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FAQ – EFI Fuel Pumps

After 30 minutes or so of driving, fuel pressure starts falling, then the fuel pump gets louder and/or seems to quit running altogether. The pump also seems hot to touch. What’s wrong?
You may be experiencing EFI vapor lock. Fuel system hot spots, the cause of vapor lock in carbureted engines of the past, and modern EFI engines where no return line is employed, are eliminated with the use of a bypass regulator placed after the fuel rail, in the engine compartment. Still, thanks to underhood engine heat, fuel in the EFI bypass system does slowly warm up as it is recycled through the fuel rail(s) and back to the tank. The longer an EFI engine runs, the higher fuel tank temperatures can become. Unlike the more common carburetor vapor lock, where fuel is heated to boiling in the float bowl(s) or fuel line(s) under the hood, EFI vapor lock is often caused by hot fuel in the tank and/or cavitation. Excess fuel pump noise combined with fluctuating and/or falling fuel pressure, often indicate cavitation is occurring. The exact point where gasoline changes from liquid to vapor depends on its temperature and pressure. A combination of high fuel temperature and low pressure are the primary cause of cavitation. In a return style EFI fuel system, the most likely place for these conditions to exist in the same place, at the same time, is at fuel pump inlet. The most common cause of cavitation is installation error. Once cavitation starts, it will feed upon itself. As vapor enters the pump, it displaces liquid fuel required to lubricate the mechanism, allowing metal to touch metal, creating even more friction and heat. Once the pump begins to super heat, a complete vapor lock may develop. In order to prevent cavitation and vapor lock, correct fuel system design and installation are vital. Ensure inlet filters meet hi-flow, low restriction requirements and that both inlet and outlet filters are kept clean. Keep the tank full on hot days. Reduce fuel pump speed during low load, idle and cruise conditions. Carefully route fuel lines and plan component placement to avoid exhaust heat. Do not overlook proper tank ventilation, if the vent line or vent valve do not allow ample air to move freely in both directions, fuel delivery problems will never fully resolve. Any conditions that restrict the pump’s access to fuel in the tank must be addressed. Read Tech Bulletin #101, Fuel Filtration and call the Aeromotive Tech Line if you need further assistance.

My fuel pump sounds louder and louder, now it seems to turn on and off or it blows fuses, why?
Your fuel pump may be experiencing significant cavitation caused by conditions described in earlier FAQs. It may also have experienced damage from debris, or it may have a motor problem. If normal steps to minimize inlet restrictions and cavitation do not resolve the issue, contact the Aeromotive Tech Support staff for assistance in diagnosing the problem and obtaining service if necessary. In the event your pump should need service or repair, an RGA is required, so be sure to call first before shipping.

Why are Aeromotive fuel pumps rated for less HP on a forced induction engine than they are for a naturally aspirated engine?
Two factors effect an electric fuel pump’ rated ability to support flywheel HP, one is the pressure at which the fuel pump is asked to deliver fuel volume and two is the HP consumed by engine accessories ahead of the flywheel. Higher fuel pressures created by “boost reference” fuel systems, common to forced induction EFI engines, load and slow down electric pumps, reducing available fuel pump volume. Forced induction also requires more fuel to support HP developed in the cylinder but lost to the devices used to create the added airflow and make boost. For example, supercharged engines consume HP to drive the compressor via a belt, turbo chargers create “pumping losses” by building exhaust backpressure in the cylinder to drive the compressor. As a result, the electric fuel pump is de-rated for forced induction and is said to support less flywheel HP. Of course, in fact, the pump supports similar developed HP, but due to the nature of a forced induction engine, less of what is developed in the cylinder remains to be measured at the flywheel. See Aeromotive Tech Bulletin #501 for a detailed discussion of these issues and how to plan your forced induction fuel system to account adequately for these factors.