Fuel Pressure – Checking

Overview

This procedure checks the fuel pressure in your Porsche 944 S / S2’s 16v engine to ensure the fuel system is working correctly. Proper fuel pressure is critical for engine performance, and this test helps diagnose issues like a faulty fuel pump, clogged filter, or leaking regulator.

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Tools Required

• Fuel Pressure Gauge (P 378 or VW 1318)
• Adapter for Fuel Rail Connection (12mm x 1.50mm threads) – required if using a non-factory gauge
• 10mm (3/8 inch) wrench
• Catch rags

Parts Required

• Sealing ball (if lost during removal) – 944 110 901 00

Step-by-Step Procedure

1. Prepare for Fuel Pressure Testing

1. Gather catch rags and place them under the fuel rail to catch any fuel that might spill. Be cautious, as the fuel rail may be under high pressure.

2. Remove the Fuel Rail Cap

2. Locate the fuel rail cap on the side of the fuel rail closest to the intake manifold, between intake runners 1 and 2 (specific to the 944 S / S2).
3. Using a 10mm (3/8 inch) wrench, carefully remove the cap. Catch the sealing ball inside the fuel rail as it may fall out when the cap is removed.

3. Attach the Fuel Pressure Gauge

4. Connect the fuel pressure gauge to the fuel rail using an adapter with 12mm x 1.50mm threads if your gauge isn’t a factory model (P 378 or VW 1318). If needed, follow the – Fabricating a Fuel Pressure Gauge Adapter -procedure to create a compatible adapter.

4. Jumper the Fuel Pump Relay

5. Locate the DME relay on the relay/fuse panel under the hood (specific to late-model 944 S / S2).
6. Remove the DME relay.
7. Jumper terminals 30 and 87b on the panel where the relay was removed. Check the relay’s underside for terminal labels if unsure. The fuel pump should start running once the jumper is in place.

5. Check Fuel Pressure (Engine Stopped)

8. Read the fuel pressure gauge and compare it to the specification for the 944 S2: 3.8 bar ± 0.2 bar (55 psig ± 3 psig) with the engine stopped and relay jumper.

6. Test Fuel Pressure (Engine Running)

9. Remove the jumper and reinstall the DME relay.
10. Start the engine and let it idle.
11. Check the gauge reading against the 944 S2 specification: 3.3 bar ± 0.2 bar (48 psig ± 3 psig) at idle.
12. If the pressure is much higher than 3.3 bar (48 psig), suspect a faulty fuel pressure regulator. If much lower, check the fuel pump or fuel filter for issues.

7. Perform Leakdown Test

13. Turn off the engine after idling.
14. Let the car sit for 20 minutes.
15. After 20 minutes, check the gauge against the 944 S / S2 leakdown specification: 2 bar (29 psig). A reading much lower suggests a faulty check valve in the fuel pump’s supply side.

8. Reassemble the Fuel Rail

16. Remove the fuel pressure gauge from the fuel rail, using rags to catch any fuel.
17. Place the sealing ball back into the fuel rail if it was removed.
18. Reinstall the fuel rail cap and tighten it to 22 Nm (16 ft-lbs) using a torque wrench.

Post Procedure Checklist

• Check for fuel leaks around the fuel rail cap.
• Ensure the sealing ball is in place and the cap is secure.
• Verify the engine starts and runs smoothly after reassembly.

Sources

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