error is stored, then everything is actually fine (a helpless-diplomatic formulation for You are simply imagining the error.). We could replace xyz on suspicion, but without any guarantee that the problem will be solved . . ."

The next, more insightful stage of diagnosis, namely the reading and evaluation of measurement data during operation (especially when the fault is occurring), requires
a) understanding of the functions of the individual subsystems and the overall system
b) some familiarity with digital diagnostic equipment
c) a degree of common sense
d) focused thinking
and seems to overwhelm many workshops hopelessly.For example, in the case of the missing Vmax, the fuel injection quantities could be evaluated, and depending on the result, further investigation would be carried out in the direction of fuel supply, accelerator pedal position, turbocharger control, injection start, mass airflow meter, etc.

Based on such and similar (successful) diagnoses, experience values for frequently occurring symptoms and their common causes have emerged over time, which are suitable for initially directing troubleshooting efforts in often promising directions.

:!: The following "rules of thumb" however do not guarantee that the symptoms observed actually correspond to the described cause :!:

It is assumed that each engine is in good working order (compression, filters within the specified maintenance interval, air intake passages clear, fuel lines not kinked, etc.) and that there is sufficient diesel fuel in the tank.

Symptoms: Performance issues, typically starting in the upper RPM range, occasional "acceleration dips" only in specific RPM ranges, or wave-like acceleration. Slight to moderate fuel consumption may be possible.
No other noticeable issues, usually no entry in the error memory.
Common Causes[/b]: Dirty or faulty mass airflow sensor (MAF), loose/detached connectors
Checks[/b]: MAF connector seating, measuring airflow at full load, or logging airflow limits with VCDS, VAS505x, 1551, etc.
Repair[/b]: verschandeln the MAF sensor (brake cleaner spray, etc.) or replace the MAF sensor
Caution[/b]: Too low boost pressure always pulls down the MAF value, even if the MAF sensor is perfectly intact. Therefore, always check the boost pressure first (or at the same time), with VCDS providing the target values for boost pressure during test drives in the appropriate measurement block, along with the actual values.
Achieving the MAF target value in the AGR (aggressive ride) block at full throttle does not prove that the MAF value is sufficient for releasing the full injection amount! To check this, the limits must be read out: the MAF or opacity limit must not mark the lowest value (at full throttle, full boost pressure, and without tuning)![/u]In tuned engines, the soot limit may always be active at full throttle, even if no error exists, depending on the tuner's philosophy (or capabilities)!
Further information[/b]: /viewtopic.php?t=3347,
/viewtopic.php?t=3128
-> Airflow at full load

Symptoms: Intermittent or constant loss of power in some or all engine speeds, occasional "acceleration drop" in certain engine speeds.
Full power is sometimes only available for a short time after engine restart or when releasing the gas pedal.
Occasionally, an error message such as "Intake Manifold Pressure Control - Control Limit Exceeded" or similar appears.
The boost pressure gauge (LDA, if present) shows unusual/incorrect values.
Common Causes: Turbocharger or intake manifold pressure control system malfunction.
If the boost pressure is too low, also consider leaks in the intake manifold (-> boost pressure is too low in the low speed range)!
Tests:
- Read the boost pressure with a diagnostic system (if no LDA is present)
- Check the VTG (Variable Turbine Geometry) system with VCDS basic settings in MWB 11 (engine idle):
Is the difference between VTG max and min boost pressure at ~ 1400 rpm at least 100 mbar, preferably 150 mbar (does VW provide a minimum boost pressure of 30 mbar in the 3L Lupo)? If not: Does the VTG rod move smoothly and without hesitation between the maximum and minimum positions every ~ 10 seconds (total stroke of about 1 cm)?
- If yes: Remove the intake manifold to the turbocharger and check the turbine wheel for smooth operation and adequate, precise bearing: the turbine must not rub against anything when pivoting.
- If the VTG rod has a jerky or too small stroke:
- Check the turbocharger and wastegate for smooth operation.
- Check the hoses of the boost system for secure fit, kinks, soft spots and leaks, including the pressure hose (if present) between the intake manifold and the engine control unit / boost pressure sensor.
- Check the available vacuum in the pneumatic system at idle, for example, using the LDA with the intake manifold. If at least 600 mbar of vacuum does not arrive at the wastegate, the VTG (depending on the tolerances of the wastegate and the adjustment of the screw) may no longer be able to reach the maximum stroke!
- Temporarily replace the boost pressure solenoid valve (for VTG turbochargers, it is often possible to replace it with the "own" AGR solenoid valve, but this should not be a permanent solution due to the differences in the valves, which can change the control characteristics).

Troubleshooting: Replacement of faulty and suspected hose sections and the lift pressure solenoid valve, Adjustment of the VTG mechanism, Installation of a large VTG loading hopper, or loading hopper replacement if necessary.

To narrow down the issue with due to excessive pressure, you can disconnect the vacuum hose from the canister on the VTG loader and observe the boost pressure during acceleration to at least 4000 rpm. Some pressure buildup is normal, but the normal maximum pressure must not be exceeded!

If this happens, the possible causes are:
1) Movement of the membrane canister due to internal defect (partially) blocked -> replace the canister
2) Membrane canister mechanism sticks (intermittently) due to oil residue -> verschandeln the canister with diesel or gasoline
3) Movement of the guide flaps in the loader due to deposits (partially) blocked -> clean
4) Guide flap mechanism jams due to overheating (over-tuning) -> save for a new loader
5) VTG or Wastegate rod is set too short -> extend in increments of half turns.

If the boost pressure remains significantly below the normal maximum value without activating the loader, the problem lies in the vacuum system (hoses / vacuum pump / solenoid valve / cables / engine control unit (ECU)).
[b]More information: /viewtopic.php?t=3101, /viewtopic.php?t=10810,
/viewtopic.php?t=3004, /viewtopic.php?t=2992,
/viewtopic.php?t=3011

Symptom: Low boost pressure in the low RPM range
Note: Below approximately 1300 rpm, the boost target (logged with VCDS) is practically not reached in an unmodified engine! It is best to identify errors by comparing them with other similar vehicles.
Possible causes:
-> LMM reports too little air mass (e.g., due to a defect, electrical problems, or a leak between the LMM and the compressor), which reduces the injection quantity and therefore the boost pressure, and shifts the boost pressure increase to higher speeds
-> Leaks between the compressor and the engine (often associated with whistling/noise when pressing the accelerator and oil traces in the intake paths, with larger leaks also increased soot formation)
-> Sticking or faulty compressor
-> VTG is not brought into position for maximum boost pressure, e.g., due to:
stuck VTG,
incorrectly adjusted VTG,
faulty solenoid valve,
faulty, detached, leaking or incorrectly connected vacuum lines,
faulty vacuum pump,
vacuum loss due to leaking "consumers": VTG, AGR valve, AGR cooler, brake booster
-> Wastegate does not close completely
-> Narrowed intake passages (e.g., AGR sludge in the intake manifold)
-> Insufficient exhaust cross-section (e.g., defective cat, crushed pipes)
Tests: see under causes
-> Check LMM values with a log drive and target value table.
-> See above under basic setup in MWB 11
-> See above under vacuum in the pneumatic system.
Solutions: depending on the cause
Further information: /viewtopic.php?t=3101, /viewtopic.php?t=10810,
/viewtopic.php?t=3004, /viewtopic.php?t=2992,
/viewtopic.php?t=3011

Symptoms: Wave-like acceleration, usually only within specific RPM ranges, often accompanied by power loss.
Common causes: Defective LMM (Lambda Control Module), Turbocharger boost valve or turbocharger mechanism is stuck.
Tests: LMM, Turbocharger system
Troubleshooting: Depending on the test results
More information:
/viewtopic.php?t=6529

Symptoms: Occasional to frequent "jerky" acceleration, often only within certain speed ranges, sometimes with partial loss of power. Sometimes, an entry regarding the needle position sensor (NBF) is found in the fault memory.

Common causes: Defective NBF

Tests: Have a helper bring the engine to approximately 3000 rpm while stationary and then tap the NBF nozzle with a tool: if the engine stutters, the NBF is defective.
Disconnect the NBF connector and take a test drive: if the jerking stops, this confirms the suspicion of a faulty NBF; a general loss of power (limp mode) is normal in this case. Afterwards, clear the fault memory.
Measure the fuel injector voltage and observe for jumps during the test drive using an analog meter.

Troubleshooting by: Replacing the NBF nozzle

Further information: /viewtopic.php?t=3195

Symptoms: Intermittent to frequent "hard" jolting, often only in certain engine speed ranges. The engine may stall and then often only restart after a waiting period. Occasionally, error codes may appear for implausible voltages or nonsensical entries in the memory. The pre-ignition indicator may be dimmer than usual, or may not light up at all.
Typical fault: The engine will not start until the pre-ignition indicator is not / barely lit.

Common causes: Main relay "109" of the EDC is defective (cold solder joints, contact resistance of the load contacts)
Checks: Open relay 109 and examine, test drive with a properly functioning relay
Repair: Replace the relay, open and reflow the old relay; if the relay is noticeably hot: verschandeln the load contacts (low chance of permanent improvement, therefore it is better to replace the relay)

[b]Symptoms: Engine shaking/vibration and/or rougher engine running, mainly during acceleration in the lower RPM range, sometimes also at idle. U.U. also clattering noises synchronized with engine speed.
Symptoms often decrease or disappear completely as engine speed increases.
Common causes: Two-mass flywheel (ZMS) defective. Can be easily mistaken for other rough running problems, especially in PD engines!
Checks: Push the vehicle forward and backward on a level surface with the highest gear engaged.
In this case, a functioning ZMS will rotate within itself, and you will feel like you are pushing against a spring before the engine (poorly controllable and jerky) revs up; load change noises are only dampened and mainly audible from the gearbox (shifting of the gear flank).
Depending on the size and position of the OT (output shaft) opening, the internal rotation of the ZMS can also be observed through the opening.
Defective ZMS can show different symptoms, e.g. hard noises during internal rotation, hard impacts at the ends of the play, or immediate rotation of the engine, with a (relatively small) intermediate spring effect only occurring in the engine suspension.
Given the repair costs (separating the engine and gearbox), it is advisable to try a different vehicle in case of doubt!
Troubleshooting by: Replacing the ZMS

[b]Symptoms: Idle engine vibration at PD motors
Common causes: PD elements (PDE) with higher manufacturing tolerances relative to each other can disrupt the idle vibration control. This process can be further exacerbated by a worn or faulty two-mass flywheel (ZMS).
Similar symptoms can also occur due to worn or faulty PDEs, which may be indicated by the warning light.
Tests: Observe the individual cylinder values for significant fluctuations in the idle vibration control, for example, using VCDS.
Unfortunately, it has not yet been possible to specifically assign certain symptoms and their causes – except when individual PDEs are reported as defective by the EDC.
Troubleshooting through: Replace faulty PDEs, or install a complete set of selected PDEs with minimal variations between them,
Replace the ZMS,
Replace the MSG with a device with a newer software version (currently only available for the AXR engine)

Symptoms: Idle engine rattling with PD motors, occasionally accompanied by misfires and/or reduced performance
Common causes: Clogged diesel filter
Tests: Replace diesel filter with a new one for testing
Troubleshooting by: Installing a new/verschandeln diesel filter

Symptoms: Stalling/Intermittent failures in PD engines under high load/full load
Common causes: The tandem pump doesn't provide enough pressure or flow.
Due to slow circulation, the diesel in the ZK can get so hot that steam bubbles form, which disrupt the function of the PDE.
Tests: Measure the pump outlet pressure with a pressure gauge (VAG tool), also remove the sealing screw of the measuring connection (on the front view of the pump, on the left lower side, above the return connection). Target pressure for the ASZ at 1500 rpm: at least 3.5 bar.
If the target value is not reached, clamp the return line before the diesel filter. If the target pressure is then reached, the O-rings of the PDE between the inlet and return (= the 2nd ring from above) are at least partially defective -> replace them.
If the target pressure is not reached when testing the tandem pump, even with the return line clamped -> replace the pump.
If a pressure gauge is not available, VCDS can provide information by reading MWB 13 and 23 (running time control and switching times of the PDE):
If the values of the individual cylinders increase increasingly with rising temperature and engine load, while they are normal at cold and partially warm engine, this can indicate steam bubbles.
If the problems are exacerbated when the pre-feed pump (if present) is switched off/deactivated, this also points in the same direction.
[b]Troubleshooting by: Replacing or repairing the tandem pump
Warning: Narrow points in the diesel circuit (inlet and return) including partially clogged filters can have similar consequences or cause the same symptoms!
More information: /viewtopic.php?t=161

Symptoms: Jerking/Intermittent issues with PD motors. Error logs due to exceeding PDE rule limits or other electrical problems in the PDE circuit.
Common causes: Contact resistance between the connector on the cylinder head and the relevant PDE, for example, due to factory-installed cable crimps with insufficient pressure.
Testing: Experienced technicians replace the PDE that is reported as faulty with a new one. If the error persists, the PDE is faulty. Otherwise, the problem is usually in the wiring harness.
Alternative: Remove the ZK cover, disconnect the PDE connector, and measure the individual wires with a multimeter (pay attention to safe contact, preferably use matching metal pieces to insert and remove from the contacts). If the resistance changes when the cable is moved, the cause has been found.
Troubleshooting: Replace the PDE wiring harness. This is also recommended as a precautionary repair, as the material costs and effort are minimal compared to replacing the PDE.

Symptoms: PD motor starts only after several attempts, but then runs normally
Common causes: NW position sensor (Hall sensor) defective, incorrectly mounted, or wiring error
Checks: Hall sensor, cable, and connector
Troubleshooting: depending on the defect

Symptoms: Idle engine vibration with PD motors
Common causes: Engine sensitive to AGR
Tests: Temporarily disable AGR,
Run the engine in the default AGR/air mass value block setting and observe whether the vibration increases when the AGR is open
Troubleshooting/reduction by: Adapt the AGR/air mass value block to the highest air mass value (= lowest AGR rate).

Symptoms [b]143_: Stalling at engine speeds between 1200rpm and 1800rpm and moderate engine load, accompanied by "clatter-clatter" or "tick-tick" noises (keywords: Kaudern, turkey), may occur in engines with DPF.
[b]144_ Common causes [b]145_: Flow separation at the compressor of the turbocharger (pump) due to high boost pressure at low air mass flow during DPF regeneration. This flow separation affects the fuel injection limit based on boost pressure/air mass, resulting in a decrease in engine power.
[b]146_ Tests [b]147_: Acoustic test (typical noise), environmental conditions (low ambient air pressure, relatively high boost pressure, DPF regeneration mode active).
[b]148_ Troubleshooting/reduction by [b]149_: Increasing the air mass flow (DPF should be cleaned if full, see /viewtopic.php?t=24243). A software update for the engine control unit may be available. The "error" is relatively non-critical and is usually only a cosmetic issue that appears under unfavorable conditions such as low air pressure and a full DPF.

[b]Symptoms: Poor starting behavior
Common causes: Loosened timing belt tensioner on the crankshaft (Caution: Major engine damage is likely!) as well as various other
Tests: Depending on the individual symptom (cold or warm start affected, etc.) and suspicion
Troubleshooting by: Depending on the cause
More information: /viewtopic.php?t=3094, /viewtopic.php?t=2440

Symptoms: Hissing noise at high engine load and/or noise/hissing from the engine compartment, partial loss of power and increased fuel consumption
Common causes: Leak between cargo outlet and engine
Checks: Visual inspection for detached/burst/torn air hoses, damaged LLK (presumably referring to a Landfill Liner Kit or similar).
Additional tips:

1. Because escaping coolant always contains some oil from the KGE, leaks can usually be detected by observing oil stains, or by following oil drips upwards to their source.

2. Areas that are difficult to inspect can be checked by blocking the intake path between the intake and the intake manifold (or parts thereof) on both sides and sealing it airtight with tight plugs, with one of the plugs fitted with a bicycle valve. The intake path is then pressurized via the valve. If the leak is small enough that pressure can be built up by pumping, it will usually be revealed by hissing or whistling sounds.

3. The "rabid" method involves an AU-like test: When the throttle is suddenly opened from idle to approximately 4500 rpm (of course, with a warm engine!), the charger's flow rate in the upper speed range increases for fractions of a second to approximately the full-load value in normal operation. In this case, most leaks are revealed by the escaping noises, which, however, are more difficult to locate in the engine noise.

It is possible that hearing protection can help, as the ear (similar to that in the passenger compartment) can better differentiate between the overall quieter sound mixture.

Troubleshooting: Replace defective parts, clamp sleeves with hose clamps. Symptoms: Increased backfire at higher engine load, partial loss of power, and increased fuel consumption
Common causes: AGR valve does not close properly
Tests: Visual and functional check of the AGR valve and the vacuum control
Troubleshooting by: Repair or replace the AGR valve, repair the vacuum control
More info: /viewtopic.php?t=3101, /viewtopic.php?t=3095

Symptoms: Stuttering in certain engine speeds without other noticeable issues
Common causes: Only VP 37: Worn injection pump, quantity control valve tends to stick due to internal pump wear, or is misadjusted due to tuning attempts
Tests: Read idle injection quantity with VCDS, should not be below 3 mg
Troubleshooting: Adjust injection quantity, correct valve position, verschandeln pump internally, if necessary, install a new pump
More info: /viewtopic.php?t=658, /viewtopic.php?t=8537

Symptoms: Engine starts normally but stalls after approximately 1 second
Common causes: Immobilizer (Immobilizer) blocks the MSG
Checks: Read fault memory, check cables and connections around the immobilizer, if necessary, check soldered connections in the immobilizer module, if there is suspicion of damage to the ignition key (integrated transponder!), try another key
Troubleshooting by: Use intact keys, repair / replace faulty components

Symptoms: PD motor loses significant bite at full throttle in 3rd gear around 3500 rpm (despite intact LMM, correct boost pressure, etc.)
Common causes : Incorrect camshaft timing (from factory or after head/cylinder head replacement)
Checks : Check camshaft timing
Repair : Adjust camshaft timing
Further information : /viewtopic.php?t=5408

Symptoms: Performance loss mainly in the upper RPM range despite a seemingly intact LMM and correct boost pressure, sometimes delayed boost pressure build-up in the lower RPM range.
Possible increased fuel consumption, usually no increased soot emissions.
When reading air mass values at full load, there may be an apparently inexplicable drop in the upper RPM range.
The problem develops or worsens slowly over tens of thousands of kilometers.
Common causes: Intake manifold narrowed due to oil sludge (from the KGE) and soot (from the AGR).
Checks: Visual inspection of the intake manifold.
Repair by: verschandeln the intake manifold and, if necessary, the channels in the cylinder head.

Symptoms: Performance loss mainly in the upper RPM range despite a seemingly intact LMM and correct boost pressure, sometimes delayed boost pressure build-up in the lower RPM range.
Possible increased fuel consumption, usually no increased soot emissions.
When reading air mass values at full load, there may be an apparently inexplicable drop in the upper RPM range.
Clattering noises (resonances) from the exhaust direction are possible.
The problem tends to occur suddenly.
Common causes: Exhaust passage narrowed due to fragments of a broken catalytic converter or obstruction.
Tests: Knock test (clattering?) on the exhaust in the area of the KAT.
Troubleshooting by: Replace defective exhaust parts.

Symptoms: Constant DPF regeneration attempts, high consumption, DPF constantly fails and does not regenerate.
Common causes: AGR problems, excessive oil consumption, leaks in the intake system, DPF overloaded with ash, temperature sensors on the DPF or engine showing incorrect values.
Tests: See above.
Troubleshooting by: Repairing the identified cause.
More information: /viewtopic.php?t=29120[/b][/b] [/b] [/b][/b] Suggestions and corrections are welcome!