Fuel pump woes...

[CaliTrav]

Hi all

So the other day I went to start and it died pretty quickly. I was getting about 30 seconds or so of running - a normal start but would just stop.

Diagnosed it as the fuel pump which would work intermittently if I tapped it but not consistently.

Bought a repair kit from burlen and have stripped/rebuilt it (new points, diaphragm etc)

I'm now having exactly the same behaviour (works erratically and intermittently, needs tapping)

I have eliminated any issues with weak earth and it's getting between 10-11v on the live. It behaves the same with the inlet connected or not but I did check for blocked pipes etc.

Any ideas what I should be checking for?

[pgp001]

Why is it only getting 10 to 11 Volts is the question I would be asking.
You battery should be giving more than that, and it may not be enough to activate the pump solenoid.

Have you tried your voltmeter directly across the battery terminals ?
If you are getting a higher voltage there, you might have a high resistance connection somewhere in the pump wiring.

Phil

[CaliTrav]

I've given the battery a charge - it is now getting 12.5v but with the same results.

Frustratingly, when I test it off the car on the workbench it seems to run fine and doesn't "stick"

[pgp001]

Logically that would suggest the problem is still on the car in that case.

Have you checked the fuel line going back to the tank to make sure there are no blockages, if there was something stuck in the pipe it might make the pump stall with the increased vacuum.

I am assuming at this point the filter on the pump inlet has been checked and is clean.

Phil

[Trickydicky]

The simplest way to test the pump and to prove if there is high resistance in the circuit is to connect the pump directly to the battery and start the car. If the problem still persists then check the fuel line for blockages.

[myoldjalopy]

'CaliTrav' - you say it runs fine on the workbench. Have you actually set it up to pump fuel whilst out of the car? If it merrily pumps fuel from a container (taking all usual safety precautions) then it does suggest a blockage in the fuel line somewhere. If it does pump fuel OK on the bench, re-fit to the car but take the pipe off the carb float chamber and see if it briskly pumps into a bottle or jar from the tank (again, taking all usual safety precautions). It its already fitted to the car, try these tests in reverse order, if necessary.

[geoberni]

Where are you checking the voltage? Always at the same spots?
The fuel pump is earthed via it's body being a good clean connection to the car structure.
You say you've eliminated any

eliminated any issues with weak earth

but... I don't know how good your understanding of electrics is, so apologies if you know all this; but basically, 'voltage' is a difference between 2 points in the circuit. Different resistance values around the circuit will 'drop' a portion of the overall voltage available.

You said it runs fine at the bench, where presumably you are applying the 12v direct to the terminal and a connection to the pump body.
But the problem comes when you have it back in the car, so as far as the electrics are concerned, that effectively brings it back to any voltage lost in the circuit.

So if there is a good 12v at the connection to the pump, then you have to eliminate the connection after the pump, i.e. the connection down to the car structure.

For example, I just nipped out and checked on Basil.
Battery 'post to post' is currently 11.78v
Voltage from connection onto pump back to the battery Neg Post is 11.63v, so a drop of 0.15 volt up to the pump.
Voltage from a clean connection on the body of the pump to the Bat Neg is 3.5mv, which is 0.0035v
so I'm getting 11.6265v out of the available 11.78v actually where I want it.

If you have already cleaned up the terminal on the pump and cleaned up the connecting surfaces of where the pump bolts to the body, I would still check it with a multimeter just to confirm.

After that, it's down to checking fuel issues instead as the others are suggesting.

[CaliTrav]

Thanks for the replies

I'm checking it at the same places. After charging still 12.5v between any measurement so the circuit is fine.

It is pumping air as I can feel that. I haven't tried pumping fuel from a jar yet as cant get it to run consistently. After saying it was running on the next day it decided not to.

So at a complete loss I've stripped it back down, re followed the instructions and rebuilt it. Set the points etc. The only thing I can think is that I'm mis setting the diaphragm somehow.

Which way up does the inbetween metal spacer go? With the raised edge toward the diaphragm (which at present causes throw over but the points won't move) or with that facing down? In either case it wasn't running at all now so I've given up for the evening.

Re. Blockages, possible but unlikely as I can move air through the line quite freely. Fuel suction at present is the second problem as I can't get the pump to run on it's own yet!

[jagnut66]

Hi,
I believe this is the 'spacer' you are referring to:

DSCF3601(1).JPG (1001.53 KiB) Viewed 845 times

DSCF3602(1)_LI.jpg (2.94 MiB) Viewed 845 times

The brass rings I refer to may also be plastic.
Also I have found the spring needs to be inverted, as in the narrow end facing the base of the pump.
My pump also refused to draw fuel, in the end I found that I had wound my new diaphragm in too tight, it would spring to life after dismantling and reassembly, then stop.
I was following advice from an old post on here which recommended doing it up fully, then loosening it by four turns.
However a lightbulb moment was prompted by another member on here and I loosened it another four turns (eight in total), which must have freed it up enough, as it then started pumping properly. My diaphragm is now just at the point where you must watch you don't tip the bearings out of their groove upon reassembly.
I held the diaphragm into the body whilst placing it on the spacer and base, before securing it with the screws.
Not necessarily your issue but just an idea.
Hope this is of some use.
Best wishes,
Mike.

[myoldjalopy]

This article is a useful guide: https://www.mg-cars.org.uk/imgytr/sufuelpump.shtml

[CaliTrav]

Thanks! Yes that's the spacer I meant. I thought I'd had a eureka moment but I had assembled it right first time.

I'll have a fiddle around with the diaphragm setting later and see how I get on. Thanks for the MG guide too - it's a bit more in depth than the SU instructions so should help.

Jobs like this are doing wonders for my patience! Put it on. Take it off. Think about it. Try again.

[newagetraveller]

When changing the points on a fuel pump it is best to count the number of turns it took to unscrew the old ones so that you can screw up the new ones by the same amount.