Question about low fuel pump current -- Opera House or anyone else

[MrShorty]

Opera House has mentioned on several occasions that a fuel pump current below ~5 amps is indicative of a fault in the fuel delivery system.
For fun, I hooked up an ammeter to mine and checked. Got a steady 2.8 to 3.2 amps. Current varied as expected (2.8-2.9 A at high vacuum conditions, 3.1-3.2 A for low vacuum conditions). As a further check, I put fuel pressure gauge on it to see the fuel pressure at idle (Didn't want to try to drive around with the gauge hooked up). Fuel pressure at idle is right where it should be, and didn't drop off when the engine RPM was raised. Other than leakdown (I think the check valve in the pump is bad), the fuel pressure test showed everything perfect.
I may have just answered my own question: a bad check valve. Isn't the check valve a standard ball + spring type check valve? The pump has to generate a certain pressure to overcome the spring in the check valve. If the check valve isn't closing (bad spring or otherwise stuck), then the pump doesn't have to work to overcome the check valve. Since pump current is a measure of how hard the pump has to work, you eliminate one source of resistance to fuel flow and the pump current will go down. Seem reasonable?
To date, I have resisted the idea of replacing the pump solely because of a bad check valve. The cost and pain of changing a pump to avoid priming the pump two or three times on cold starts just hasn't seemed worth it.

 

[Opera House]

I got that information from Dr. Phils new book, "More ideas I just pulled out of my a$$." First measured the current on my 92 w/140K a few years ago that was having regulator problems. The engine would die at about 2 1/2 amps. Last measured my 97 about 6 months ago at about 140K and it was 4.89A at idle. Reminds me that I need to measure it again, doing a study on the current over time. My meters are very accurate. Could be a very worn pump and deserves further investigation. Think there is a flow test for the exit of the regulator. Is this the factory regulator? Leakdown problems usually don't affect engine running since these are high bypass back to the tank. Assume you are having starting problems.

 

[Opera House]

Did a little experiment. I inserted a current meter and a two ohm resistor in place of the fuel pump fuse. This dropped the idle pump current to 2.93A (by coincidence) operating 8.29 volts at the hood. That would be with a battery voltage of 14.3V. Engine started and ran fine although I never drove it or measured the rail pressure. This is at least an example of what resistance in the electrical circuit can do. Isn't science great! Normal pump current is now 4.57A. Previous readings have been 4.89, 4.66, and 4.71V. I should be saying, much below 5A.

 

[MrShorty]

Interesting stuff, Opera House. And, yes, science is great.
I'm only having starting problems if I don't cycle the key a couple of times before starting. I'm going to have to investigate the return flow thing. I know with the engine off and the FP test lead grounded, I can hear the return flow in the gas tank, but I haven't disconnected anything in an attempt to quantify the return flow.
Does it make sense that the current draw would decrease over time? From Ohm's law (E=IR), if the current is decreasing, then the resistance must be increasing, assuming a constant voltage. That makes sense, doesn't it? Extended over a long enough time, eventually the resistance through the motor would be so great, that there isn't enough power to run it. Assuming this is right, then I wonder if there's some threshold resistance/current value above/below which the pump ceases to function. Could this be made into a reliabe measure of fuel pump operation? May have to explore this when I get back from vacation.
On a side note, while I was measuring the current alongside the pressure, I decided to run my own little experiment. Since I was under the hood with the ammeter in line with the fuel pump, I just disconnected one of the leads while the engine was running and watched the pressure drop. Engine began to stumble ~20-25 psig. Repeated it a few times, and it seemed pretty consistent. This was all done at idle; don't know what would happen at higher RPM, but it does suggest a lower threshold value for the fuel pressure.

 

[Opera House]

The resistance of the pump shouldn't increase much. As the brushes wear, contact force will decrease and cause a slight increas. When they wear enough, they will become intermitant and wear even faster. You can look at the pump as positive displacement, and the motor with a series resistance. Theoretically the motor draws current till it reaches speed. At that point it no longer draws current because it generates the same voltage as the supply. Pumps usually just die. An electrical series resistance will decrease the pump flow but the pressure will remain the same as long as there is enough voltage to overcome the internal pump leakage and injector flow (if engine is running). I would be sticking some pins in the pump leads to see what the voltage is at the motor. For regulator leakage, a good experiment would be this. Jumper the pump relaywith some combination of small lamps. Find a value that will run the pump at about 20# rail pressure. This should not be enough to open the regulator. Listen for fluid noises or even disconnect the regulator return line to see if you have flow.