Help!!! - Maddening fuel pump related problem

Thoughts appreciated please - '96 mk1 1.8

The car conked out a couple of weekends ago and I had to be recovered home.

It was immediately apparent the problem was fuel pump related as I couldn't hear the pump when I turned the ignition on.

I won't lead you through my extensive diagnostic process however this is where I am now:

1. With the ECU relay pack bypassed via a jumper cable in the 8 pin connector there is 12v at the fuel pump connector.  Therefore the wiring and inertial switch would appear to be OK.
2. With the relay pack connected there isn't 12v at the pump.  This despite replacing the relay pack with a second hand one from MGFnTFBitz. (A 970 instead of the 710 originally fitted but I assume they are compatible).
3. The fuel pump will not fire up either with the system rigged as per (1) or with 12v injected directly from a charger.

(3) above would suggest that pump is not working, and I'm about to get messy and pull it out of the tank and have a look.  However (2) suggests that something else is at work too.

I am of course highly sceptical that two things have failed simultaneously though there is the possibility that one thing failing has caused another to.

One thing that I don't really understand is as follows: When you turn the ignition on (under normal circumstances) the pump whirrs for a second or two then stops.  why is this?  Is there a sensor somewhere that tells the ECU that the system is up to pressure and turns the pump off?  Could there be pressure in the system explaining why with the relay pack in place there is no 12v?

Many thanks in anticipation of your help.

There is no any sensor providing info to engine ECU regarding fuel presure. On ignition engine ECU will fire fuel pump a fraction of time to build up a pressure in the fuel system to be ready when you turn starter on. Engine ECU will fire fuel pump to build presure by default.
Fuel pump is lubricated by fuel, so you should not let fuel pump to starve from fuel in the tank.

It is alittle bit odd that you do not have 12V with multirelay attached. Have you checked that you have clean contacts on the relay and multiplug?

EDIT: Can you also check that you do not have bare or broken wires near left hand side tailgate hinge?

Can I clarify something please?
Under normal circumstances with everything working as it should and turning the ignition on the fuel pump can be heard to run then stop.
Is that because the ECU stops it from pumping or is there an internal pressure switch in the pump that temporarily switches the pump off?

If the latter and connecting 12 volts to the pump then that pressure would need to collapse back through the pump before any further 12 volt supply would have any consequence.

Secondly if you have changed parts associated with the ECU dont these need to be programmed to match other components in the car as illustrated in one of the other topics and posted by “pscan”?

Wrt to your second question, the relay pack for my car (an early one don't forget, '96, chassis number less than 1900) has no discernible digital content whatsoever.

Roverlike: yes hinge wires, I was aware of this issue and have not checked yet. What connected with the engine management goes through this bit of the loom?

I found this on the Internet:

”The Rover MEMS energises the fuel pump for a few seconds when the key is turned to ignition. It then waits for activity on the crank sensor showing the engine being turned over before it energises the pump again.”

it comes frim someone calling himself Sprocket; consequently, you’ll want to be circumspect as to how much weight you want to give it!!!    I’m assuming, of course, that this is relevant to your MG through its Rover DNA.

JeremyC wrote:

Wrt to your second question, the relay pack for my car (an early one don't forget, '96, chassis number less than 1900) has no discernible digital content whatsoever.

Roverlike: yes hinge wires, I was aware of this issue and have not checked yet. What connected with the engine management goes through this bit of the loom?

None of wires connected with the engine management goes through this bit of loom, however some other wires passing through this bit of loom share the same ground point as engine management circuit. If these wires passing through hinge loom have ground wire compromised, and since one of these wires is ground wire connected to same ground point as engine management circuit, by that connection engine menagement can also be compromised.

EDIT: You can check from this thread I created on this topic:  https://www.the-t-bar.com/forum/roverlike-s-guides/97856-mg-f-tf-boot-hinge-lid-wiring-loom-inspection-guide

The initial running of the pump pressurise the fuel rail to enable fuel to be delivered the instant cranking begins, irrespective of where in the cycle the engine has stopped. I’m unsure what stalls the pump once pressure has been built if the car isn’t started immediately. A visit to the circuits might be enlightening.
Like the Freelander wiper motor question, boot hinge wiring is a constant leitmotif, the effects of intermittent failure can be felt far & wide.
The previous owner of my car had “done the wire job” & yes he had so I repaired it with a good quality wire with several times the number of thinner inner strands; it also has a generous amount of slack.
This might not resolve your problem, however it might shine a light into a dusty & dim corner.
M

One more tought on your fuel pump: I see that you brought bypassed 12V power to the pump. But have you brought independent solid earth connection to the pump as well? I would try to bring 12V and independent earth connection to the pump in order to fully test the pump and see if it is working or not.
If the pump is working, then your problem might all the time be in broken wires at the hinge in the boot.

Thanks for your further thoughts.

1. I haven't tried an independent earth but I have tested the earth in the plug and it's showing 0.0 ohms to one of the engine cover bolts.  I did clean up the earthing point near the inertia switch last night.
2. Thinking further on my suspicions of the relay I realise I've not ben thinking clearly.  Any measurement I take with the relay in circuit is invalid as by the time I've turned the key and connected the multimeter the initial priming phase will be over.
3. I've uncovered the hinge wiring from about 2" from the loom to 2" to where it goes in to the boot lid and it looks very good.  I've also pulled the wires and none are broken
4. Pumps are not expensive and this one sat for long periods of time (700 miles in 6 years prior to my ownership) so I've decided to change the pump.

One more note: when you are testing this earth to the pump it is not important to test Ohms but also power/voltage on that same earth connection. Voltage can be passed from bare or broken wires to earth point and thus making earth connection not 0 Volt but with some higher amount and by that pump might not work.