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My grandmother's 2001 (mk4 facelift) Fiesta is underused these days, but I ran it on Saturday without problems. On Monday, presumably after getting fully hot, it stalled out and had to be limped home. After being stopped for a while, it would restart, only to stall again. As we got it onto the driveway, it seemed to shake off whatever was causing the issue, but it's now back. I have the car's front wheels up on ramps and it won't start.

This is now happening whether the engine is hot or cold. The engine turns over correctly and will intermittently fire, but immediately stalls. There are new HT leads, ignition coil pack, fuel pump and relay. When the ignition is turned on, the fuel pump buzzes to pressurise the system as expected. The battery is charged. Every few starts, if I give it some throttle, sometimes up to 50%, the engine will stay running for maybe 1 second, then stop again.

The local mechanic recommended the crank position sensor. I bought a replacement; it won't fit in the hole, but I have compared the resistance readings to the original sensor and they are both within tolerance. Additionally I have an OBD reader, and while cranking I could see an RPM reading when the sensor was connected, which stays zero if the sensor is unplugged. Unplugging also prevents the engine firing at all and raises an error code as expected, so I don't think it's the crank sensor.

The ECU had logged a code P0131, indicating the lambda sensor could be faulty, but as I understand that won't stop the engine from running. There are no other error codes logged, and clearing the P0131 hasn't caused new ones to be added.

The car has had a history of fuel-delivery problems last year, requiring the new fuel pump relay and pump itself. Unburnt fuel does seem to be going through the engine so the injectors are firing and the fuel pressure seems correct, as the engine will sometimes run with the throttle opened. I can't tell if it's losing spark, fuel or both intermittently. Any suggestions appreciated.

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Heh, I forgot to close this question.

The answer was so insanely simple I didn't even think about it.

The car was out of fuel!

Turns out there is an electrical problem with the fuel gauge. Somehow, and I don't know how, the gauge reads backwards until ~50% and then stops. Ergo, if the tank is full, the low-fuel warning light comes on, while if it drops below 50%, the gauge sticks. As a result, I never suspected it was out of fuel. I have no possible explanation for how it's started doing this, but it started up perfectly once there was fuel in the tank.

The last mechanic to work on this has some awkward questions to answer...!

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injectors must be clogged one or more.

Try use some injector cleaning like Winns or Lucas or something.

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It sounds like your mechanic hooked something up wrong to the new fuel pump when it was installed. The gauges work one of two ways: 0 to infinite resistance or infinite to 0 while the tank is emptying. If you got an aftermarket pump and it came with a universal sending unit, it could have both types of connectors on it and it was hooked to the wrong ones. (also I don't know why this is getting downvotes. I was an auto mechanic for 15 years and I've seen this happen)

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