Fuel System Leakdown Failure

Overview

This procedure helps you diagnose why your Porsche 944 S / S2 (16v engine) might be hard to start after sitting for a while. The issue, called fuel system leakdown, happens when pressure in the fuel rail drops due to leaks. This could be from a faulty fuel injector, a bad fuel pressure regulator, or a leaking fuel pump check valve. Identifying and fixing the leak ensures your car starts reliably and runs smoothly.

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Tools Required

• Metric wrench set (sizes like 10mm (3/8 inch), 13mm, etc., depending on fittings)
• Metric socket set
• Hose clamping tool
• Fuel pressure gauge (with adapter for Porsche 944 S2 fuel rail if needed)
• DME/Fuel Pump relay jumper
• Catch rags

Parts Required

• Fuel pressure regulator (944 110 198 03) – if found faulty
• Fuel pump check valve (944 110 920 00) – if found faulty
• Fuel injector (944 606 110 01) – if found faulty, per injector

Required Additional Procedures

Steps in these procedures are required to complete this process:

• Fuel Pressure Test
• Fuel System Component Locations

Step-by-Step Procedure

1. Step 1: Set Up the Fuel Pressure Test

1. Follow this link to ensure this step – Fuel Pressure Test – of the process is followed accurately to connect a fuel pressure gauge to the fuel rail on your Porsche 944 S / S2.
2. Install a DME/Fuel Pump relay jumper to run the fuel pump and pressurize the fuel system to its normal operating pressure, which should be around 3.8 bar (55 psi) for the S2’s 3.0L engine.

2. Step 2: Test the Fuel Pressure Regulator

3. With the system pressurized, remove the DME/Fuel Pump relay jumper to stop the fuel pump.
4. Right away, use a hose clamping tool to clamp the rubber fuel line downstream of the fuel pressure regulator. If you’re not sure where it is, follow this link to ensure this step – Fuel System Component Locations – of the process is followed accurately.
5. Note the fuel pressure on the gauge and the exact time you clamped the hose—this is your starting point.
6. Wait 20 minutes, then check the pressure again. Compare it to the spec from the Fuel Pressure Test (should hold above 3.0 bar (43.5 psi) for the S / S2). If it’s still too low, the fuel pressure regulator isn’t the problem—move to the next step. If it holds steady now (but didn’t before), replace the fuel pressure regulator (944 110 198 03).
7. Remove the hose clamp from the fuel line.

3. Step 3: Test the Fuel Pump Check Valve

8. Reinstall the DME/Fuel Pump relay jumper to pressurize the fuel system again.
9. Remove the jumper to stop the pump.
10. Immediately clamp the rubber fuel line upstream of the fuel dampener using the hose clamping tool. Check its location using Fuel System Component Locations if needed—it’s near or opposite the fuel pressure regulator on the fuel rail.
11. Note the pressure and the time you clamped the hose.
12. After 20 minutes, check the pressure against the Fuel Pressure Test spec. If it’s still low, the fuel pump check valve isn’t the issue—move to the next step. If it holds steady now (but didn’t before), replace the fuel pump check valve (944 110 920 00), which is at the fuel pump inlet.

4. Step 4: Check the Fuel Injectors

13. If neither the regulator nor the check valve is leaking, the problem is likely a fuel injector. Start by removing the fuel rail with the injectors still attached. You’ll need to disconnect the fuel supply and return lines first—use a 13mm wrench for the fittings.
14. Once the fuel rail is off the intake manifold (so you can see the injector tips), reconnect the fuel supply and return lines.
15. Place catch rags under each injector tip to catch any leaks.
16. Install the DME/Fuel Pump relay jumper to pressurize the system and watch for leaks. Note which injector(s) drip fuel.
17. Replace any leaking injector(s) with a new one (944 606 110 01) or have them refurbished.

Post Procedure Checklist

• Check for leaks at all fuel line connections after reassembling.
• Start the engine and listen for smooth operation—no stuttering or misfires.
• Watch the exhaust—no black smoke should appear, which would indicate a still-leaking injector.
• Run a final fuel pressure test to confirm the system holds pressure per the spec (above 3.0 bar (43.5 psi) after 20 minutes).

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manual, Volume 1
• YouTube