Car will turn over but wont start

Intro

Sorry the subject isnt very specific but I'm totally lost as to what the problem could be as I'm not getting any error codes on OBDII Code Reader. Its an MG F 1.8 non VCC from 2001. I know its OBDII compatible due to the double lambda sensors and its a post 2000 production car.

The problem

Car suddenly cut out on a 3 hour drive, no warnings just died while moving in slow traffic. Electrics were fine and would turn over but wouldn't start. Waited 20 minutes and car started and could drive for another 15 minutes before dying again. After I got my mg recovered home she'd start from cold fine 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 this kept happening for about a week.

This fault has now developed to the point the car only turns over and will not start for love or money whether engines hot or cold.

What i've replaced/tried

Alternator, battery, HT leads, Sparks, Coolant, oil, post cat lamda sensor, camshaft sensor, fuel pump. Fuel filter was done about 15k miles ago.

My mechanic has a theory as to what the problem is:

He thinks the engine coolant temperature gauge is giving a faulty reading which inadvertently stops the ECU supplying fuel to the engine. He wants me to swap it out before going to him. Mark.L's post here explains this clearly: https://www.mg-rover.org/threads/engine-coolant-temp-sensor.218389/

I'm not so sure myself as she won't start even after leaving her overnight and trying in the morning. Maybe the ECU is fried? Any idea's?

Thanks

Edit: I should add, another reason i don't think its the coolant temp gauge is that when I could get the MG to start she ran utterly flawlessly. No spluttering, no black smoke and no misfiring.

The article describes separate temperature senders for the ECU and the gauge. You have mentioned the gauge but Im assuming it's the ECU one he wants you to change. It could have an intermittent fault. Also check the wiring to it's connector

As it appears to be temperature related then my first choice is always coil packs breaking down, but heat related ignition faults normally clear when cooled down. Same with water temp sender, the ECU one not the gauge one. Should start when cold just miss fuel when hot.

I would suggest its back to basics time. You need 3 basic things for engine running, Fuel, Air and Spark. and then put them together at the right time.

Have you got a spark when you crank engine, Have you got fuel when cranking, Is airway free to engine.

The Mk2 MGF has MEMS3 with twin coil packs, that is two coil packs each supplying spark to 2 cylinders. If one of these is goosed you may be only getting sparks to 2 cylinders. You would be very unlucky to have 2 dead coil packs. I would check spark first on all 4 spark plugs to make sure your ignition electrics are working. The number of working plugs from 0 to 4 will give you a direction to fault find in rather just firing the parts cannon and hoping .. yes we have all done it lol.

You say the fuel pump was changed, but was power supply to it checked and is it actually pumping pressurised fuel, it may be a fuel pump relay that was dying and now dead. That is more inline with your first symptom of "dying" rather than misfiring.

So , fuel or spark? Should we start a poll? Just kidding. Good luck in your endeavours.

Yes, no misfiring or issues when it was dying. I know a bad HT lead cuz the mil light woud flash under high acceleration. The HT leads and sparks have been done so it's not electric, the fuel pump was a cock up on my behalf. I replaced the rear lambda sensor but broke the wire bracket. Wire melted itself into hot engine and short circuited the fuel pump. Mechanic replaced the fuel pump but kept blowing the fuse when he ran the pump/car. He then found the lambda wire melted to the engine block and properly insulated it.

Air filter intake is white clean but I could open it up and see if it works without the filter. Thanks for the suggestion.

Rule #1……ALWAYS suspect the new part!

Just because a component has been recently replaced doesn’t mean it’s still good, it’s a case of trust but verify.
Test it anyway.

It might sound obvious and quite possibly not the cause but have you tried the fuel cut off switch is not popped 🤔

Sent from my SM-G981B using Tapatalk

I referenced my Rover SD1’s earlier in another post & it was the first time I’d come across such a thing, it took me ages to twig what it was & what was adrift.
The cut out was linked out when I bought the f, it was buried under a load of gash wiring in the colour scheme of mains cable & rainbows of insulation tape. I never did find out what that multicoloured rats nest was for. It went binwards.

@Cobber The newest part (prior to the fault) was the fuel pump but it was repaired around 2 years ago. Since then i've done two one hour journeys and all other journeys shorter than 20 minutes. The issue only reared its head after I took her out for a long journey; 2 hours of solid motorway driving (at the speed limit) and then hitting slow traffic where she died when i accelerated up hill. At the time I thought it must be an overheat issue but the temp guages hadn't moved for the duration of the trip (I did have the heater matrix on full whack - the tried and tested way to prevent MGFs from overheating).

I'm guessing to test the fuel pump I test the pressure whilst key's in the ignition position? Is there a pressure test valve on the fuel rail I can press with a screwdriver (obviously i'll clean up the fuel once i've tested)?

Edit: I should add that I can hear the pump doing its usual priming whir when turning the key to ON (Position 2)

@Steve I've not had any sudden braking or run the tank anywhere close to zero so i dont think fuel cut off switch will have popped but i'll check and see.

@Airportable are you adding credence to steve's suggestion or is this a different "cut out" component I need to look for?

Sorry if any of these questions are blockheaded :bat:

I think the cut off switch others are referring to is the emergency inertia cut off switch, in a crash it cuts off the fuel pump. There is a button under engine grill on the far LH side. Black button press to reset. These are designed to trip and stay tripped in an accident and should not be intermittent or heat related. If you can hear pump priming on start up then pump is getting volts. However pressing it down only takes one finger , costs nothing, just to be sure.

No, Steve & I are referring to the same switch which is mounted, certainly on my car, down adjacent to the ECU, to the left as you look at the boot & under the grill. My comment was posted to highlight this as a possible cause of irritation as it was when I first encountered them.
Prior to these being introduced & after a collision the old SU type pump would continue to run if the fuel pipe ruptured pumping fuel into a fire, if one had started or being the catalyst for one starting.
M

Yes that is the one, under the grill LH side as I said. Always worth a reset , just in case.

Have you tried applying voltage directly to the fuel pump to bypass any relays and interlocks ? If doing this its best to run it from a circuit that is only live with the ignition (or if running from an unswitched live then best to have an inline switch you can operate whilst trying to start the car) so it's only on when the car is running/ trying to be started.