Shop manual diagnosis charts should be referred to when diagnosing for air problems to determine the presence of air. First install a short clear plastic hose into the return line at the top of the injection pump (Figure 4-12).
Start the engine and observe for air bubbles or foam in the line. If foam or bubbles are present, proceed as follows:
1. Raise vehicle and disconnect both fuel lines at the tank unit.
2. Plug the smaller disconnected return line.
3. Attach a low pressure (preferably hand operated pump) air pressure source to the larger 3/8 fuel hose and apply 8-12 P.S.I.
a. Diagnosing trucks equipped with dual tanks will require a check of the right fuel lines with the dash switch in the right tank position and a check of the left fuel lines with the dash switch in the left tank position. The switching valve could be a source.
4. Observe the pressure pump reading of 8-10 P.S.I. A decrease in pressure will indicate the presence of a leak. The pressure will push fuel out at the leak point indicating the location of the leak.
5. Repair as necessary.
In checking for air problems, the proper size clamps on all hoses should be checked. Also, a burr on the edge of a pipe could rip the inside of a line and create air ingestion. Particular attention should be given to improper installation or defective auxiliary filters or water separators.
Since operation of the hydraulic advance mechanism is dependent on transfer pump pressure and pump housing pressure, any deviation from pre-set tolerances can affect the advance mechanism and therefore, the injection timing. Fuel pump delivery less than 51/2 lbs. to 6 Vi lbs. pressure, for example, will reduce total advance directly proportional to pressure loss. Leaks, plugged filters, air ingestion restriced lines etc. will all reduce pressure delivery. Return line restriction can raise housing pressure to as high as transfer pump pressure depending on the degree of restriction and eventually stall the engine by upsetting the balance of transfer pump and housing pressures.
