djkeenan

I loathe to do this in case I can't get the hose back onto the fuel pump. Can I do what Badger did in this thread?

https://www.the-t-bar.com/forum/9-mgf-tf-pitstop/91391-low-fuel-level-warning-light

Bridge/short the terminals (white/purple and brown/grey) at the fuel pump relay and drain the fuel after the filter? Badger just means a bit of copper wire between the white/purple wire to the earth terminal next to it (as shown in his 3rd picture), right?

Also, I'm getting voltages to the pump but are the voltages correct?

Where is the the fuel relay? I think it's the one pictured here.
I don't know what i've shown in that picture but its not the relay. MGFcar.de explains that the location is: "The fuel pump relay is located in the engine management relay module, which is positioned behind the ECM mounting bracket. The relay is normally open when the ignition is off. When the ignition is switched on to position II, the ECM provides an earth path for the relay coil. With the ignition on, the relay receives a feed, via the ignition switch, from fuse 14 in the passenger compartment fusebox, which energises the relay coil, closing the contacts. A permanent battery supply is provided to the relay contacts from fuse 2 in the under bonnet fusebox, via the inertia switch. The feed passes through the relay contacts and operates the fuel pump to pressurise the fuel system. The relay will be energised for a short time only to pressurise the fuel system.

When the ignition switch is moved to the crank position III, the ECM will energise the relay when the engine starts cranking and will remain energised until the engine stops."

i.e. The "feed" [to the pump relay] not only turns on the fuel pump [by turning on the fuel pump relay] but also provides a couple seconds of charge to prime/pressurise the fuel pump while a different, "permanent battery supply is provided" to run the pump until the car is switched off. The ECM will keep the pump relay energised to maintain the battery electrical power to the fuel pump unless it decides to remove the earth for situations such as the car stalling. Interpreted this wrong. See correction below.
I.e. Ignition position 2: the "feed" closes the relay contacts for a few seconds to run the fuel pump to prime/pressurise the fuel line. Ignition position 3: The ECM will keep the pump relay energised (re-close the contacts) to maintain power to the fuel pump unless it decides to stop the feed by disconnecting the earth [breaking the electrical connection] for situations such as the car stalling.

Or, if I shouldn't bypass the relay, attach a 12v wire direct from the positive terminal on the battery to the purple white connector on the fuel pump?

Edit: I think this has been done before as stated by the OP in this post https://www.mg-rover.org/threads/help-required-fuel-pump-relay.346521/. I also have a spare car battery if you think this would be a way to run the pump?

Edit2: The electrical library on the mgftf owners website that describes fuel pump operation as such: "The fuel pump relay is energised by the ECM for a few seconds after the ignition is switched to position 2 then CONTINUTIOUSLY during cranking and once the engine has started.* The ECM energises the fuel pump relay by connecting an earth to the BP wire between the fuel pump relay and the ECM.

When the fuel pump relay is energised the fuel pump is supplied with battery power from the switch contacts of the fuel pump relay on the WP wire."

*I'm not sure but I think the next section goes on to describe exactly which wire does what to achieve this.

Edit3: After further thinking about what I just said it still doesn't explain why i'm reading 6v instead of 12v. I did read on another forum that: "a stuck pump can load down the circuit enough to show only 6 volts" so maybe i've got a gunged up pump that only occasionally works? It also doesn't explain why it works (sometimes) on cold engine starts but is always dead on warm starts. I think I need to get some readings while the engine is running and record the voltage of each wire.