Mechanically Speaking: How many fuel pumps does it take?

"Do not underestimate the value of that annoying check engine light in clueing you into a burgeoning fuel pressure problem."

The lowly fuel pump has proliferated in the modern day vehicle.  Until the late 2000s most gasoline powered automobiles had only one fuel pump.

Today many vehicles are sporting Gasoline Direct Injection (GDI). As such these vehicles have a low pressure fuel supply pump feeding a very high pressure fuel pump.

We all know our vehicles need fuel to run. Some of us know that fuel is supplied to the fuel injectors by a pump. In the carburetor era (pre 1980) that fuel pump was likely a mechanically driven device mounted on the side of the engine. It sucked fuel out of the tank up to itself and then pushed the fuel into the carburetor where it was then metered into the engine.

Along comes fuel injection and the fuel pump becomes an electromechanical device that in most cases is mounted in or very close to the fuel tank. The fuel injector (a much more accurate metering device)  requires some serious pressure (thirty to sixty pounds per square inch) compared to the lowly carburetor’s demand for three to six pounds per square inch.

Now, with GDI, our fuel injectors are diesel like and must spray fuel directly into the combustion chamber during your engine’s compression stroke. Pressures are now in the thousands of pounds per square inch.

These pressures require a very special pump familiar to most diesel aficionados, a high pressure mechanical pump driven by the engine. Sound familiar? This thing is a lot more intricate than the one that was feeding low pressure fuel to the old carburetor.

This high pressure pump in fact still requires 30 to 60 pounds per square inch of fuel from the electromechanical fuel pump in your gas tank. So now you have two fuel pumps.

If the high pressure pump fails your engine stops running, plain and simple. When it comes to replacing a failed high pressure pump it is a very good idea to know why.  If the in-tank electromechanical pump is weak or fuel filters are clogged and the high pressure pump is not getting the lower pressure fuel it needs, another failure is imminent.

Many diesel owners have been familiar with this scenario for some time.  Their high pressure diesel injection pumps and common rail pumps produce tens of thousands of pounds per square inch pressure and are very costly to replace. They are quickly destroyed by a malfunctioning low pressure lift pump or clogged fuel filter.

Do not underestimate the value of that annoying check engine light in clueing you into a burgeoning fuel pressure problem. Your vehicle now incorporates fuel pressure sensors to control and monitor the high and low fuel pressure.

Driving around with that orange light on does not always mean to check your gas cap. It may be an indication of more serious problems. The sooner you react, a simple fuel filter replacement may save you the cost of two expensive fuel pumps.

Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC. E-mail: nutechauto@telus.net

 

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