Hi Guys. Need some assistance with this one.

Vehicle presented with a loss of power under load at around 3800 - 4500rpm. Only when you plant the accelerator. If you back off and gradually feed in the power it's possible to drive the engine to its rev limit. It only seems to do it when it's warmed up and it's very hard to get it to do it in 1st. Oh and NO DTC's logged!

Looking at live data when the vehicle faults, the rail pressure drops from 1800b at 3850rpm to around 100 - 120b. Obviously far to low! The MAF voltage is steady at 4.19v while it's losing power. While I was testing, the ECU did flag a DTC for Rail Pressure relief valve but after a few minutes it cleared and hasn't come back. I can't see anything else obvious on Live Data. The injector duration stays constant, the Manifold pressure drops from 236kpa to around 200 and the Baro pressure fluctuates from .997 to .991 while it's faulting (but I can't see 6 millibars making any difference).

After making some enquiries and taking some advice from a local diesel specialist I decided to replace the fuel pump as the Denso units fitted to these motors apparently do fail with this type of symptom. Not a job for the faint hearted, I can tell you!!

Needless to say, the pump has NOT cured the problem! Now I've really got to fix it. Even though I advised the owner I couldn't 100% guarantee my diagnosis due to the intermittant nature of the fault I've still Spent £475+ VAT on a pump!!!

I've tried in turn disconnecting the MAF sensor, the EGR valve and even clamped the fuel return pipe from the pressure relief valve on the common rail, figuring that if the valve is releasing the fuel pressure it will blow the pipe off under pressure (but it didn't).

I'm now running out of ideas! Any suggestions (or silver bullets) would be very welcome!!

Thanks for reading this incredibly long post!!

Cheers - Paul.