Topic

After swearing off the car and calling the scrap man I've laid awake at night wondering what could cause my MGF to not start. Today, curiosity got the the better of me and I've started troubleshooting once again. The issue is the car would start from cold but wouldn't start from warm i.e. I'd drive for 5 minutes, turn off engine then attempt to turn engine back on and the car will crank but wont start at all. This issue has progressed to fully dead i.e. I can crank it but it wont ever start from cold. I've taken the fuel pump inspection lid off and had a listen while someone cranks the engine and I cant hear squat. Current theory is the fuel pump (done less than 1000 miles) has died.

Tried testing the electrical wiring to the pump using info from the second post of this thread:

https://www.mgfregister.org/forum/viewtopic.php?t=25603

Tested the the white and purple wire and i'm getting about 12v for about 2 seconds when the key is in the ignition position. Getting 6v over the green wire when in the ignition position and when cranking the ignition which I think is also acceptable.

Tried testing with the inertia switch disconnected and i get no voltage across the white and purple wire but still get 6v over the green wire even though the inertia switch is disconnected. Also Tried bypassing inertia switch (copper wire over the pins) but get the same readings so I don't think the inertia switch is faulty.

P.S. I've tested the sparks on the exhaust manifold, got a good shock up my arm from through the HT rubber sleeve from being so reckless.
P.P.S Exact model is an MG F 1.8 non VCC from 2001.

As always thanks so much for reading this. Random support from strangers on the internet always makes my day.

Back off the fuel filter pipe & see if you have any flow. Not with too much enthusiasm as it can deliver a lot quickly if allowed to.
M

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.

I can understand your reticence in potential having petrol pissing out all over the shop but when I recommended backing of the pipe I was thinking of no more than a couple of turns, just enough to see a constant flow & not just a release of pressure. Obviously take precautions & have rags & a container handy, what with the cost of petrol!
M

Just seen you've replied to my many. many times edited response. I've been researching for about 5 hours and just thrown everything at the wall to try and work out how the fuel system works in an MGF (I still dont know what the green wire does. Goes to Instrument panel (100). Obviously there's not a "Fuel Pump failure" light on the dash cuz I've already had one of those blow up and MG didnt tell me squat. But it is drawing 6V of power. Which is weird in of itself. SO its a mystery voltage going to a mystery location. I'm losing my mind

I'll edit this post tomorrow with whatever happens after further testing.

The proper way to teat fuel pressure is to use a fuel injection system pressure test gauge, suitable for the high pressures involved.
Flow rate can be determined by disconnecting a fuel line and fitting a suitably length fuel hose to reach a suitably sized container.
You run the pump for a set time and measure the amount of fuel delivered into the aforementioned suitably sized container.

Are you referring to the green/black wire from the tank sender unit? If you are then that's the signal for the fuel gauge & the voltage there will be governed by the amount of fuel in your tank.
M

@Cobber I did the test you suggested. Results: 90ml when priming. 210ml when cranked for 5 seconds. Total 300ml fuel displaced by pump.

@Airportable Yes, the green ad black wire from the top of the fuel pump (is that called the tank sender unit?). It reads 6V I guess because I have a full tank?

After doing the test Cobber suggested I reassembled the fuel line. Replies to above posts ends here.



After I reconnected the fuel line the car started first time but then doesn't start the second like it always does. I don't hear the fuel pump prime when I attempt to restart the car, I'm guessing the ECU/fuel relay skips that part cuz it knows the engine is still hot [and full of fuel]. I tried bypassing/overriding the relay by running a copper wire off the post of the 12v battery to the fuel pump tank sender unit and then cranking so I had maximum fuel going into the engine. Still nothing. I have the horrible suspicion I'm barking up the wrong tree.

Speaking of other trees, I have tested the HT leads by sparking them off the exhaust manifold (swapped the plugs for new ones then used one of the old plugs to check for a spark).

What I haven't done is test the coil packs that the HT leads run off of. I don't really want to take these out of the spark plug chambers due to all the warnings about dangerous voltages and damaging the ECU. What I have done is tested the resistance when cold (1.3ohms) and the resistance immediatly after running the car for 10 minutes. Still 1.3ohms after running hot. I know this resistance is a lot higher than the 0.7 recommended ohms but the parts place said they'd be fine and to be fair they have been for the past 2 years. FYI I destroyed the OEM coil packs when I blew up my engine (they'd work but occasionally not send a spark).

I've also done all these tests with the air intake filter removed (makes a great noise) so it's probably not an air issue.

Battery is new, charged to 13.4V and the correct type and CCA for the car.

So yeah. At a loss as to why I can start the car cold but cant start it hot. I don't know what else to test...

Edit: unless my fuel displacement is far too low? Just seen on Mike Satur's website that "the stock fuel pump flows approx 2 litres per minute" and by that math my car is pumping 2.5 litres a minute so I'm golden there.

Edit2: added a subtitle (in bold) to separate replies to users and my own further testing.

djkeenan wrote:

..
@Airportable Yes, the green ad black wire from the top of the fuel pump (is that called the tank sender unit?). It reads 6V I guess because I have a full tank?

... I tried bypassing/overriding the relay by running a copper wire off the post of the 12v battery to the tank sender unit and then cranking so I had maximum fuel going into the engine. Still nothing. I have the horrible suspicion I'm barking up the wrong tree

Unless i have misunderstood this you have applied 12 volts to the sensor that reports the level of fuel in the tank back to the fuel gauge. Have you tried a permanent live to the fuel pump itself ?

Don’t rely on your fuel gauge under experimental conditions, the meter retains its last reading & if that is erroneous it can not be considered accurate.
At the end of a very long job list I have to dig out a test rig & see if by a simple modifications I can create an indication more in line with what is now expected.
M

No no. Only the first sentence was directed at AirPortable. I've not done anything with instrument panel wire except read the 6v reading. The next paragraph is unrelated info i.e. running the live feed direct from the battery positive terminal to the socket on the sender unit that contains the white purple white wire (which is what powers the sender unit NOT the fuel gauge).

I'm sorry, I should have double spaced after I finished @ replying or used a subtitle to separate the two.

Edit: I've gone back and edited my earlier post so its properly formatted and added a subtitle for the sake of clarity.

I’ve had another idea of things to test. So the camshaft position sensor (CPS) tells the ECU when to fire the sparks. Maybe this information is not getting to the ECU when the engine gets hot. i.e. the wire that connects the CPS to the ECU is shorting? But then I’ve looked into this and according to post 3 of this thread the “ECU reverts to batch injections” when not receiving a signal. So should still run without it. https://www.mg-rover.org/threads/camshaft-position-sensor-problem.484531/

Then I thought maybe the crankshaft position sensor is faulty. Anyone know the location on a car like mine?

I’ve read that I’m looking for about 800ohm across the sensor. Could it be shorting once it reaches a certain temperature?

Note I’m not getting rough idling, misfires or hard starts so its not showing the usual signs of a temperamental/faulty crankshaft position sensor. I’m thinking maybe its either working, when engine cold, and shorting when engine hot. What do you think?