How to read a 1.8T boost log in VCDS (groups 115 and 118)

Updated: 2026-11-05

Is your 1.8T losing power, hesitating or building boost inconsistently? Before replacing the turbocharger or N75 valve, record a VCDS log. Measuring group 115 shows requested and actual boost pressure, while group 118 helps you relate the result to intake air temperature and boost control.

This guide covers older 1.8T 20V petrol engines, including selected Audi A3 8L, A4 and TT 8N models, and the VW Golf IV and Passat B5. Measuring groups depend on the ECU — always verify the field labels in your version of VCDS. Older engines with Motronic M3.8, such as the AEB, may show boost in a different group (e.g. 011). If you do not have a CSV file yet, start with How to record a VCDS log.

At a glance: what a healthy log looks like

• Under load: actual boost approaches the requested pressure and continues to follow it.
• At the start of acceleration: actual pressure may briefly lag behind the request while the turbo spools up.
• Short deviations: these can occur when load changes. How long they last, and whether they repeat, matters.
• N75: assess its duty cycle together with boost pressure, not as a standalone valve test.
• Intake air temperature: higher temperatures after several runs can affect engine performance.

How to record the log

1. Warm up the engine, check the oil level and read any stored fault codes.
2. In VCDS, go to Select → 01-Engine → Meas. Blocks - 08.
3. Select 115, optionally also 118. Fewer groups generally mean a higher sampling rate.
4. Click Log, enter a filename such as 18t-115-118.csv, then click Start.
5. With a passenger operating the laptop, accelerate under load in a safe, legal location, ideally under controlled conditions. Audi's test procedure for certain 1.8T engines uses third gear from around 2000 rpm.
6. Finish with Stop → Done, Close. If necessary, repeat the test to check whether the result is consistent.

Do not attempt full-load testing if the engine is misfiring severely, knocking audibly, overheating or has another serious fault. The driver must not operate the laptop.

Group 115: requested and actual boost

Typical fields in group 115 on many ME7 ECUs:

VCDS normally displays absolute pressure. At an atmospheric pressure of 1000 mbar, a reading of 2000 mbar corresponds to approximately 1.0 bar of boost above atmospheric pressure. At higher altitudes, subtract the actual ambient pressure rather than automatically subtracting 1000 mbar.

The important comparison is between the two pressure curves while the ECU is requesting boost — not at idle or immediately after lifting off the throttle.

Specific values from Audi workshop documentation

Audi A3 8L workshop instructions give the following check values for selected engine versions during a third-gear test:

These are test values for particular engines at specific operating points, not the maximum boost pressure for every 1.8T. Requested pressure may differ after an ECU remap. Sources: Audi A3 8L — checking charge pressure and 154–165 kW versions.

Group 118: N75 and intake air temperature

On many 1.8T ECUs, group 118 contains:

On the petrol 1.8T, N75 controls the wastegate, not the variable-geometry turbo vanes used on many TDI engines. On a typical ME7 configuration, a higher duty cycle can indicate a greater boost-control request, but never diagnose the valve from the percentage alone without understanding the ECU configuration.

Typical fault patterns

None of these patterns is a definitive diagnosis. Several faults can produce similar symptoms. If a leak is suspected, a separate pressure or smoke test following the procedure for that engine may be needed.

Worked example

The following values are hypothetical and illustrate the method only:

The large difference at the beginning may be due to turbo spool-up. At higher rpm, the values track each other closely. A proper assessment still requires the full time series, not just four readings.

How vaglogs judges group 115

vaglogs diagnostics compares actual boost with the request from 2500 rpm, in stretches of at least 1 second with a steady request. If the log has the accelerator pedal, underboost is judged at full throttle only (pedal at 80% or more). While the boost is still climbing quickly towards the request, the turbo is spooling up, and that is not reported. It reports:

• 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.

Related fault codes

The absence of fault codes does not rule out a problem. Do not replace a turbocharger based only on one log.

Common mistakes

• Confusing absolute pressure with gauge boost pressure.
• Using the same reference table for stock and remapped engines.
• Logging too many groups at once. Short pressure fluctuations can be missed.
• Judging the graph after the throttle has closed. That is a different operating condition.
• Assuming every boost-pressure drop means a failed turbocharger.

Frequently asked questions

Is group 115 enough to test the N75 valve?

No. It shows the result of boost regulation, not the physical condition of the valve. Include group 118 and, where necessary, mechanical tests.

Does a brief boost overshoot mean something is broken?

Not necessarily. Consider the duration and size of the deviation, as well as engine load and speed.

How can I analyse the CSV file?

Open it in vaglogs. The diagnostics mark the stretches where actual boost drifts away from the request, and the chart shows both traces. Related guides: Ignition timing retard on the 1.8T and How to find an intake leak.

Sources

• Audi A3 8L: checking charge pressure control, group 115.
• Audi A3 8L: checking charge pressure on 154–165 kW versions.
• Ross-Tech Wiki: 17705/P1297 — pressure drop between turbo and throttle.
• Ross-Tech: measuring blocks and logging in VCDS.