P0251 Ford Focus under pressure

This 2003 Ford Focus 1.8TDCi had refused to start one morning, the owner had coaxed it into life with a sniff of easy start.
It had been playing up for a while, under load the glow plug light would flash and the car would loose all performance, now it refuses to start without the help of easy start.

A quick visual check and it looked like the previous garage had thrown some parts at it in an attempt to diagnose the cause. It sported a new fuel filter, new inlet metering valve and a couple of cans of injector cleaner. None of these had cured the fault and the final diagnosis was the high pressure pump.

What had made the garage come to this conclusion?


This code was stored, but after a test drive more codes

These suggest that the rail pressure cannot be controlled within the limits programmed into the ECU. A new fuel metering valve has been fitted so it must be the pump at fault.

Or is it? Rail pressure deviation has a number of causes, and we split them into supply pressure faults, high pressure faults, fuel delivery and return faults.

Using a graphing scan tool it is possible to test the high pressure generation using serial data. The system needs around 200-250 bar before it will switch the injectors.

We had 84 bar cranking, the fuel metering valve can be disconnected on this system and the test repeated and should result in full system pressure (no control) this resulted in 194 bar. We had a high pressure generation fault. But it can not be the metering valve causing the problem.

The scan tool can provide data about fuel delivery and the PID to check is the correction factor for the individual cylinders. Common rail injectors normally have a high return flow rate when they are in trouble so with the excessive correction factor in cylinder 1 we expected the worst when carrying out the back leak test. So it proved, cylinder 1 had twice the return flow of the other injectors.

A new injector was fitted and coded into the ECU. The injector is about 1/10th the price of the pump. But did it fix the pressure fault?

Cranking with the cam sensor disconnected prevents the ECU switching the injectors, and allows the pressure to build in the rail. We now had just over 600 bar cranking.This test proves the pump and low pressure stage as any faults would not generate such high pressures.