A Volkswagen Golf Mk5 (also known as a Rabbit in some markets) equipped with a Stage 1 REVO tune suddenly developed performance issues after a starter motor and battery replacement. The engine starts and idles normally, but misfires above 2500 RPM and won’t rev past 4000 RPM. Prior to the battery change, the car ran with full boost. This article walks through troubleshooting the problem using VCDS (Vag-Com Diagnostic System) with a focus on potential turbo-related issues.
Initial Troubleshooting Steps and VCDS Scan Results
The owner initially suspected a circuit problem due to multiple fault codes appearing simultaneously after the battery replacement. Several components were checked:
- Battery: Confirmed fully charged.
- Relays: Engine compartment and under-dash ECU relays visually inspected, tested for clicking, and bench tested. All appeared functional.
- Fuses: All fuses checked and found to be intact.
- Connectors: Visual inspection of connectors for the N276 (Fuel Pressure Regulator Valve) and N80 (EVAP Purge Control Valve) revealed no obvious issues.
- Voltage Measurements: Voltage at N276 and N80 measured with the ignition on and engine off. Readings were:
- N276: Terminal 1 – 3.4V, Terminal 2 – 2.4V
- N80: Terminal 1 – 3.5V, Terminal 2 – 3.5V (The correctness of these readings needs further investigation.)
The following components have not yet been tested:
- N249 Diverter Valve (recently replaced with OEM part)
- O2 sensor heating circuit
- N75 Turbo Boost Control Valve
- Voltage Supply Relay (identification uncertain)
A VCDS scan revealed the following fault codes:
- 008852 – Fuel Pressure Regulator Valve (N276): P2294 – Open Circuit – Intermittent
- 012423 – Voltage Supply Relay for Engine Components: P3087 – Electrical Malfunction
- 001092 – EVAP Purge Control Valve (N80): P0444 – Circuit Open
- 000051 – Turbocharger Bypass Valve Control Circuit: P0033 – Malfunction / Open Circuit
- 000579 – Solenoid Valve for Boost Pressure Control (N75): P0243 – Malfunction
Focusing on the Turbocharger System with VCDS
Given the symptoms (limited RPM and misfires under boost) and the fault codes related to boost control (P0033, P0243), further diagnostics should focus on the turbocharger system. Here’s how VCDS can help:
1. N75 Valve Testing: Use VCDS Output Tests to activate the N75 valve. Listen for an audible click and monitor specified values in measuring blocks to confirm proper operation. A faulty N75 can cause overboost or underboost conditions.
2. Turbo Actuator Movement: Visually inspect the turbo actuator rod for free movement while performing the N75 output test. Sticking or binding can indicate a mechanical issue with the turbocharger.
3. Boost Pressure Readings: Log boost pressure using VCDS while driving the car (within safe parameters). Compare actual boost pressure to specified boost pressure to identify discrepancies. Low boost can point to vacuum leaks, a faulty N75, or a failing turbocharger.
Conclusion: Next Steps in Diagnosis
The multiple fault codes and symptoms suggest a potential issue within the turbocharger control system or a related electrical problem triggered by the battery replacement. Using VCDS to perform targeted tests on the N75 valve, turbo actuator, and boost pressure readings will help pinpoint the root cause of the problem. Further investigation into the “Voltage Supply Relay for Engine Components” fault (P3087) is also crucial, as it could be the underlying cause of the other issues. Refer to the factory wiring diagrams and VCDS measuring block documentation for specific procedures and expected values for the Golf Mk5.
