Fuel Pressure – Checking – 2.5L NA

Overview

This procedure outlines how to check the fuel pressure in the Porsche 944 (8-valve engine) for both pre-1985.5 and 1985.5 onwards models. Proper fuel pressure ensures optimal engine performance and helps diagnose issues with components like the fuel pump, fuel pressure regulator, or fuel filter. This guide is tailored specifically for the 8-valve engine and it’s variants (1983–1985.5 and 1986–1988).

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Tools Required

• Fuel Pressure Gauge (P 378 or VW 1318)
• Adapter for Fuel Rail Connection (12mm x 1.50mm threads) (if non-factory gauge is used)
• Catch Rags
• 10mm (3/8 inch) wrench (for fuel rail end cap, based on Porsche 944 Workshop Manual)

Parts Required

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

Step-by-Step Procedure

Step 1: Prepare for Fuel Pressure Testing

1. Place catch rags around the fuel rail to catch any fuel that may escape when removing the end cap, as the fuel rail may be under high pressure.
2. Locate the fuel rail end cap:
   For Pre-85.5 models:
   The cap is at the end of the fuel rail.

   For 85.5 Onwards models:
   The cap is on the side of the fuel rail closest to the intake manifold, between intake runners 1 and 2.

3. Using a 10mm (3/8 inch) wrench, carefully remove the fuel rail end cap. Catch the sealing ball (a small ball bearing) to prevent it from being lost.

Step 2: Attach the Fuel Pressure Gauge

4. Connect the fuel pressure gauge to the end of the fuel rail. If using a non-factory gauge, use an adapter with 12mm x 1.50mm threads. Ensure the connection is secure to prevent leaks.

Step 3: Jumper the Fuel Pump Relay

5. Locate and remove the appropriate relay:
   For Pre-85.5 models:
   Remove the fuel pump relay from the relay/fuse panel under the dash on the driver’s side (left-hand drive).

   For 85.5 Onwards models:
   Remove the DME relay from the relay/fuse panel under the hood.

6. Jumper terminals 30 and 87b on the panel where the relay was removed. Check the terminal labels on the bottom of the relay if unsure. The fuel pump should start when the jumper is installed.

Step 4: Check Fuel Pressure with Engine Stopped

7. Read the fuel pressure gauge and compare it to the “Engine Stopped – Relay Jumpered” specification for the 8-valve engine: 2.5 bar ± 0.2 bar (36 psig ± 3 psig, or 250 kPa ± 20 kPa).
8. If the pressure is significantly higher, suspect a faulty fuel pressure regulator. If significantly lower, check the fuel pump or fuel filter for issues.

Step 5: Check Fuel Pressure with Engine Running

9. Remove the jumper and reinstall the relay.
10. Start the engine and let it idle.
11. Read the fuel pressure gauge and compare it to the “Engine Running at Idle” specification for the 8-valve engine: 2.0 bar ± 0.2 bar (29 psig ± 3 psig, or 200 kPa ± 20 kPa).

Step 6: Perform Leakdown Test

12. Stop the engine and let the car sit for 20 minutes.
13. After 20 minutes, read the fuel pressure gauge and compare it to the “20 Minute Leakdown” specification for the 8-valve engine: 1.0 bar (14.5 psig, or 100 kPa).
14. If the pressure is significantly below the specification, suspect a faulty check valve on the supply side of the fuel pump.

Step 7: Reassemble the Fuel Rail

15. Remove the fuel pressure gauge from the fuel rail, using catch rags to collect any escaping fuel.
16. Replace the sealing ball in the fuel rail end.
17. Reinstall the fuel rail end cap and tighten it to 22 Nm (16 ft-lbs) using a 10mm (3/8 inch) wrench.

Post Procedure Checklist

• Check for fuel leaks around the fuel rail end cap.
• Ensure the sealing ball is properly seated and the end cap is tightened to 22 Nm (16 ft-lbs).
• Verify that the fuel pump relay (Pre-85.5) or DME relay (85.5 Onwards) is reinstalled correctly.
• Start the engine and confirm it runs smoothly with no fuel-related issues.
• Inspect the work area for tools or debris left behind.

Sources

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