Fuel Pressure - Checking

Tools

• Fuel Pressure Gauge (P 378 or VW 1318)
• Adapter for Fuel Rail Connection (12 x 1.50 mm threads) — if factory gauge is not used *
• Catch rags

* See FUEL-20 for instructions on making an adapter for fuel pressure gauges with non-metric threads.

Fuel Pressure Specifications

Condition	Specification
All 8V 944s and 944 Turbos
Engine Stopped — Relay Jumpered	2.5 bar ± 0.2 bar (36 psig ± 3 psig)
Engine Running at Idle	2.0 bar ± 0.2 bar (29 psig ± 3 psig)
20-Minute Leakdown Pressure	1.0 bar (14.5 psig)
All 16V 944s
Engine Stopped — Relay Jumpered	3.8 bar ± 0.2 bar (55 psig ± 3 psig)
Engine Running at Idle	3.3 bar ± 0.2 bar (48 psig ± 3 psig)
20-Minute Leakdown Pressure	2.0 bar (29 psig)

Procedure

Caution: Have rags ready to catch any fuel that escapes from the fuel rail when the end cap is removed. The fuel rail may be under high pressure — remove the end cap carefully.

1. Remove the cap from the end of the fuel rail (8V cars). There is a sealing ball (essentially a ball bearing) inside the end of the fuel rail — catch it when removing the cap. On 16V cars, the cap is on the side of the fuel rail closest to the intake manifold, between intake runners 1 and 2.
2. Attach the fuel pressure gauge to the end of the fuel rail. Most gauges require an adapter. Metric adapter kits are available with more expensive gauge sets. A shop-fabricated adapter can be made from an old fuel rail end cap drilled and tapped to accept a pipe-thread-to-SAE-thread adapter.
3. On early (pre-1985.5) cars, remove the fuel pump relay from the relay/fuse panel under the dash on the driver's side (left-hand drive).
4. On later-model cars, remove the DME relay from the relay/fuse panel under the hood.
5. At the panel where the relay was removed, jumper terminals 30 and 87b. (Check terminal numbers on the underside of the relay if unsure.) The fuel pump should start when the jumper is installed.
6. Check the gauge reading against the Engine Stopped — Relay Jumpered value in the specification table for your vehicle.
7. Remove the jumper, reinstall the relay, and start the engine.
8. Check the gauge reading against the Engine Running at Idle value in the specification table.
9. Readings significantly above specification typically indicate a faulty fuel pressure regulator. Readings significantly below specification typically indicate a faulty fuel pump or clogged fuel filter.
10. After running the engine at idle, stop the engine and allow the car to sit for 20 minutes to check fuel system leakdown.
11. After 20 minutes, check the gauge reading against the 20-Minute Leakdown value for your vehicle.
12. Readings significantly below the leakdown specification are typically caused by a bad check valve on the supply side of the fuel pump.

Caution: Have rags ready when removing the fuel pressure gauge from the fuel rail — residual pressure may remain.

13. When testing is complete, remove the fuel pressure gauge from the fuel rail.
14. Replace the sealing ball and end cap on the end of the fuel rail. Tighten the cap to 22 Nm (16 ft-lb).