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I get that the main effect of a bad fuel pump is that the engine will cut out at higher speeds, and that this is often not easily noticeable until that happens because there are little or no prior indications that I am aware of. And that this doesn't mean that the car fails to start (in most cases). I also assume that the answer must be that for some reason it must be harder to maintain a good fuel pressure at higher speeds but I don't understand why.

Why is the effect of a bad fuel pump more noticeable at higher speeds?

UPDATE

Just to clarify, by "higher speeds" I meant an increase in miles per hour (mph) of the car when in motion.

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2 Answers 2

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Background

You asked

Why is the effect of a bad fuel pump more noticeable at higher speeds?

I assume that by 'higher speeds' you mean engine speeds or RPM's.

During high RPM operation an ICE consumes more fuel so the flow rate into the fuel injection system or carburetor increases at higher RPM's.

This taxes the fuel system and puts more load on the pump requiring it to fulfill the need for more fuel.

If a fuel pump is nearing the end of it's service life and is not running optimally this can lead to situation called 'fuel starvation' where the requirements for fuel for the ICE are not being met.

This would result in a lean condition to the vehicle where it may not be able to achieve a higher RPM due to the fuel starvation and the resulting power loss.

Answer

The effect of a fuel pump going bad will result in a lean condition to the ICE preventing it from maximizing conversion from fuel to torque at it's maximum possible conversion rate.

Update: From OP Edit of Original Question

Since you actually mean higher speed as in higher rate of distance covered over time I will add this.

Higher speeds equate to higher RPM's which in turn consumes more fuel.

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  • Higher speeds translate to higher loads
    – Zaid
    Commented Jan 20, 2016 at 19:09
  • Would you say that 7,000 RPM with the throttle wide open in 2nd gear is would consume less fuel at 7,000 RPM with the throttle wide open in 6th gear?
    – DucatiKiller
    Commented Jan 20, 2016 at 19:10
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It's a demand vs supply deal

The fuel pumps found in most OEM fuel delivery setups today are driven by an electric motor that is running at a fixed speed. This effectively fixes the flow rate provided by the fuel pump.

The majority of fuel-delivery systems found in fuel-injected vehicles are return-style. What this means is that the fuel injectors will pulse open to utilize the amount of fuel required, and the remainder is recycled back to the fuel tank.

Under high loads (which usually tally with higher speeds), the fuel injectors are commanded to stay open for longer, meaning that less fuel is returned back to the fuel tank.

If the fuel pump can't deliver enough fuel flow, the engine may end up being starved of the fuel it needs, so the injectors are commanded to stay open but there is not enough fuel to go around, resulting in misfires.

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