2000 AUDI A4 fuel

20-134

EVAP system leak detection


Function of leak detection system
The leak detection system can recognize leaks in
the entire fuel system, including the main fuel
tank, the EVAP canister and EVAP canister
purge regulator valve -N80.
On the mechanical side, the leak detection
system consists of a vacuum-operated
diaphragm pump (LDP) with a reed contact on
the pump diaphragm rod. The pump receives
vacuum from the intake manifold.

If the leak detection system is triggered; the
diaphragm pump pressurizes the fuel system
between 20 and 30 mbar.
If specified pressure is reached, the pump
diaphragm remains extended and the reed
contact remains in the "open" position.
If the pressure drops (i.e. there is a leak), the
diaphragm moves back and the reed contact
closes.
During diagnosis, first there is a measurement
of whether the reed contact has closed within
10 seconds. If this does NOT occur, the tank
system is then sealed.
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Fuel tank and EVAP system, checking
for leaks


Note:
In the following, individual questionable points
will be considered that could be leaks in the fuel
tank and tank ventilation system. After
eliminating a possible leak, it is effective to then
perform a tank leak diagnosis to determine
whether the leak was successfully sealed.

- Switch OFF ignition.

- Check if filler cap was completely closed for the
test.

- Check if filler cap seal is damaged or crimped,
replace if necessary.

- Check if the gaskets under the cover flanges are
OK and if the union nut on the fuel tank cover
flange is correctly tightened.

- Check hose connections for tightness and
integrity repair or replace as necessary.

- Remove luggage compartment trim.
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20-137

Note:
Observe the cover flange installation instructions.

Front wheel drive Page 20
-44


All wheel drive Page 20
-97
.

- Visually inspect all fuel tank system line and
hose connections and replace any leaking lines,
for example:


Line from EVAP canister to EVAP canister
purge regulator valve in engine
compartment.

Lines from EVAP canister to the LDP (under
left wheel house liner).

Line from the fuel tank to the EVAP canister
(under the cover for the EVAP canister).

Lines of the tank ventilation system (under
right wheel house liner).

Fuel lines to the engine.

Note:
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20-138

- Visually inspect the fuel supply system
components:


Fuel tank

EVAP canister

LDP

EVAP canister purge regulator valve -N80

Note:
At times, the above components cannot be
effectively leak tested using a leak detection
spray, either because of component location or
function. In such cases, clamp off the lines to the
component involved and perform a leak check. If
the leak is no longer displayed, the clamped off
component is faulty. If the LDP is clamped off
and there is still a leak displayed, the LDP is
faulty.

- Check EVAP canister purge regulator valve -
N80

Repair Manual, Fuel Injection & Ignition,
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20-151

Checking voltage supply (all vehicles)

Note:
Voltage supply to the LDP is via the fuel pump
relay.

Test condition:

Fuse for LDP is OK

- Disconnect connector from LDP.

- Connect voltage tester VAG1527 B as follows:Connector terminal
Measure
3 Engine Ground (GND)
- Briefly operate starter motor.
LED must light up.
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If LED does not light up:

Electrical Wiring Diagrams, Troubleshooting & Component Locations
If wiring is OK:
Repair Manual, Fuel Injection & Ignition, Repair Group 24 - Check electrical wiring from terminal 3 via fuse for fuel pump relay for
open circuit. - Repair wiring as necessary.
- Check fuel pump relay:
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If you did not find any malfunction, however the
Leak Detection Pump (LDP) did not function
during the output Diagnostic Test Mode (DTM):

- Replace Leak Detection Pump (LDP) -V144
Page 20
-162
.
If the LED does not blink, although wiring is OK:

- Replace Engine Control Module (ECM):

Repair Manual, Fuel Injection & Ignition,
Repair Group 24
- Generate readiness code:

Repair Manual, Fuel Injection & Ignition,
Repair Group 24
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20-159
If you did not find any malfunction, however the
Leak Detection Pump (LDP) did not function
during the Output Diagnostic Test Mode (DTM):

- Replace LDP Page 20
-162
If the LED does not blink, although wiring is OK:

- Replace the Engine Control Module (ECM)

Repair Manual, Fuel Injection & Ignition,
Repair Group 24
- Generate readiness code:

Repair Manual, Fuel Injection & Ignition,
Repair Group 24
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