Rotary Pump Priming Problems

Stanadyne DBO injection pump

Injection pump priming information for Stanadyne and Delphi (CAV, Lucas) pumps.

We receive a number of troubleshooting calls from people that have just installed a rebuilt pump, but can’t get fuel to the injectors. Most of the problems occur due to creating an airlock inside the injection pump.

The high pressure portion of the injection pump will have residual calibration fluid in it from testing the injection pump. The injection pump has to deliver the correct amount of fuel on the test stand during calibration, to pass testing. Once you get clean fuel with no air to the high pressure side, you should have no problem getting fuel to the injectors. Airlocks are usually created when all the injection lines are open at once or if you do not prime the system to get clean non aerated fuel to the high pressure portion of the pump, prior to cranking the engine.

Delphi (Lucas CAV) pump components
Delphi DPA injection pump with top cover fuel vent
Stanadyne DBO injection pump
Stanadyne (Roosa Master) DB series pump. Note; when replacing the injection pump, leave the return connector in the rebuilt pump. It is a check ball style fitting and the pump was calibrated with it. If the governor came apart inside the pump, then the return connector could be restricted and cause running issues.


  1. Use the primer pump to prime the fuel filter. Caution: Do not fill the fuel filter with fuel, because the filter will remove contamination that you can’t see. Slacken the filter outlet, or the fuel pump inlet, whichever one is higher. Allow fuel to flow until it is free from air. Tighten connections
  2. Loosen the head vent screw on the injection pump, if equipped, and operate the primer until fuel free flows from the head vent screw. Then tighten the vent screw.
  3. Crank the engine over for about 5-10 seconds with the shutoff in the off position.
  4. Open the throttle lever and ensure the shutoff is in the run position, or the wire is connected to the shut-off solenoid.
  5. Loosen one high pressure line and crank the engine over until fuel free of air flows from the injection line. Tighten the injection line.
  6. Repeat step 5, one injection line at a time, until the engine starts.
  7. With the engine running at idle speed, loosen the governor vent screw, if equipped. When fuel free of air flows from the vent, tighten the vent screw. Sometimes it is necessary to run at a faster rpm than idle to vent the governor.

If you are having trouble getting fuel out of the injection lines, you may have created an airlock. Air locks can be difficult to purge. However, if you cycle the shut-off, on & off, while cranking, with only one line open, you can usually get them to purge. In some instances, we have had to put the injection pump back on the test stand and spin it over at about 300 rpm to clear an airlock.

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