[alfa] EFI fuel pumps and pressures etc

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi All,

I see a familiar thread growing about the fuel pressure and delivery volume of certain EFI
fuel pumps.  This topic I know quite well having recently done some work in this area.

One digestee quoted an Autozone pump and gave some flow rates, I assume this was in US
gallons?  A US gallon is equivalent to 3.6 litres, an Imperial gallon is equivalent to 4.5
litres, so they are quite different.  Bosch and other fuel pump manufacturers always give
the flow rates in litres, so beware when comparing pumps!

The delivery rate quoted for Bosch pumps is not at the normal working pressure.  In the
Bosch data they nearly always use a figure of 1.5 bar, which is 22 psig.  (1 bar = 14.7
psig)  Most EFI systems run at around 2.5 bar at idle and about 3 bar at full load, the
fuel pressure regulator controls the delivery pressure with the inlet manifold vacuum by
pinching off the return line.  This is to ensure the fuel pressure in the rail is constant
with respect to the manifold pressure, this eases the fuel computation calculations.

If you look at a typical Bosch fuel pump for an EFI vehicle, say a L-Jetronic or Motronic,
the maximum flow rate available with the biggest pump is about 195 litres per hour at 1.5
bar test pressure.  If you run a pump open circuit into a measuring container, the
delivery pressure is close to zero and you would expect it to flow more fuel in this
condition. Typically it will flow at least twice the amount it can at 1.5 bar.  The high
pressure quoted is what is known as the "dead-head" pressure and is the pressure the pump
can generate into a closed circuit with zero flow.  This is normally measured in the
vehicle by pinching off the fuel return line back to the tank.  Figures for Bosch pumps
vary between about 5 bar to 10 bar depending on the pump type.  10 bar is about 150 psig.
It will drop as the pump starts to wear due to dirt and water in the fuel.  Often the pump
appears OK at idle but fails the dead-head pressure test.  This is because the pump is too
far gone.  If the dead-head pressure test shows less than 4 bar the pump is definitely
shot.  For some reason unknown to us all, the Nissan fuel pumps, made by Bosch, have a
very short life.  A typical Nissan will eat a fuel pump every 18 months.  It must have
something to do with the design of the fuel tank allowing sediment to be drawn into the
pump from the tank.  Mercedes and BMW in-tank pumps are also a common problem, these last
a little longer, on average about 2.5 years.  The pump has a check valve in the outlet
port to hold the system pressure in the fuel rail when the pump is turned off.  This
should bleed down very slowly.  Typically after 30 minutes the pressure should be still
above 1 bar, if not the check valve is faulty and the pump has to be replaced or
reconditioned.  Another cause of a rapid bleed down is a weeping injector, so check these
as well before condemming the pump.  Hoses and filters can also have slight weeps which
will cause the same sysmptons, although these are usually obvious if bad enough.

A typical replacement pump sold by Bosch is the one originally made for the VW Caravelle,
Bosch only make a limited number of pumps today and have drastically downscaled the
production.  A VW Caravelle pump is spec'ed at 195 litres per hour at 2 bar test pressure,
this is about 2/3rds of its normal pressure in a vehicle, so we can extrapolate the likely
flow rate at 3 bar to be about 2/3rds of the test flow rate.  Hence, it will only be
capable of flowing about 130 litres per hour in the vehicle.  Bosch quote the flow rates
after an initial 6 hour running in period.  Prior to this the delivery flow is lower as
the pump needs to bed in before it gives maximum flow rate.  So taking a new pump off the
shelf and hooking it up to a test rig will give false results, ask me how I know!  195
litres per hour is equivalent to 3.25 litres per minute, 0.72 Imperial gallons per minute,
or 0.85 US gallons per minute.  A good pump should be able to easily deliver twice this
volume into a measuring container at zero pressure.  In the test rig I used the flow rate
can be measured at various delivery pressures with precision flow meters fitted on the
inlet and delivery side of the pump.  We do not use petrol/gasoline but the
"calibration-fluid" sold for testing injectors, because it is non-inflammable.  It has the
same viscosity as petrol but is a lot safer when you are playing with high pressure pumps!
If you don't want to spend the money on "cal-fluid" then use regular kerosene or diesel,
which is close enough for basic measurements.

Knowing these facts should make diagnosing a pump problem easier.

John
Durban
South Africa
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