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Home >> Volvo >> 2000 >> C70 Convertible L5-2.3L Turbo VIN 53 B5234T3 >> Repair and Diagnosis >> Powertrain Management >> Computers and Control Systems >> Description and Operation >> Leak Diagnostic

Leak Diagnostic






Leak diagnostic




Vapor which evaporates from the fuel in the fuel tank is routed to and stored in the EVAP canister from where it is guided into the combustion process via the canister purge (CP) valve and negative pressure in the intake manifold. A leak diagnostic has been introduced in certain markets to ensure that there are no leaks in the fuel tank system. The diagnostic is constructed in such a way that the system detects any leaks corresponding to a hole that is 1 mm (0.0394 in) or larger.
The fuel tank system consists of fuel tank, fuel filler pipe, EVAP canister, canister purge (CP) valve and lines between these components. To be able to diagnose the fuel tank system, it is also equipped with fuel tank pressure sensor and EVAP canister shut-off valve.
The diagnostic is divided into different phases in which the various components are tested. If a fault is detected in any of the phases the diagnostic is interrupted and the diagnostic trouble code (DTC) for the component identified is stored.
The diagnostic test works as follows:
1. The fuel tank pressure sensor is checked for an unstable signal. Diagnostic trouble code (DTC) ECM-4308 (faulty signal) is stored if the signal oscillates more than 5 times +/- 10 hPa (+/- 0.145 psi) during 5 seconds. The fuel tank pressure (A) is checked so for stability and that the short-term fuel trim is not too low.
2. EVAP canister shut-off valve (B) is shut and an evaporation check is carried out by gauging how much fuel tank pressure increases. This provides a value for how much fuel evaporates, and this value is used later to calculate leakage flow. If fuel tank pressure sinks, this indicates that the canister purge (CP) valve is leaking and diagnostic trouble code (DTC) ECM-4008 canister purge (CP) valve open is stored.
3. EVAP canister shut-off valve (B) is opened, the tank system is open. Canister purge (CP) valve (C) starts to pulse. As a result of the vacuum in the intake manifold the engine starts to suck in air through the EVAP canister. Because EVAP canister shut-off valve (B) is open, the fuel tank (A) pressure drops slowly. If fuel tank pressure sinks rapidly this indicates that the EVAP canister shut-off valve is clogged and diagnostic trouble code (DTC) ECM-4019 EVAP canister shut-off valve shut is stored.
4. EVAP canister shut-off valve (B) closes the canister purge (CP) valve (C) which pulses with a duty cycle of approximately 17%. Fuel tank pressure (A) sinks to -10 hPa (-0.145 psi). Diagnostic trouble code (DTC) ECM-4308 large leak (signal too low) is stored if this pressure is not reached within ten seconds. This indicates a larger leak in the fuel tank. Diagnostic trouble code (DTC) ECM-4308 large leak (faulty signal) is stored if the fuel tank pressure does not change within 2 seconds. This indicates a defective fuel tank pressure sensor or clogged piping.
5. Canister purge (CP) valve (C) closes, EVAP canister shut-off valve (B) is shut. There is now a negative pressure in the fuel tank. This negative pressure will decrease slowly. The decrease rate depends on fuel level, fuel evaporation and any leaks. Leakage flow is calculated by comparing pressure increase speed with the pressure decrease speed from stage 4 and by compensating for the evaporation measured in stage 2. Diagnostic trouble code (DTC) ECM-430D small leak is stored if the calculated leakage flow exceeds a certain level. This indicates a smaller leakage in the fuel tank system.
6. EVAP canister shut-off valve (B) is opened, the diagnostic test is concluded.
During the different phases when the system gauges whether the fuel tank system pressure acts normally or not, there are a number of circumstances which are taken account of, for example:
- the amount of fuel in the tank
- height above sea level
- fuel temperature and evaporation
The system can calculate this information. Therefore it is not possible to describe how quickly/much the pressure is permitted to increase or fall in the different phases.
To carry out the diagnostic it is necessary that:
- there are no diagnostic trouble codes (DTCs) registered for: Vehicle speed signal, canister purge (CP) valve, EVAP canister shut-off valve and fuel tank pressure sensor.
- fuel trim must be active
- the engine must be idling
- speed 0 km/h (0 mph)
- vehicle below 2500m above sea level
- outside temperature must be above -8°C (17.6°F)
- engine coolant temperature (ECT) must be above -8°C (17.6°F) and below 120°C (248°F)
- pressure in fuel tank must be above -10 hPa (-0.145 psi)
- the concentration of fuel fumes in the EVAP canister must not be too high
The diagnostic starts 17 minutes after starting the engine at the earliest, when all conditions have been met. It takes approximately 30 seconds. If the diagnostic is interrupted for any reason, it will try to start again the next time all conditions are met. The control module performs a maximum of 4 diagnostic attempts during an operating cycle. If no fault is detected the diagnostic will not run again in the same operating cycle. If a fault is detected two further attempts are made to evaluate the fault.

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