lost on an
Join Date: Sep 2011
County: Australian Capital Territory
159 in-tank pump symptoms and behaviours
I'm writing in the context of the 3.2 V6 rather than the TBi, but I believe the in-tank fuel pumps are similar if not identical.
The high pressure pump is able to compensate for weakness in the pressure from the tank pump in normal running. In the V6 the high pressure pump is run off the front bank camshaft and a solenoid is used to control the pressure, but higher pressures are used at higher revs more than varying the injector duty cycle. If the pump tank is not delivering the full pressure required to the engine the duty cycle of the solenoid is longer to increase the required pressure to the direct injectors. But this can only work up to a point, and so overtaking and other high load situations can result in a loss of combustion as not enough fuel gets to the cylinders. So the fault codes tend to fuel pressure and lambda sensor related.
When the engine recognises that it needs to run longer high pressure pump duty cycles it learns this as part of map adjustments. So if the pump tank returns to health, you can get a running rich characteristic. Which can steer people in the direction of a dodgy MAF sensor (including my mechanic).
The pump is immersed in fuel and this acts to help keep the pump cool. As the fuel level reduces there is less to keep the pump cool. So if the failure mode for the pump is heat related then low fuel levels will exacerbate this. If your mechanic test the car from cold with a reasonably full tank of fuel, then it is not difficult for the problems to remain hidden.