EVAP & Leak Detection Pump

To comply with United States Environmental Protection Agency (EPA Tier 1) and California Air Resources Board (CARB ULEV) regulations, all North American Volkswagen Golf MK4 and Jetta MK4 1.8T models (AWP, AWW) are equipped with an active, automated evaporative fuel vapor diagnostic system. The system must verify that the fuel tank, charcoal vapor canister, and purge lines remain hermetically sealed against hydrocarbon vapor release, capable of detecting leaks as small as 0.020 inches (0.5 mm).

To execute this test, VAG integrated the vacuum-driven Leak Detection Pump (LDP / V144) mounted inside the right rear wheel arch behind the fender liner. In contrast, European models (AUM, AUQ) utilize passive engine depression purge without an active pump.

code

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   BOSCH ME7.5 LEAK DETECTION PUMP (LDP V144) OPERATIONAL CYCLE                   │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                     [ Engine Intake Vacuum ]                                     │
│                                                │                                                 │
│                                                ▼                                                 │
│                         ┌─────────────────────────────────────────────┐                          │
│                         │   LDP Solenoid Valve (Driven by Pin 113)    │                          │
│                         └──────┬───────────────────────────────┬──────┘                          │
│                                │                               │                                 │
│          ECU Grounds Pin 113   │                               │ ECU Releases Pin 113            │
│          Vacuum pulls spring-  │                               │ Calibrated return spring pushes │
│          loaded diaphragm up   │                               │ diaphragm down, pumping filtered│
│          into upper chamber    │                               │ air into fuel tank (25 mbar)    │
│                                └───────────────┬───────────────┘                                 │
│                                                ▼                                                 │
│                                  [ Magnetic Reed Switch (F144) ]                                 │
│                                  Feedback signal on Pins 25 & 26                                 │
│                                                │                                                 │
│                        ┌───────────────────────┴───────────────────────┐                         │
│                        ▼ Rapid Diaphragm Drop                          ▼ Slow Diaphragm Drop     │
│                 Pressure Escaping Fast:                         Pressure Holds Steady:           │
│                 DTC P0455 (Gross Leak)                          EVAP System Airtight!            │
│                 DTC P0442 (Minor Leak 0.020")                   Readiness Sets to PASSED         │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

28.1. Mechanical & Electrical Deconstruction of the `V144` Pump

The V144 pump assembly integrates three operational components into a single composite housing:

Electro-Pneumatic Vacuum Solenoid:
- Wired to ECU Pin 25 and Pin 26.
- The reed switch contacts close when the diaphragm is at the top of its stroke and open as the diaphragm approaches the bottom of its stroke.
- The ECU determines fuel tank pressure decay rates by measuring the reed switch open-duration time (treed) using internal timer capture.
Spring-Loaded Pumping Diaphragm:
Internal Magnetic Reed Switch (F144):

28.2. The Three Phases of the LDP Diagnostic Test Cycle

The ECU initiates the EVAP diagnostic cycle automatically during warm engine idle (Tmot > 70°C, road speed v = 0 km/h, fuel level between 15% and 85%):

1. Phase 1: Atmospheric Reference Check

The ECU energizes the canister regenerative purge valve (N80 on Pin 15) to pull the charcoal canister down to manifold vacuum, verifying that the purge valve opens and seals properly.

2. Phase 2: Fuel Tank Pressurization

The ECU pulses Pin 113 at a rate of approximately 1 Hz to 2 Hz. Each pulse pulls the diaphragm up and lets the spring drive a stroke of air into the fuel filler neck. The pump continues cycling until the fuel tank reaches 25 mbar, at which point air pressure balances the internal spring, holding the diaphragm at the top of its travel.

3. Phase 3: Pressure Decay Measurement

The ECU halts pumping pulses and monitors how quickly the diaphragm drops:

Airtight System (No Leak): The 25 mbar pressure holds against the diaphragm. The reed switch remains closed for longer than a calibrated threshold time (tdecay > 28.0 seconds). The ECU records Test Passed and sets the OBD-II EVAP Readiness bit to 0 (Ready).
Minor / Micro Leak (0.020'' Orifice): Pressure bleeds off slowly (12.0 s < tdecay < 28.0 s). The ECU queues DTC P0442 / 16826 (Small Leak Detected).
Gross Leak (Missing Gas Cap or Split Filler Neck): The diaphragm drops instantly (tdecay < 4.0 seconds). The ECU triggers DTC P0455 / 16839 (Gross Leak Detected).
Electrical Circuit Malfunction: If the reed switch never closes or solenoid resistance is outside 25… 35 Ω, the ECU flags DTC P1472 (Short to Ground) or P1473 (Open Circuit).

28.3. Software Delete Protocol for Track & Clean Engine Bay Builds

When deleting the evaporative system for motorsport builds or engine swaps:

Disable Diagnostic Testing via Codewords:
- In array CLA*, set fault classes for P0442, P0455, P1472, P1473, and P0441 to 0x00 to prevent MIL illumination.
Configure ESKONF Hardware Driver Mask:
Zero Diagnostic Error Classes:

28.1. Mechanical & Electrical Deconstruction of the V144 Pump