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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?

I have no idea what I'm doing
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3 Answers3

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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.

Robert S. Barnes
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vini_i
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Preamble

(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."

Zaid
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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).

DucatiKiller
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KLuuppo
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