Fuel flow is increased to feed the engine's demand, as fuel pumps adjust for heavy loads. Read More >> Extra fuel is needed in order to produce extra power from the engine while under heavy loads, such as towing or accelerating rapidly up a steep hill. In standard or performance vehicle fuel pumps can usually deliver anywhere from 90 to up to and over 255 litres per hour (LPH), the difference at heavy load will increase these levels significantly. For example, a high-performance vehicle may require the fuel pump to move as much as 400 LPH in order to keep the engine from running lean and losing up to 20% power.
The engine control unit (ECU) monitors throttle position and oxygen levels in real time to adjust the fuel flow. Following these specifications, the ECU inform fuel pump to change its output. The goal of this is to keep the balance in air-fuel ratio, usually 14.7 :1 for most common gasoline engines. Under large load conditions, insufficient fuel can result in a lean air-fuel mixture causing performance problems such as misfires or hesitation.
Fuel pumps now days simply keep a constant pressure, usually between 40 to 60 PSI in modern fuel systems regardless of the load. The ECU will keep this pressure constant, no matter how much the engine has to work. In the case of high load, if the fuel pressure drops below this range there may be lack of power and stalling in an engine. For example, during a 2017 recall, an automaker discovered fuel pumps in some trucks were not delivering sufficient fuel when the engines required it under heavy loads and could stall at high speeds;
Many high performance vehicles come with optional boosted voltage fuel pumps, that can go directly to two or more heavy duty pumps. The new pumps feature more flow and pressure capability to support engines putting out over 500 horsepower. A fuel pump not rated for the power level can hurt your engines efficiency (especially under heavy load, turbo or supercharged) by up to 15% as a rough guide.
Answering the question "how do fuel pumps operate under load?" The pump compensates by increasing fuel delivery to match engine demand, which keeps the car running properly and prevents damage. If you want to discover extra about fuel pump, read it here: Fuel Pump.