fuel pumps

My ZT is a diesel face lift with both UBP and ITP. it had 87k on it when bought and the mileage stands at 170k at present. No record of fuel pump changes in the service history or since I've owned it. A week ago the UBP started making horrible noises so I bought a replacement pump and fitted it. Car started first time and although it's not as quiet as I'd hoped, the car runs ok. However, (and there's always a however in these threads!) on a couple of occasions the engine turns over but doesn't start and it's always started on the button when circomstances are normal. I've turned the ignition off, waited a few seconds, and then turned the key again. This time the pump operates and the car starts ok. The only electric connection that I disconnected was the battery and the pump connection itself, and that can only be fitted one way. The tank is half full at present.



I understand that the UBP will function by itself if the ITP fails and carry on working for some time, but not the other way round. Could it be that the ITP has failed some time ago and the UBP has eventually passed away having worked on its own for some time. But if this was the case, then surely the new UBP would work in the same way as the one I replaced. Or have I bought a faulty pump? Or could something else be amiss and I've simply misdiagnosed the original fault.



Any help and advice welcome.

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