Fuel Pressure Regulator - General Information and Replacement

General Information

Faulty fuel pressure regulators (FPR) are a known cause of misdiagnosed fuel system failures on 944s — DMEs, fuel pumps, and other components have been unnecessarily replaced when the FPR was the actual culprit.

The fuel damper is located on the supply side of the fuel rail and smooths out pressure pulsations caused by the fuel injectors. The fuel pressure regulator is located on the return side and controls the fuel rail pressure. The two components look similar and are often confused.

Failure symptoms:

• Engine runs rough
• Engine stumbles or sputters
• Engine runs very rich — black smoke from the exhaust
• Engine will not start
• Engine starts but stalls shortly after startup
• Noisy fuel pump
• Noticeable decrease in fuel economy

Tools

• 10 mm socket and ratchet
• Flat-tip or Phillips head screwdriver
• Catch rags
• Safety glasses
• 0–100 psi fuel pressure gauge (recommended)
• Adapter for fuel pressure gauge to fuel rail (11 × 1.50 mm female threads)

Part Numbers

Part No.	Model	Year	Engine Code
944 110 198 01	944	1982–1985	M44.02, M44.04
944 110 198 03	944	1985.5–1988	M44.5-10
944 110 198 04	944S / 944 (2.7L)	1987–1989	M44.40; M44.11 thru 46K 02979; M44.12 thru 46K 61255
944 110 198 06	944 (2.7L) / 944 S2 / 968	1989–1995	M44.11 46K 02980 forward; M44.12 46K 61256 forward; M44.41; M44.43/44
944 110 198 03 and 944 110 198 05	944 Turbo	1986–1989	M44.51 / M44.52 thru 47K 02563
944 110 198 05	944 Turbo	1989–1991	M44.52 47K 02564 forward

Testing

The most reliable check for a faulty FPR is to measure fuel rail pressure using FUEL-01. A failing FPR typically causes abnormally high fuel rail pressure.

Injector driver shutdown test: A common FPR failure mode is for the regulator to close, driving rail pressure extremely high. The resulting high differential pressure across the injectors causes them to draw excessive current, which the injector drivers detect and then shut down. To test for this:

1. With the engine not starting (or starting and immediately dying), disconnect the wiring connector from one injector.
2. Attempt to start the engine.
3. If the engine starts and continues to run (roughly, on three cylinders), the FPR is likely failed closed. The single disconnected injector reduces total current load enough to keep the drivers from shutting down.

Replacement

1. Disconnect the FPR vacuum line.
2. Place a catch rag under the regulator fuel line connection.
3. Disconnect the fuel line from the FPR. This is typically a hose clamp connection removable with a flat-tip or Phillips screwdriver.

Caution: The fuel rail may be under pressure. Wear safety glasses and cover the regulator with a rag before removing it — fuel can spray toward your face and eyes.

4. Remove the FPR retaining bolts (typically 10 mm socket).
5. Rock the regulator gently until it is free from the fuel rail.
6. Install the new regulator in the fuel rail. Install and tighten the retaining bolts.
7. Connect the fuel line to the regulator.
8. Connect the vacuum line to the regulator.