Originally Posted by EVO8LTW

When the car is in closed loop operation at low loads, it uses the factory narrowband O2 sensor to increase/decrease injector pulsewidth to maintain ~14.7:1 AFR. So, unless the new pump dramatically increased fuel pressure, this will continue to work even with a new pump and the computer would continue to make little adjustments through the fuel trims. However, when the car goes into higher load cells, fuel is added to enrich the AFR and those fuel additions are made without regard to the O2 sensor's readings. If you car was tuned with a pump that delivered less volume at higher boost pressures (and fuel pressure is regulated by boost pressure at the FPR) than the Walbro, then these enrichments are going to be too generous and your car will run too rich. The fact that the higher load cells are essentially open loop since the car doesn't use a wideband O2 sensor is the problem. The car does have a return line FWIW. Make sense?