Leak Detector

Overview

A leak detector is a specialised diagnostic tool used to identify leaks across various automotive systems. In vehicle manufacturing and workshop maintenance, "leakage" encompasses multiple areas: from the airtightness of engine blocks and gearbox casings, to the sealing integrity of evaporative emission control systems, as well as the weatherproofing and soundproofing of the vehicle body. The primary advantage of a leak detector is its ability to pinpoint leak locations quickly and accurately without dismantling or compromising component assemblies, thereby preventing safety risks or performance degradation caused by minute leaks.

Based on their operating principles and application scenarios, leak detectors are primarily categorised into differential pressure leak detectors, ultrasonic leak detectors, and smoke leak detectors. In addition, modern vehicles are equipped with an on-board Diagnostic Module Tank Leakage (DMTL) system to actively monitor the fuel system for leaks.

Differential Pressure Leak Detector

Differential pressure leak detectors are the most widely utilised leak testing equipment in automotive component manufacturing, primarily employed to verify the airtightness of parts such as engine cylinder blocks, cylinder heads, gearbox casings, and radiators.

Its operating principle relies on the differential pressure comparison method, functioning much like a balance scale. During testing, the device simultaneously pressurises a reference standard and the test workpiece with equal air pressure, keeping the differential pressure sensor between them in equilibrium. If the test workpiece leaks, this balance is disrupted, allowing the sensor to detect the resulting pressure difference and calculate the leak rate.

The key advantage of this technology lies in its symmetrical design and synchronous testing, which effectively neutralises the effects of adiabatic temperature rise from pressurisation, pipework deformation, and ambient temperature fluctuations, achieving a measurement precision of up to ±0.1 Pa. The testing cycle of a differential pressure leak detector typically comprises three stages: inflation, stabilisation, and testing, and is widely deployed for seal testing across automotive production lines.

Ultrasonic Leak Detector

Ultrasonic leak detectors are mainly used for full-vehicle airtightness testing, accurately pinpointing subtle leak points around doors, windows, sunroofs, and wiring harness grommets.

The system operates on ultrasonic detection technology. During testing, an ultrasonic transmitter is placed inside the cabin, while a high-precision ultrasonic receiver probe scans the exterior of the vehicle. If body leaks exist, ultrasonic signals escape through the gaps, enabling the receiver probe to capture these high-frequency signals beyond human hearing and pinpoint the exact leak location.

Featuring non-contact operation and high sensitivity, ultrasonic leak detectors offer significantly higher testing efficiency than conventional water ingress tests. They can detect microscopic leaks undetectable by other methods, directly safeguarding the vehicle's NVH (Noise, Vibration, and Harshness) performance and weatherproofing quality. Currently, this technology is utilised by mainstream carmakers such as SAIC, Wuling, and SINOTRUK for sealing quality control during assembly.

Smoke Leak Detector

Smoke leak detectors are primarily utilised for evaporative emission control (EVAP) system leak testing. China 6 emission standards mandate that vehicles detect leaks as small as 0.5 mm in diameter, triggering the check engine light once a leak is identified.

The unit operates by introducing pressurised smoke infused with a fluorescent dye into the fuel system (typically pressurised to 3.49 kPa). If a leak is present, the smoke escapes through the breach, allowing technicians to locate the leak quickly under UV light. This approach is particularly effective for detecting minute leaks across fuel tanks, fuel lines, and carbon canister purge solenoid valves.

On-Board Leak Diagnostic System (DMTL)

Beyond workshop diagnostic equipment, modern vehicles feature an integrated Diagnostic Module Tank Leakage (DMTL) system, which actively monitors the fuel system's sealing integrity during vehicle operation.

Mandated under China 6 emission regulations as an enhancement to the EVAP system, this diagnostic module comprises an electric vacuum pump, a changeover valve, and a reference orifice (throttle orifice). Its operating cycle runs through four modes: fuel tank ventilation, carbon canister regeneration, reference measurement mode, and fuel tank measurement mode.

In reference measurement mode, the system measures the pump's current draw under standard load via a 0.1 mm reference orifice as a baseline. In fuel tank measurement mode, the system pressurises the fuel tank to 2–3 kPa and evaluates system integrity by monitoring pump current variations. If the canister or lines are severely blocked, the pump current rises sharply; if a leak or breach is present, the current remains consistently below the reference baseline.

Once a fault is detected, the system logs the corresponding diagnostic trouble code (DTC) and illuminates the check engine light to alert the driver for servicing.

Correlation with Vehicle Inspection Regulations

At the vehicle inspection level, standalone leak detection is not a mandatory test item for vehicle owners, but its diagnostic outcomes have an indirect bearing on inspection clearance.

For internal combustion engine (ICE) vehicles, an EVAP system leak will prompt the on-board DMTL system to illuminate the check engine light and log DTCs. Under current OBD inspection guidelines, diagnostic scanners read emission-related fault codes during testing; any active emission fault codes will result in an immediate inspection failure.

For new energy vehicles (NEVs), the "Specification for Operational Safety Performance Inspection of New Energy Vehicles" (GB/T 44500—2024), implemented on 1 March 2025, mandates rigorous checks on traction battery safety and electrical systems. Among these, testing the insulation resistance of charging ports and equipotential bonding functions fundamentally as electrical leak detection—identifying potential electrical leakage hazards across high-voltage components.