Attention: TDR Forum Junkies 
I ran some tests on my lift pump to try and find out why the pressure was getting down to single digits when my truck is working. Since the pump was out of the truck, (replaced with the latest rev at 50K miles), I had the opportunity to measure the pressure and flow at different voltages and using a variable restriction at the output.
The set-up I used was to pump diesel from a 1-quart container using a 3/8" suction line. The pump output pressure was measured by using a tee with a mechanical pressure gauge and a needle valve to adjust the restriction. The fuel was then returned to the container so it would be re-circulated. The pump power came from a regulated DC power supply and was set at 13. 6 volts to simulate a typical voltage level when the truck is running. An oscilloscope was connected to monitor the current draw of the pump as it operated. This allowed me to measure the RPM and current draw under the various pressure loading.
Here are my findings:
The pump has a 6 pole permanent magnet motor and turns at 5000 RPM. The current draw at 13. 6 volts averages 3. 6 amps with no restriction at the output of the pump. The motor shows the current varying as much as 1. 5 amps as the commutator rotates. As I close off the needle valve to build pressure on the gauge, the gauge needle will vibrate as if the pump is "cogging" and causing the pressure to pulse. With the needle valve fully closed, the maximum pressure was 11 PSI. (Remember, this was my bad pump. ) I opened the valve just enough to get an 8 PSI reading on the gauge. This is about the right pressure for a good pump when cruising on a level road. The pump flows about 75 gallons per hour under these conditions. I also monitored the temperature of the diesel as I ran this test for about 2 hours. The fuel, at the start of the test was at 60*F and after about 1 hour stabilized at 100*F. The pressure remained the same throughout this test however the gauge needle continued to vibrate + or - 2 PSI at the 8 PSI mark. This test could have gone on for days but I stopped it.
Next I dissembled the pump by removing the cover plate, which is held on with the 3 mounting studs. There is a rubber gasket below the cover and removing this reveals a nicely machined rotary vane pump. Looking closely at the construction shows that fuel enters from the input port through a hole of about ¼ inch. A stainless steel screen catches the big stuff before it enters the internal rotor housing. The output port is also about ¼ inch. What I found interesting is at the output port; the pressurized fuel is forced down into the motor housing. This fuel must be used to cool and lubricate the electric motor. The fuel in the motor housing then exits by popping off a spring-loaded check valve and is returned to the inlet side of the pump. It is this valve that controls the pump pressure. I defeated the spring-loaded valve by inserting a small nylon rod that would push against the valve and hold it closed with the cover re-installed. Retesting the pump showed the maximum pressure at 29 PSI with the needle valve at the output closed. Current draw now was at 6. 5 amps and pump speed was down to about 4000 RPM. The gauge needle now was bouncing + or - 4 PSI.
So what does this all mean? The problem with the lift pump pressure is most likely to be caused by the relief valve spring fatiguing due to the pressure pulsations. This spring, if weak will drop the pressure. If you have no pressure and can still hear the pump running, the spring has collapsed and is unable to keep the relief valve closed. A quick way to determine if your pump is weak is to measure the lift pump current draw. At 3. 6 amps the pump would only make 11 PSI. It draws almost 6 amps at 20 PSI.
Hope this helps.
The set-up I used was to pump diesel from a 1-quart container using a 3/8" suction line. The pump output pressure was measured by using a tee with a mechanical pressure gauge and a needle valve to adjust the restriction. The fuel was then returned to the container so it would be re-circulated. The pump power came from a regulated DC power supply and was set at 13. 6 volts to simulate a typical voltage level when the truck is running. An oscilloscope was connected to monitor the current draw of the pump as it operated. This allowed me to measure the RPM and current draw under the various pressure loading.
Here are my findings:
The pump has a 6 pole permanent magnet motor and turns at 5000 RPM. The current draw at 13. 6 volts averages 3. 6 amps with no restriction at the output of the pump. The motor shows the current varying as much as 1. 5 amps as the commutator rotates. As I close off the needle valve to build pressure on the gauge, the gauge needle will vibrate as if the pump is "cogging" and causing the pressure to pulse. With the needle valve fully closed, the maximum pressure was 11 PSI. (Remember, this was my bad pump. ) I opened the valve just enough to get an 8 PSI reading on the gauge. This is about the right pressure for a good pump when cruising on a level road. The pump flows about 75 gallons per hour under these conditions. I also monitored the temperature of the diesel as I ran this test for about 2 hours. The fuel, at the start of the test was at 60*F and after about 1 hour stabilized at 100*F. The pressure remained the same throughout this test however the gauge needle continued to vibrate + or - 2 PSI at the 8 PSI mark. This test could have gone on for days but I stopped it.
Next I dissembled the pump by removing the cover plate, which is held on with the 3 mounting studs. There is a rubber gasket below the cover and removing this reveals a nicely machined rotary vane pump. Looking closely at the construction shows that fuel enters from the input port through a hole of about ¼ inch. A stainless steel screen catches the big stuff before it enters the internal rotor housing. The output port is also about ¼ inch. What I found interesting is at the output port; the pressurized fuel is forced down into the motor housing. This fuel must be used to cool and lubricate the electric motor. The fuel in the motor housing then exits by popping off a spring-loaded check valve and is returned to the inlet side of the pump. It is this valve that controls the pump pressure. I defeated the spring-loaded valve by inserting a small nylon rod that would push against the valve and hold it closed with the cover re-installed. Retesting the pump showed the maximum pressure at 29 PSI with the needle valve at the output closed. Current draw now was at 6. 5 amps and pump speed was down to about 4000 RPM. The gauge needle now was bouncing + or - 4 PSI.
So what does this all mean? The problem with the lift pump pressure is most likely to be caused by the relief valve spring fatiguing due to the pressure pulsations. This spring, if weak will drop the pressure. If you have no pressure and can still hear the pump running, the spring has collapsed and is unable to keep the relief valve closed. A quick way to determine if your pump is weak is to measure the lift pump current draw. At 3. 6 amps the pump would only make 11 PSI. It draws almost 6 amps at 20 PSI.
Hope this helps.
Kevin
... ... ... ... . Kevin
