How to read a TDI boost log in VCDS (groups 011, 003, 008)
You have a VCDS log and columns full of numbers. This guide shows how to read groups 011, 003 and 008 to tell whether the turbo, the air mass meter and boost control work as they should. It covers 1.9 and 2.0 TDI engines with a distributor pump (VE) and with unit injectors (PD), with Bosch EDC15 and EDC16 control units. A section on common rail TDIs is at the end.
If you do not have a log yet, start with How to record a VCDS log. To judge boost you need a full-throttle pull on a warm engine.
In short: what a healthy log looks like
- Idle: specified and actual boost are almost equal, around 1000 mbar, which is the atmospheric pressure.
- Full throttle: from about 1800–2000 rpm the actual boost reaches the specified value and keeps following it. A short spike above the request right after the turbo spools up is normal.
- The N75 valve works within its range and does not sit at its limit for the whole pull.
- Air mass: the actual value (group 003) reaches the specified one. On a 1.9 TDI 90 hp (ALH) it should be at least 850 mg per stroke at 3000 rpm.
VCDS shows absolute pressure
VCDS shows absolute pressure, i.e. including the atmospheric pressure (about 1000 mbar). A boost gauge shows the pressure above atmospheric. To compare the two, subtract about 1000 mbar: 2000 mbar in VCDS is about 1.0 bar (14.5 psi) on the gauge.
Newer control units may show pressure in hPa. 1 hPa is 1 mbar, so you read the numbers the same way.
Group 011: specified boost, actual boost and N75
| Field | What it shows |
|---|---|
| 1 | engine speed |
| 2 | specified boost (mbar): what the control unit asks for |
| 3 | actual boost (mbar): what the intake pressure sensor (MAP) measures |
| 4 | N75 valve duty cycle (%): the valve that drives the turbo vanes |
Column names depend on the label file and the VCDS language. Without labels both pressures may have the same name, e.g. "Absolute Pres.". In that case the first column is the specified value and the second the actual one.
Workshop values
Full load, about 3000 rpm, warm engine (coolant at least 80 °C):
| Engine | Power | Specified (mbar) | Actual (mbar) | N75 (%) |
|---|---|---|---|---|
| ALH, AGR (VE) | 90 hp | 1850–1950 | 1850–2250 | 45–95 |
| AHF, ASV (VE) | 110 hp | 1850–1950 | 1850–2250 | 45–95 |
| ATD, AXR (PD) | 100 hp | 1900–2100 | 1850–2250 | 45–80 |
| AVB (PD) | 101 hp | 1900–2150 | 1900–2150 | 55–75 |
| AJM (PD) | 115 hp | 2000–2150 | 2000–2200 | 45–95 |
| ASZ (PD) | 130 hp | 2200–2400 | 2100–2600 | 40–95 |
The values come from the workshop tables on the Dieselschrauber forum (links at the end). After a remap the specified boost is higher, so compare the actual value with the specified one in your own log, not with the table.
When a difference means a fault
Service documentation gives no single tolerance. In practice:
- Below about 1800 rpm the turbo is still spooling and the actual boost can be well below the request. That is normal.
- A short spike above the request right after spool-up, lasting a fraction of a second, is normal too.
- At part throttle the actual boost often sits below the request, by as much as 200–300 mbar, with N75 at its limit. Usually that is not a fault. Only judge stretches with the pedal floored.
vaglogs diagnostics reports a problem on a diesel when, at full throttle from 1800 rpm, the difference lasts at least 1.5 seconds:
- underboost: actual at least 200 mbar below the request, critical from 400 mbar;
- overboost: actual at least 150 mbar above the request, critical from 300 mbar.
These are our thresholds, not a factory standard. The control unit has its own and stores a fault code when they are exceeded (list below).
N75: which way it works
The turbo on a TDI has variable geometry (VNT). The N75 valve sends vacuum to the vane actuator. The more vacuum, the more the vanes close and the higher the boost. With no vacuum, a spring opens the vanes and boost is at its lowest.
The percentage in group 011 does not mean the same on every control unit:
- EDC16 and EDC17 (later PD engines such as BKC, BXE, BLS, BKD, and common rail): a higher percentage means more vacuum and more boost. It typically sits around 80% at idle and drops once boost reaches the request.
- EDC15 (VE engines such as ALH, AHF, ASV, and early PD engines such as AJM, ATD, ASZ): VCDS shows it the other way round, a low percentage means a lot of vacuum. During a pull the percentage usually rises from about 20% to 80%.
There are exceptions, so check the direction on your own log. At the start of a pull, before the turbo spools up, the control unit asks for the maximum. The N75 value at that moment means "vanes closed, full boost" on your car.
N75 at its limit while underboosting: if the actual boost is below the request and N75 sits at its "full boost" value (around 95% on EDC16 and EDC17), the control unit is already doing all it can. The cause is mechanical: a leak, the turbo vanes, the vacuum supply or the N75 valve itself, not the software.
Group 003: air mass
| Field | What it shows |
|---|---|
| 1 | engine speed |
| 2 | specified air mass (mg/stroke) |
| 3 | actual air mass (mg/stroke), from the air mass meter (MAF) |
| 4 | EGR valve duty cycle (%) |
The unit "mg/H" in VCDS means milligrams of air per stroke (H for the German Hub). On some EDC16 control units group 003 is labelled "Exhaust Gas Recirculat. (spec.)/(actual)", but the values are the same air mass.
- Idle: both values are low (around 230–370 mg on an ALH) and the EGR is open. Part of the exhaust goes back into the intake, so the meter measures less air. That is normal.
- Full throttle: the EGR closes and the actual air mass should reach the specified one. According to Ross-Tech, an ALH should show at least 850–900 mg at 3000 rpm. A worn meter on these engines often does not go above about 600 mg.
- The specified air mass is often capped at a fixed value, e.g. about 850 mg, also on overrun. So only compare the two at full throttle, from about 1800 rpm.
- Actual air mass stuck at one value that does not change with rpm? The control unit has probably rejected the meter and runs on a substitute value. Read the fault codes.
vaglogs diagnostics flags the air mass meter when, at full throttle between 1800 and 4000 rpm, the actual air mass stays below 75% of the specified value for at least 2 seconds.
Group 008: injection quantity and its limits
On EDC15 all values are in mg per stroke:
| Field | What it shows |
|---|---|
| 1 | engine speed |
| 2 | quantity requested by the driver (accelerator pedal) |
| 3 | torque limitation |
| 4 | smoke limitation (from the air mass) |
The control unit injects the lowest of these quantities. If the smoke limitation is the lowest at full throttle, the engine is short of air, and the cause is in groups 011 and 003. The torque limitation being the lowest is what the manufacturer intended.
Example for an ALH at about 3000 rpm: driver request 38–42, torque limitation 34.2–38.2, smoke limitation 37–40 mg/stroke. The torque limitation is the lowest, as it should be.
On EDC16, group 008 shows the same in newton metres (Nm): requested torque, torque limitation and smoke limitation. You read it the same way.
Typical fault patterns
| What the log shows | Most common cause |
|---|---|
| Actual boost below the request, N75 at "full boost", air mass clearly high | a leak after the turbo: intercooler, hoses, clamps |
| Actual boost below the request, N75 at "full boost", air mass low as well | sooted or seized turbo vanes, weak vacuum supply, leaking vacuum hoses, a worn N75 valve or turbo |
| A clear spike above the request, then a drop to about 1000 mbar and no power | sticking turbo vanes or N75, the control unit goes into limp mode |
| A stepped, jerky actual boost curve | turbo vanes moving stiffly |
| Boost is fine, air mass below the request | a worn air mass meter, a leak between the meter and the turbo, or a blocked air filter |
| Actual air mass always below the request, also when cruising | an EGR valve that does not close |
| About 1000 mbar at full throttle and no power | limp mode, read the fault codes. Switching the ignition off and on usually clears it until next time |
The pressure sensor (MAP) can be checked standing still. Switch the ignition on without starting the engine and open group 010. Atmospheric pressure (field 2) and intake pressure (field 3) should be almost equal. A large difference points to the sensor.
Fault codes
| VAG code | OBD code | Text in VCDS | Meaning |
|---|---|---|---|
| 17964 | P1556 | Charge Pressure Control: Negative Deviation | boost below the request |
| 17965 | P1557 | Charge Pressure Control: Positive Deviation | boost above the request |
| 17963 | P1555 | Boost Pressure Control: Upper Limit Exceeded | upper boost limit exceeded |
| 16683 | P0299 | Boost Pressure Regulation: Control Range Not Reached | underboost (newer control units) |
| 16618 | P0234 | Boost Pressure Regulation: Limit Exceeded (Overboost Condition) | overboost (newer control units) |
| 17958 | P1550 | Boost Pressure Control Valve (N75): Control Deviation | N75 control deviation |
| 16486 | P0102 | Mass Air Flow Sensor (MAF/G70): Signal too Low | air mass meter signal too low |
Common rail TDI
Some early common rail engines still have classic measuring groups, which you read as above. Most EDC17 control units are UDS, though: instead of groups you pick named values in Adv. Meas. Values. Look for pairs of specified and actual values, e.g. "Charge air pressure: specified value" and "Charge air pressure: actual value", "Air mass: specified value" and "Actual air mass", plus the charge pressure actuator activation.
- Pressure is usually in hPa and is absolute too.
- Actuator activation: a higher percentage means more boost. Around 95% with boost below the request is the same case as above: the control unit gives the maximum and the cause is mechanical.
- The specified air mass is often capped at a fixed value when cruising, and at high rpm the actual air mass naturally falls. So judge air mass at full throttle in the middle of the rpm range.
- Number of values: pick 4–6. With 12, each one refreshes too rarely to catch boost changes.
Frequently asked questions
Is 2000 mbar in VCDS 2 bar of boost?
No. VCDS shows absolute pressure, so the boost above atmospheric is about 1 bar.
Actual boost is slightly above the request. Is that a problem?
A short spike after spool-up is not. Workshop tables also allow the actual boost slightly above the request, e.g. up to 2250 mbar against a 1850–1950 request on an ALH. Long overboost, or a spike that ends in limp mode, is a problem.
N75 shows 95%. Is the valve broken?
Not necessarily. On EDC16 and EDC17, 95% means the control unit asks for full boost. If the actual boost still does not reach the request, the cause is mechanical (see the typical fault patterns). On EDC15 the same state shows as a low percentage.
Why is boost below the request when cruising?
At part throttle that is common and usually not a fault. Judge boost at full throttle.
Can I compare a remapped car with the table?
No. A remap changes the specified boost and air mass. Compare the actual values with the specified ones from the same log.
Can this be checked automatically?
Yes. Upload the CSV file to vaglogs. The diagnostics compare boost and air mass with the requested values, take spool-up and pedal position into account, and show where in the log and by how much the values drifted apart.
Sources
- Ross-Tech: VAG-COM and TDI engines
- Dieselschrauber: measuring block values for VE engines and for PD engines
- Ross-Tech Wiki: fault codes 17964, 17963 and 16683