Walbro 255 pump vs. Standard O/E pump

Same on my car - I got fuelling probelms AFTER fitting the 255 pump, that I did not have before (though unfortunately I dont know if there were other contributing factors). we know the walbro 255 pump is not designed for the coupe - so I'm trying to find out what needs / does not need to be done in order to use it efficiently on our cars.

here's my current understanding - anyone that knows better please correct me!

the O/E fuelling system is designed to drop the voltage when the fuel rail is fully pressurised and under load to reduce strain on the pump and also get more power at the top end from leaning out the mixture slightly.

On paper the Walbro pump supplies more fuel - however if this pump has a higher impedance than the O/E pump then any voltage drop could be exagerated and less fuel will be supplied. How much less? - I dont know, but is it feasible that to upgrade to a 255lr pump on the standard wiring / power supply could result in less / the same fuel getting to the rail under high demand than with the O/E pump?


Jim - thanks for checking the O/E pump. Does anyone have a Walbro 255 pump that they have not installed yet that they could check the impedance of? Not being lazy - but I'd rather not have to take mine out of the car to check it myself...

I think Im confusing you.

What Im saying is that the voltage supplied wont alter but the voltage will still be dropped across the pump due to the current increasing at high loads.

V = I X R. Resistance/ Impedance of the pump and the wiring to it is constant. As current draw increases at high flow conditions the voltage drop across the pump and wiring increases also. Nothing to do with voltage supplied which will remain constant.

The reason some Coops will have voltage drop and some not is both due to:

1/ The health of the pump - A knackered pump will draw more current than a healthy one and

2/ the state of the wiring - dodgy wiring will have more resistance and more voltage drop.


Thinking about it you really need to know the current draw of the Walbro at full load as this will dictate the voltage drop and hence whether you need to upgrade the wiring.

This is my understanding anyway.

I wouldn't say so much on the wiring causing this, well it is partily the wiring, but its the earthing of the wiring which causes the drop, hence when the relay is fitted, you are earthing in the boot rather than using the wire that trails from the back of the car to the front of the car as an earthing point.

Thanks guys. Begbie - did fiat mean to do this, or is it a design flaw?

*shrugs

I dont work for Fiat so wouldn't have a clue

I'll rephrase - is there any reason why they would earth at the front of the car rather than in the boot?

The voltage doesn't drop at the pump when the correct pressure is reached, the fuel pressure regulator just returns the extra fuel to the tank so the fuel rail stays at a constant pressure.

As Begbie said, the crash switch was the problem with the standard fuel pump, it was probably dirty contacts inside the switch causing voltage to be dropped.
Like Cosmo said V= I x R so a more current drawn through the dirty contacts causes a lower voltage at the pump because the voltage drop is being shared through the pump and the resistance of the contacts.

An uprated pump will probably have a higher resistance than the standard pump because it will have more windings on the pump motor hence a higher resistance, thats why the uprated wiring mod should really be done if you fit an uprated fuel pump to cope with the extra current drawn by the pump.

Hope this clears it up slightly Suba ?

off topic...is the fuel pressure regulatore the little shinney thing with spout at the right end of the fuel rail?

The only real way to test it is to read the voltage whilst your driving and make sure it doesn't drop below 12v.

I remember Hanny only having 9v at the standard pump and this was probably why his piston melted.

I just pushed 2 bits of wire into the fuel pump plug and extended them to my volt meter on the passenger seat, my voltage was 13v at tick over but dropped to 11v at about 5000rpm when the pump was under load.
This was cured with the fuel pump voltage mod, it read 13.5v all the time.

i couldnt use the meter suba i dont know how to read ohms? what should it read roughly as i had a couple of readings but not sure if i was doing it right or putting on right connections....

Turn the multimeter to ohm, see sign in the picture.
http://www.pro-ohm.org/online/pic/logo.gif

Meagy,
You will need to unplug the pump from everything else then just put the 2 meter probes on the 2 wires to the pump, that will give you a reading of one set of coils in the pump.

A DC pump uses 3 sets of coils though and an armature to reverse the current through the coils, your reading will vary depending on where the brushes are on the armature.

I just removed mine from the tank and measured. And I still get 0.6-0.7 ohm.
Bear in mind that the multimeter itself gives about 0.3 ohm in the cables/probes.