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Thread: Where should I measure fuel flow rate?

  1. #1
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    Where should I measure fuel flow rate?

    I want to measure fuel flow rate. I believe this would be best done by measuring the flow rate at the fuel return line (FD to fuel tank, via accumulator), with pump running and engine off. Where is the easiest place to do this? Page D:01:07 says this should be approximately 29 gal/hour. I think it would be less meaningful to measure the flow rate by disconnecting the fuel supply line to the FD. An old post said this rate should be ~1.15 gal/minute. The 'fuel injector' test is also probably valid, and I've done it several times, but it isn't very easy.

    This issue arises from the following. On a recent excursion at freeway speeds (the first in a year), after about 15 minutes I found I couldn't maintain 70 mph. While limping home, I couldn't give it full throttle, although it ran smoothly until it hit what I presume to be the fuel limit. The idle is smooth, and I can rev the engine (no load) with no coughs or sputters.

    Of course I also need to measure the fuel pressure at the FD, but I'm waiting on repair parts for my tester.

    Thanks in advance,
    Robert
    Robert
    1981 DeLorean #1890
    1976 Datsun 280Z
    1968 Pontiac Le Mans convertible

  2. #2
    On Vacation to North Pole Ron's Avatar
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    No idea why the difference in the two flow rates...
    I'm thinking the flow should be the same (between main components), so it wouldn't matter where really. The symptoms could fit a pump or filter failing and fuel has to get to the FD first, so I'd check it at one of the two with whatever plumbing was handy while I waited.

    To be sure- You checked the Primary and Pump pressure before finding the leak??

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    Even if you could accurately measure the flow rate, all that will tell you is you don't have enough. The engine sputtering told you that. It won't tell you why. Here is what I would do. Empty the fuel tank and take out the parts, and inspect and clean everything. Replace anything suspect. Replace the fuel filter by the motor. My guess is the pick-up hose is collapsing and/or a filter is dirty. Replace the air filter. If that doesn't fix you up maybe the coil is breaking down or the ignition wires. Replace the wires, cap, rotor, and spark plugs unless they have been recently replaced.
    David Teitelbaum

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    Senior Member Bitsyncmaster's Avatar
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    I ran a flow test. This is the amount from the return line with the pump running for 60 seconds. This was with 5 gal. in the tank. Flow was just a little less than 2 quarts for 60 seconds. So that is the flow at 75 PSI since my primary pressure regulator has been calibrated for 75 PSI.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  5. #5
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    Thanks for confirming that Dave. A little less than 2 qts in 60 seconds sounds close enough to 29 gal/hr to me.

    I'm assuming that 70 mph cruising takes 2-3 gal/hr to sustain, and moderate acceleration at that speed would take double or triple that.
    Robert
    1981 DeLorean #1890
    1976 Datsun 280Z
    1968 Pontiac Le Mans convertible

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    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by DMC1890 View Post
    Thanks for confirming that Dave. A little less than 2 qts in 60 seconds sounds close enough to 29 gal/hr to me.

    I'm assuming that 70 mph cruising takes 2-3 gal/hr to sustain, and moderate acceleration at that speed would take double or triple that.
    That was with my GM pump but since it's draw was around 6 amps and the OEM pumps were 7 amps I would expect the same flow rate from either. With the new type pumps, it's real easy to pull the return line off the pump since it's a quick connect. I have about 25 feet of spare hose and an extra quick connect so no effort to test. That test I did when I was measuring the fuel level sender resistance with each half gallon added to the tank.

    I'm not doing much driving yearly so if my tank of gas gets about 6 months old I can drain it via the return line and use that gas in my daily driver.
    Last edited by Bitsyncmaster; 10-23-2019 at 05:41 PM.
    Dave M vin 03572
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  7. #7
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    Just to follow up... I did get my replacement connector hose for my TU pressure tester, and was able to make some measurements. Pressure is 72 psi, with flow of 26 fl oz in 30 seconds (slightly over 24 gal/hr). The pump was drawing 5.4 A under battery voltage. I don't know what the voltage at the pump was. I think I have seen in the past that primary pressure will rise slightly with the engine running (e.g. with alternator output).

    So I set out on a test run to see if I could repeat the earlier symptoms. It ran fine at 75 mph, and could accelerate on the overpasses. I'm not sure if that is a success, or failure. However, on the way home, running at a steady 35 mph, it died totally. Analysis today shows the fuel pump 'locked', drawing 11 A, and subsequently dropping the voltage at the pump to 7 v. I'm not really sure if I can directly connect the dots between that outcome, and the earlier performance, but obviously a new pump will change things.

    I suppose the 7 v at the pump with the 11 A load has been discussed before. In this case, if all the voltage drops in the circuitry were eliminated, I suspect the outcome would be to blow the 20 A fuse.
    Robert
    1981 DeLorean #1890
    1976 Datsun 280Z
    1968 Pontiac Le Mans convertible

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    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by DMC1890 View Post
    I suppose the 7 v at the pump with the 11 A load has been discussed before. In this case, if all the voltage drops in the circuitry were eliminated, I suspect the outcome would be to blow the 20 A fuse.
    Only time I've heard of the fuel pump fuse blowing is when a frequency valve was shorted. The long wiring also limits the current draw. I know of a few warmup regulators that have shorted the heater and that also does not blow that fuel pump fuse.

    Your primary pressure will not change when the fuel pump voltage goes higher with the engine running. Most of them I've checked run around 70 PSI unless you re shim the PPR.
    Last edited by Bitsyncmaster; 10-28-2019 at 07:29 PM.
    Dave M vin 03572
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  9. #9
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    I'm calculating .47 ohms for the wiring, fuse, etc, and .63 ohms for the locked pump. So Dave, with your pump pulling 6 A, do you recall what voltage you got at the pump, or rather how much drop you have in your pump wiring?
    Robert
    1981 DeLorean #1890
    1976 Datsun 280Z
    1968 Pontiac Le Mans convertible

  10. #10
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by DMC1890 View Post
    I'm calculating .47 ohms for the wiring, fuse, etc, and .63 ohms for the locked pump. So Dave, with your pump pulling 6 A, do you recall what voltage you got at the pump, or rather how much drop you have in your pump wiring?
    When I was testing the pump and sender I used my bench power supply. That gave me accurate current and voltage. In the past I did do testing on the OEM pump and found a 1 volt drop at the two pin connector to the pump. There is also the ground wiring due to the inertia switch. My pump is wired directly to frame ground because I have my battery relay switches off if the inertia switch gets tripped.

    Here is the data I took when testing the new pump and sender. The pump current seemed to increase as the tank got more gas in it.

    Note: pump current at 12.6 volts

    Empty 39.8 ohms
    2 Qt 39.8 ohms 4.8 amps
    4 Qt 39.8 ohms 4.6 amps
    6 Qt 48.0 ohms 5.5 amps
    8 Qt 55.9 ohms 5.5 amps
    10 Qt 63.7 ohms 5.9 amps
    12 Qt 71.3 ohms 6.1 amps
    14 Qt 78.4 ohms ?
    16 Qt 85.8 ohms 6.5 amps
    18 Qt 93.4 ohms 6.6 amps
    20 Qt 101.3 ohms 6.8 amps
    Last edited by Bitsyncmaster; 10-28-2019 at 09:07 PM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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