I suspect my fuel pump causing intermittent driveability issues. I checked the fuel pressure at the fuel rail and it seems like the pressure is fine: ~2.4 bar at idle, 3.1 bar at engine off (3.09 bar pressure regulator). However I was told that the pressure test by itself is not sufficient for diagnosing fuel pump problems and a volume test must be performed - measuring the volume of the fuel pushed by the pump in a set amount of time by disconnecting the line and directing the flow into some kind of a vessel.

I do not understand why a separate volume test is necessary if the pressure shows up fine. Doesn't the fuel pressure depend on the volume of fuel being pushed? Doesn't low volume equal low pressure? Is this only required when measuring pressure immediately after the pump, rather than at the rail?

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    I would have expected a roughly constant pressure are all levels of demand. Your injectors will be designed to inject fuel at 3.1 bar if that is what your regulator is spec'd at. If you are only seeing 2.4 bar at idle, then I would say your fuel pump is not delivering correctly. A volume test would highlight if the pump is not capable of producing the volume of fuel required for high load on the engine.
    – HandyHowie
    Commented Dec 22, 2015 at 12:13
  • Are you absolutely sure? The fuel pressure regulator vacuum line is connected to the intake manifold. The regulator maintains 3.1 bar only when the pump primes (engine off), and when the regulator vacuum line is disconnected. At idle the vacuum in the cylinders "suck out" the fuel out of the injectors and at high load boost pressure the fuel is "pushed in", so the regulator compensates for this by reducing pressure at idle and increasing at load. Should this difference not be observed when measuring at the fuel rail? Commented Dec 22, 2015 at 12:37
  • I have never heard that before. Where did you get that info from? Is this direct injection or does it inject into the inlet manifold? How would the ECM know how long to open the injectors for if the engine was sucking the fuel out at unknown pressure. I am interested if what you say is correct.
    – HandyHowie
    Commented Dec 22, 2015 at 12:54
  • I have just read this - howstuffinmycarworks.com/Fuel_pressure_regulator.html which does say that the pressure is altered depending on vacuum. I guess it makes sense that the pressure difference will affect fuel flow.
    – HandyHowie
    Commented Dec 22, 2015 at 13:10
  • @HandyHowie Well, these are the working principles of fuel systems injecting into the manifold. Can be found in the book "Automotive Service – Inspection, Maintenance, Repair" by Tim Gilles, for example. The injector timing does not have to be altered because the FPR compensates for the pressure differences just as I described. Commented Dec 22, 2015 at 14:04

3 Answers 3


A fuel pump is designed to provide 3 to 4 times the needed maximum volume of fuel to the engine. The excess fuel is then returned back to the tank through the pressure regulator and return line.

When the fuel pressure is low it is very obvious that something in the fuel system is having a problem. It could be a bad fuel pressure regulator, plugged fuel filter, bad fuel pump, just to name a few.

When the pump can only provide lets say half the maximum volume the pump can still provide the pressure making this condition much harder to diagnose. Further the symptoms of this kind of failure are usually drivability related. What happens is that at idle the engine only uses a small amount of fuel and it runs fine. When the fuel consumption hits the spot where the consumption is greater than what the pump can provide the pressure drops off and some type of drivability issue occurs.

This type of issue can be caught one of two ways. The first is a volume test. The second is by connecting a fuel pressure gauge in such a way that it could be looked at while driving. If under load the fuel pressure drops off then you found your problem.

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    So if I understand correctly the volume test is mostly indended for seeing how the pump will behave under high load? As in, if the flow volume is inadequate, then in idle/low load the fuel pressure might be fine, but under high load the pressure will drop, but it will be difficult to see without a gauge sticking out of the hood? Commented Dec 22, 2015 at 14:19
  • @IhavenoideawhatI'mdoing You are correct.
    – vini_i
    Commented Dec 22, 2015 at 14:30


(Because it's important)

Let's talk about fuel pumps

The fuel pumps found in most OEM fuel delivery setups today are driven by an electric motor that is running at a fixed speed. This effectively fixes the flow rate and discharge pressure provided by the fuel pump.

The flow rate and pressure deviate over time due to several reasons which include:

  • clogging of the inlet filter
  • worn seals
  • the brushes in the electric motor wear out (though this wouldn't explain a low flow rate)

Fuel regulators in a return-style setup

Without going into the nitty-gritty of how fuel regulators operate, the key thing to remember here is that it alters the amount of fuel returned back to the fuel tank to ensure that a fixed pressure drop is maintained across the fuel injectors.

This means that the classical relationship between pressure drop and flow rate is not respected. In fact, fuel rail pressure is almost constant regardless of flow rate.

Why measuring pressure isn't sufficient

A low fuel pressure measurement at the rail might indicate a clogged fuel filter, some obstruction in the fuel line or a fuel pump that just can't deliver a high enough pressure.

However, the pressure-stabilizing effect of the fuel regulator means that the fuel rail pressure measurement doesn't tell us anything about the adequacy of the fuel flow rate (from the perspective of the flow demanded by the fuel injectors).

This is the reason for the flow rate test; you can have good fuel rail pressure but insufficient flow.

For this test, my Bosch book says that most manufacturers specify:

"[fuel] delivery of one litre in 30 seconds or less, but check your manual."

  • Good read, but are you saying that the pressure measurement is inadequate because the fuel pressure regulator stabilizes pressure? If the flow is high enough then the regulator pressure will be maintained, of course. But as I understand it, if it's not then the pressure will drop, the regulator is unable to maintain pressure when the flow becomes inadequate. The explanation by vini_i seems convincing, an idle pressure test won't show high load problems, a flow test will. Commented Dec 23, 2015 at 14:41
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    @IhavenoideawhatI'mdoing I think we're on the same page. When I said insufficient flow I was referring to the flow demanded by the injectors, not the flow upstream the fuel regulator
    – Zaid
    Commented Dec 24, 2015 at 11:39

Consider the fuel rail, kept at pressure p by the regulator. The fuel pump adds some amount a of fuel into the rail, increasing the pressure in the rail by some amount. Now injectors inject amount b to be burned into the cylinders. If a is larger than b, the regulator flows amount a-b back to tank, keeping the rail still at pressure p.

However, if the pump is defective and supplies amount smaller than b the regulator can't increase the pressure, resulting in a drop in p. This will cause injectors to flow less than was intended, causing the mixture to go lean.

One final note: production of flow is probably a function of back pressure on the pump, so I would take the test volume from the regulator back-flow line when the regulator is set to maximum pressure (vacuum line disconnected).

  • You are not wrong, but the question was about the necessity of a flow test when a pressure test was already executed. Also, since the flow test is carried out with a disconnected fuel line, I don't think there is a difference how the regulator is set. The answer of vini_i adresses the question very well. Commented Dec 23, 2015 at 7:32
  • I was trying to give another view on why the volume plays a part. About the volume test, pump might give enough free flow (as in the case of disconnecting the line coming from the pump), but not when there is 3 bar pressure to work against. Easiest way to test this would be disconnect the regulator return line instead of the pump line.
    – KLuuppo
    Commented Dec 23, 2015 at 9:33
  • You may be right, flow with and without pressure should be different. Though usually the manufacturer specifies a pump rating that you can compare to (liters/hour), isn't it for a disconnected pump? Commented Dec 23, 2015 at 14:45

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