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One thing that bugs me about my car is that the fuel pump noise is fairly inconsistent. Turning on the lights, aircon, defroster, etc. lowers the fuel pump speed and gives a lower pitched noise. It also dips for a second when the RPM gets too low, sometimes when accelerator is released and clutch is pushed in, sometimes apparently for no reason at all. I had times when engine RPM would be critically low immediately after starting until the fuel pump gets to full speed (it works slower for the first few seconds on these cars for some reason) and this inconsistency might be the cause of some intermittent idle problems I have.

Is this normal behavior? I do understand that the alternator is being spun by the engine, but it should have a voltage regulator and even if generated voltage dropped, as I understand the battery should maintain system voltage above 12V (battery works great, by the way). Shouldn't constant voltage always be provided to the fuel pump?

The car is a 1999 Volvo V40 T4.

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    I'll throw this out there like any other question: What is your year/make/model/engine of your vehicle??? This is VERY important, especially with the fuel pump. Some vehicles have known issues where this type of thing happens. These same vehicles usually have a fix for it as well. It could be an old fuel pump, etc, etc, etc. Please post up the information so we can give you a correct and coherent answer. – Pᴀᴜʟsᴛᴇʀ2 Nov 21 '15 at 16:00
  • Great question, +1 – Zaid Nov 21 '15 at 16:48
  • It's a 1999 Volvo V40 T4. My fuel pump IS acting fishy besides this, but as far as I can tell if it receives constant voltage, it's speed should be constant as well, so I'm not blaming the pump itself for this behavior. – I have no idea what I'm doing Nov 23 '15 at 7:58
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It sounds very much like your alternator is not delivering a constant voltage. It should be able to cope with a change in demand. Check the voltage at the battery with different loads turned on, the alternator should be able to keep a voltage of around 14v

Alternatively you could have a slightly bad connection from the battery to the fuse box, or the battery to ground. A change in load would cause a voltage drop across the bad connection, which would then cause a lower voltage to your pump.

Chech the voltage at the battery under different loads, then do the same test at the fuse box.

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  • I can do that, but shouldn't the battery compensate if the alternator voltage drops? – I have no idea what I'm doing Nov 23 '15 at 16:15
  • When the alternator is working correctly, it should keep a constant approx 14v. Under load, the battery won't supply 14v so the voltage to any load will vary with the alternator voltage and cause your problem. – HandyHowie Nov 23 '15 at 17:15
  • Almost forgot this question. The voltage is not exactly constant, but always above 14V. For example turning on the window/mirror heater and fans drops the voltage from 14.8V to 14.3V or so. Is this normal behavior, or should the voltage be set in stone? – I have no idea what I'm doing Jan 6 '16 at 8:38
  • If your voltage reading is accurate, then 14.8v is too high, which would indicate a fault in the alternator regulator. Lead acid batteries start to gas at 14.4v so that voltage will be damaging your battery. Your battery could be the only thing that is stabilizing the voltage at the moment, this could lead to other circuits being damaged eventually. You need to get this accurately checked. You likely need a new alternator or the current one refurbishing. – HandyHowie Jan 6 '16 at 9:10
  • I don't guarantee that these were the exact values, might have been 14.6V, but it was solidly above 14V. I've heard that low temperatures increase the charging voltage. It is currently around -20 °C where I live, so I'd say the value is pretty normal (I've heard of values as low as 15V at -30 °C). – I have no idea what I'm doing Jan 7 '16 at 9:57
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Your issues may be beyond the fuel pump.

The telling sign of this is that a change in engine load is inducing a change in the noise of the fuel pump.

I assume that your fuel supply system is a "return"-style fuel system in which the fuel flow provided by the pump is constant, regardless of load; the pressure regulator determines how much fuel is recycled back to the fuel tank based on engine load.

If so, here are a few things that may be contributing to the issue:

  • Improper fuel regulation

    Lack of reference to manifold pressure, so the fuel regulator is delivering a fixed amount of fuel regardless of engine load.

    A very common culprit for this is a compromised vacuum line between the engine manifold and fuel regulator.

  • Clogged fuel filter

    The build-up of debris can accumulate over time to the point that it affects the ability of the fuel regulator to maintain a certain pressure differential between the fuel rail and the manifold.

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  • Actually, it's a strange Return Lacking Fuel System used on some Volvo turbo models. It has two fuel pressure regulators and no return line to the tank. – I have no idea what I'm doing Nov 23 '15 at 15:37
  • Here's what I know: my first fuel pressure regulator is good and the vacuum line to it is solid. What I don't know is whether my second FPR is good, as it is placed in a very inconvenient location next to the fuel tank. – I have no idea what I'm doing Nov 23 '15 at 15:40
  • My fuel filter was changed not too long ago, however I don't dismiss the possibility that it might be clogged, I was forced to run on extremely low fuel not too long ago and some odd behavior emerged since then (or it's just a coincidence). However as far as I know a clogged fuel filter would give me problems on high load and it seems to run alright when floored. The fuel rail pressure readings are quite shaky when throttle is applied, but the numbers seem realistic (3.1 bar on ignition only, 2.4 bar in idle). – I have no idea what I'm doing Nov 23 '15 at 16:13
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The noise being generated by the fuel pump will relate to how hard it's having to work to provide the fuel line pressure that is required at the time. It makes sense to me that, when you come off the throttle, your fuel demand drops as the injectors close, this leads to a "glut" of fuel pressure in the lines so that pump doesn't have to do anywhere near as much work.

That said, how loud is the fuel pump. I've had a couple of cars with fuel pumps I could hear but not so loud that I'd hear them over a revving engine.

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  • Surely the fuel pressure regulator will just any excess fuel back to the tank, so there shouldn't be any change in load on the high pressure pump whether or not the fuel is getting sent to the injectors, don't you think? – HandyHowie Nov 23 '15 at 12:28
  • If the FPR requires 4 BAR in the fuel lines, as demand is placed by the fuel injectors drawing fuel, to keep the pressure up the fuel pump will be working harder than when the injectors close. Consider a garden hose. If you want a constant pressure in it, then stab holes in its length (i.e. the fuel rail) but still want consistant pressure, you'll have to turn the top on more. – Steve Matthews Nov 23 '15 at 12:30
  • When the injectors are open, the fuel will go to the injectors, when the injectors close, the fuel will go back to the tank the pump will not know the difference. You're illustration would be more like a leaking pipe on the injector where pressure is being lost after the regulator. – HandyHowie Nov 23 '15 at 13:17
  • ...but the regulator will be providing fuel to the back of the fuel rail when there is demand. How about, watch what pressure is indicated on an inline pressure gauge at various applications of throttle. – Steve Matthews Nov 23 '15 at 13:37
  • Well, I always thought the excess fuel is just returned. It's possible, but why is there a need to slow down the pump? The pump is a stock Walbro unit that is quite noisy on these cars. I'm not sure if it's noisier than it should be, though. – I have no idea what I'm doing Nov 23 '15 at 15:34

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