Fuel System Leakdown Test – 2.5L NA

Overview

This procedure helps diagnose fuel system leakdown issues in the Porsche 944 with an 8-valve engine configuration, covering model years 1983 to 1988. A leakdown failure occurs when components like fuel injectors, the fuel pressure regulator, or the fuel pump check valve leak, causing the fuel rail to lose pressure. This can lead to hard starting, especially after the car has been sitting. The guide isolates the source of the leak for repair or replacement.

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Tools Required

• Metric Wrench Set
• Metric Socket Set
• Hose Clamping Tool
• Fuel Pressure Gauge (with adapter for fuel rail if necessary)
• DME/Fuel Pump Relay Jumper
• Catch Rags

Parts Required

• Fuel Pressure Regulator (if diagnosed as leaking): 944 110 198 01
• Fuel Pump Check Valve (if diagnosed as leaking): 928 110 229 02
• Fuel Injector (if diagnosed as leaking): 944 606 110 00

Required Additional Procedures

Steps in these procedures are required to complete this process:

• Fuel Pressure Test
• Fuel System Component Locations

Step-by-Step Procedure

Step 1: Pressurize the Fuel System

1. Follow the Fuel Pressure Test procedure to connect a fuel pressure gauge to the fuel rail. Ensure the gauge is securely attached.
2. Install a DME/Fuel Pump Relay jumper to activate the fuel pump and pressurize the fuel rail to its normal operating pressure, approximately 2.5 bar (36 psi).

Step 2: Test the Fuel Pressure Regulator

3. Remove the DME/Fuel Pump Relay jumper to stop the fuel pump.
4. Immediately clamp the rubber fuel line downstream of the Fuel Pressure Regulator using a hose clamping tool. For the Fuel Pressure Regulator location, follow this link to the Fuel System Component Locations procedure.
5. Note the fuel pressure and the exact time the clamp was applied. This marks the start of the leakdown test.
6. Wait 20 minutes, then check the fuel pressure. Compare it to the specification in the Fuel Pressure Test procedure (typically, pressure should hold above 2.0 bar or 29 psi).

For Pre-85.5 models:
If the pressure holds, the Fuel Pressure Regulator is likely leaking and should be replaced with part 944 110 198 01.

For 85.5 Onwards models:
If the pressure holds, the Fuel Pressure Regulator is likely leaking and should be replaced with part 944 110 198 01.

7. If the pressure still drops significantly, the Fuel Pressure Regulator is not the issue. Proceed to the next step.
8. Remove the hose clamp from the Fuel Pressure Regulator line.

Step 3: Test the Fuel Pump Check Valve

9. Reinstall the DME/Fuel Pump Relay jumper to pressurize the fuel system again.
10. Remove the DME/Fuel Pump Relay jumper to stop the fuel pump.
11. Immediately clamp the rubber fuel line upstream of the Fuel Dampener using a hose clamping tool. For the Fuel Dampener location, follow this link to the Fuel System Component Locations procedure.
12. Note the fuel pressure and the exact time the clamp was applied. This marks the start of the leakdown test.
13. Wait 20 minutes, then check the fuel pressure. Compare it to the specification in the Fuel Pressure Test procedure.

For Pre-85.5 models:
If the pressure holds, the fuel pump check valve is likely leaking and should be replaced with part 928 110 229 02.

For 85.5 Onwards models:
If the pressure holds, the fuel pump check valve is likely leaking and should be replaced with part 928 110 229 02.

14. If the pressure still drops significantly, the fuel pump check valve is not the issue. Proceed to the next step.
15. Remove the hose clamp from the Fuel Dampener line.

Step 4: Test the Fuel Injectors

16. If neither the Fuel Pressure Regulator nor the fuel pump check valve is the source of the leak, the issue is likely a leaking fuel injector.
17. Remove the fuel rail with the injectors still attached. Disconnect the fuel supply and return lines as needed, using a 17mm wrench for the fittings.
18. With the fuel rail removed from the intake manifold, position it so you can see the injector tips.
19. Reconnect the fuel supply and return lines to the fuel rail, ensuring tight connections to avoid leaks.
20. Place catch rags under each injector tip to catch any fuel leakage.
21. Install the DME/Fuel Pump Relay jumper to pressurize the fuel system.
22. Observe the injector tips for leaks. Note which injector(s) are leaking.
23. Replace any leaking injector(s) with part 944 606 110 00 or have them professionally refurbished.

Post Procedure Checklist

• Verify that all hose clamps and the DME/Fuel Pump Relay jumper have been removed.
• Check all fuel line connections for leaks after repressurizing the system.
• Start the engine and ensure it runs smoothly without excessive black smoke, indicating proper fuel delivery.
• Inspect the fuel rail, injectors, and surrounding areas for any residual fuel leaks.
• Confirm that all tools and debris are removed from the engine bay.

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manuals
• YouTube