Fuel System Leakdown Failure

Introduction

Fuel system leakdown failures are caused by one of three sources: a leaking fuel injector, a bad fuel pressure regulator (FPR), or a leaking fuel pump check valve. When any of these fail, the fuel rail drains down during extended sits, causing difficult starts — the fuel pump must refill the rail before the engine gets enough fuel to fire. If the cause is a leaking injector, black smoke on startup is typical.

The procedure below isolates which component is responsible.

Tools

• Metric wrench set
• Metric socket set
• Hose clamping tool
• Fuel pressure gauge and adapter for fuel rail (if needed)
• DME/Fuel pump relay jumper
• Catch rags

Test Procedure

1. Using FUEL-01, connect a fuel pressure gauge and install a DME/fuel pump relay jumper to pressurize the fuel rail to normal operating pressure.
2. With the system at normal pressure, remove the relay jumper to stop the fuel pump.
3. Immediately install a hose clamping device on the rubber portion of the fuel return line downstream of the FPR (between the FPR and the fuel tank). For FPR location, see FUEL-07. Note the current pressure and the time — this is the leakdown test start time.
4. After 20 minutes, note the fuel pressure.
   • If pressure passes the leakdown test (improved compared to a prior failed test), the FPR is leaking and should be replaced.
   • If pressure still fails the leakdown test, the FPR is not the source. Proceed to step 5.
5. Remove the hose clamp from the FPR return line.
6. Install the relay jumper to restart the pump and re-pressurize the fuel system.
7. Remove the relay jumper to stop the pump.
8. Immediately install a hose clamping device on the rubber portion of the fuel line upstream of the fuel damper (between the fuel pump discharge and the damper). The fuel damper looks nearly identical to the FPR and is located either adjacent to or at the opposite end of the fuel rail. See FUEL-07 for location photos. Note the pressure and the time.
9. After 20 minutes, note the fuel pressure.
   • If pressure passes the leakdown test, the fuel pump check valve is leaking and needs to be replaced. The check valve is threaded into the inlet of the fuel pump.
   • If pressure still fails the leakdown test, the damper-side clamp test is negative. Proceed to step 10.
10. If neither the FPR nor the fuel pump check valve is the source, one or more fuel injectors are leaking.
11. Remove the fuel rail with the injectors still attached. This will likely require disconnecting the fuel supply and return lines from the rail.
12. With the fuel rail removed from the intake manifold (so the injector tips are visible), reconnect the fuel supply and return lines to the rail.
13. Place catch rags under each injector tip.
14. Install the relay jumper to start the fuel pump. Observe which injector(s) are leaking. Replace or refurbish the leaking injectors as necessary.

What the result means

FPR leaking

If clamping the return line downstream of the FPR makes the rail hold pressure for 20 minutes, the fuel pressure regulator is bleeding the rail down through its return path. Replace the FPR — see FUEL-07.

Fuel pump check valve leaking

If the FPR clamp test still fails but clamping the line upstream of the fuel damper (pump discharge) makes the rail hold, the leak is back through the fuel pump. The check valve threaded into the pump inlet is not sealing — replace it.

Leaking injector(s)

If neither clamp holds pressure, one or more injectors are leaking past their tips. Remove the fuel rail with injectors attached, reconnect the supply and return lines with the tips visible, run the pump, and watch for the dripping injector(s). Replace or refurbish as necessary. A leaking injector typically also shows as black smoke on startup.