EFI fuel pumps
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Thread: EFI fuel pumps

  1. #1
    Fellow Frogger!
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    Default EFI fuel pumps

    This may sound like a strange question to which I should know the answer, but do EFI fuel pumps run continuously with ignition on, or are they modulated by the ECU in some way?
    roger

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    Fellow Frogger! racing405's Avatar
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    Your efi pump should start when you put the key into run. If you don't turn the key to start position, the pump should shut off again after a few seconds. If you start the engine, the ECU will maintain a signal to keep the fuel pump solenoid pulled in and the pump running. The only time it will stop is if you turn the key off, or the ECU recognises the engine stops - stall / accident - in which case the pump will be shut off again to avoid spraying fuel into a crash condition.
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    1000+ Posts bluey504's Avatar
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    The simple answer is that an EFI system is basically a full flow circuit with programmed fuel leaks.....Fuel flow and pressure are best 'controlled' with a steady state baseline ie It is what it is. The fuel delivery amount is governed by how long the injector is open, or for comparison the volume flowed in a stated time. Strangely the amount of fuel squirted is the same each time when warmed up, it is the cycle rate (RPM)/ duty rate that produces the power. (Naturally aspirated engine at the same altitude...EFI helps to correct the mixture ratio as the air gets colder/thinner via measuring the air volume and temperature.)
    Hope this helps, I basically look at pumps as conveyers. Electric pumps run at full speed/pressure in all situations whilst mechanical pumps mimic a windmill. More speed produces more flow.
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    Thanks for the replies.
    roger

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    Real cars have hydraulics DoubleChevron's Avatar
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    depends on the car. Most vehicles have a tachometric relay ... so if the engine drops below the threshold (about 200rpm) the fuel pump will be switch off. They obviously over-ride the tachometric functionality with the starter circuit so the car can intially start.

    The pump should never run with the ignition on, as in the event of an accident there is nothing to stop the fuel pump running until someone switches the ignition off...................... Having said that I've noticed old carby land rovers .... always have the pump running when the ignition is on. I'm assuming there must be an impact switch in there somewhere that'll drop the pump out in the event of an accident

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    Quote Originally Posted by DoubleChevron View Post
    depends on the car. Most vehicles have a tachometric relay ... so if the engine drops below the threshold (about 200rpm) the fuel pump will be switch off. They obviously over-ride the tachometric functionality with the starter circuit so the car can intially start.

    The pump should never run with the ignition on, as in the event of an accident there is nothing to stop the fuel pump running until someone switches the ignition off...................... Having said that I've noticed old carby land rovers .... always have the pump running when the ignition is on. I'm assuming there must be an impact switch in there somewhere that'll drop the pump out in the event of an accident

    seeya,
    Shane L.
    At risk of hijacking the thread, for which Roger has his answer, I'd observe that one of the old Land-Rover issues was actually keeping the fuel pump running. Does anyone remember SU fuel pumps inside chassis rails, covered in mud?

    Roger, you might think of cooling systems in much the same way. The water pump provides a steady pressure and potential flow for particular engine revs, and the thermostat regulates the actual flow according to temperature.

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    Now go make me a sandwich Hotrodelectric's Avatar
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    Quote Originally Posted by DoubleChevron View Post
    depends on the car. Most vehicles have a tachometric relay ... so if the engine drops below the threshold (about 200rpm) the fuel pump will be switch off. They obviously over-ride the tachometric functionality with the starter circuit so the car can intially start.

    The pump should never run with the ignition on, as in the event of an accident there is nothing to stop the fuel pump running until someone switches the ignition off...................... Having said that I've noticed old carby land rovers .... always have the pump running when the ignition is on. I'm assuming there must be an impact switch in there somewhere that'll drop the pump out in the event of an accident

    seeya,
    Shane L.
    You're thinking Bosch systems. Tachymetric relays only showed up on the K-Jet system and were retained I think through the Motronic. All of the GM, Ford and Chrysler systems I've played with most definitely do not have a tachymetric relay. Instead, they rely on a simple oil pressure switch to signal the ECU for fuel delivery. Timer circuit to "charge" the system is directly from the ECU. This carries the benefit of shutting the fuel delivery down if oil pressure is too low.

    I do know that US market Fords have had an impact switch shutting down the pump for quite awhile now. I've used them to great effect on streetrods, and their usefulness was proven to me first-hand in Los Angeles a few years ago.
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    I think that with the erudite traffic that this query has generated that I should explain why I asked the question in the first place.
    I recently replaced the starter motor in a '95 306 S16. After the replacement the engine would crank but not fire. After some investigation I found that there was no live feed to the fuel pump fuse. I replaced the tachymetric relay with no improvement and have spent hours tracing wires on a spare loom trying to track the problem.
    I can get the car going by running a wire from a ignition switched point to the fuel pump supply, bypassing the ECU and tachymetric relay, but I was concerned about the pump running continuously with ignition on and no safety cutout. Hence the question.
    roger

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    The inertia switch isn't tripped? I may be very wrong here, but thought the s16 had fuel pump controlled by the ECU rather than tachometeic relay, and that one section of the 2 part relay supplied power to the pump. As I said I may be very wrong, and am happy to be corrected.

  10. #10
    Now go make me a sandwich Hotrodelectric's Avatar
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    Quote Originally Posted by lhs2.1 View Post
    I think that with the erudite traffic that this query has generated that I should explain why I asked the question in the first place.
    I recently replaced the starter motor in a '95 306 S16. After the replacement the engine would crank but not fire. After some investigation I found that there was no live feed to the fuel pump fuse. I replaced the tachymetric relay with no improvement and have spent hours tracing wires on a spare loom trying to track the problem.
    I can get the car going by running a wire from a ignition switched point to the fuel pump supply, bypassing the ECU and tachymetric relay, but I was concerned about the pump running continuously with ignition on and no safety cutout. Hence the question.
    roger
    I'm imagining the car was running before this. First dumb question: you did replace all the wiring back to the starter, correct? You've checked the wiring to the engine, ensured nothing is amiss? All engine grounds are solid and tight? To pull the starter, you had to disconnect the battery- disturb anything there?

    I don't know if your system has a crankshaft or camshaft position sensor- I would also check there and see if there is anything obviously broken. If the system can't count teeth or see position, it can't see where 1 is.

    Does your car have one of those PSA-style keypad immobilizers? Or any style, really. Many anti-theft systems depend on killing the fuel pump.

    Like I tell everyone else, simple and stupid first, THEN dig deeper.
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