Fuel pump is crucial, but often overlooked

Hey there, time traveller!
This article was published 17/06/2005 (7463 days ago), so information in it may no longer be current.

WITH all the complexity of modern car electronics and fuel injection, the lowly fuel pump often gets forgotten when it comes to solving driveability problems. Smart technicians have learned to never overlook this small part, but unfortunately, I still see many owners spending hundreds and even thousands of dollars trying to fix their vehicle problems when perhaps it could be accomplished much faster and cheaper. Let’s look at how fuel pumps have changed and what goes wrong.

Back in the days of carburetors, most fuel pumps were mechanical devices that used a lever and internal spring to pull the fuel from the fuel tank and push it into the carburettor. Most systems operated at only 5 to 7 PSI and diagnosis was usually straight forward. Either they didn’t pump fuel or they leaked. In either case, a new fuel pump was needed.

With the advent of fuel injection, electric fuel pumps became the norm. While a few vehicles use fuel pumps mounted to the vehicle body or frame, most vehicles use an in-tank fuel pump. The fuel surrounding the pump helps to keep the pump cool as it runs. The tank or fuel pump housing is designed so that fuel is retained around the pump even when the fuel level is low. In-tank fuel pumps don’t have to create much suction. Because of their location, they only have to push the fuel to the engine. Many of these pumps operate in the 9 to 40 PSI range. Internally, they create pressure by moving the fuel with centrifugal force, similar to an engine water pump. Never operate one of these pumps outside the fuel tanks as they will quickly overheat and can damage the pump mechanism.

I often get asked why the fuel tanks don’t explode when the electric motor operates in the tanks. The simple answer is that the air fuel ratio in the tank is too rich for combustion to take place even though there may be sparks occurring at the fuel pump brushes. Never use one of these pumps to empty a full tank when it needs to be removed if the fuel tank is open or there is the possibility of an explosion.

Many current fuel injection systems operate at higher pressures. This provides a finer spray from the fuel injectors for more complete combustion. However, the electric fuel pumps now need to be positive displacement pumps to generate the higher pressures. A positive displacement pump is one where everything that goes in must come out. Engine oil pumps and power steering pumps are positive displacement types and so now are many fuel pumps. One major disadvantage of this type of pump is that the internal tolerances are small so a small piece of dirt or rust can be trapped in the pump and prevent it from turning.

Vehicles that sit a lot or get water in the fuel tank are more likely to have fuel pump problems caused by dirt or rust. The vehicle may be running fine but shut it off and it doesn’t start again. If you listen carefully at the fuel filler, you should hear the pump run when the engine is cranked. If there is no sound from the pump, then it is likely stuck. If you are replacing a fuel pump because it suddenly stopped working, have the fuel tank cleaned too so there is no debris that will cause a new pump to stop working. Manufacturers have installed finer “socks” (inlet screens) on high pressure pumps to help filter out dirt that could stop the pump but they can’t stop everything or no fuel would get through.

Some of the more difficult to diagnose problems have to do with high electrical resistance inside the fuel pump. A broken wire or poor electrical connection slows pump operation so that fuel pressure is low. Sometimes this will happen when the pump is cold. The parts contract and electrical resistance is too high. Fuel pressure is low, but if you crank long enough, the pump warms up and the parts expand, creating a better electrical connection. The engine then starts and operates fine, until it sits for a while and cools down. Then the problem occurs again.

The reverse can also happen, The pump may work fine when cold but when operating, the heat expands the wiring and a high resistance develops. These vehicles tend to operate fine until they are turned off, but then they won’t restart until the vehicle sits for several hours.

Any time you have driveability or starting problems, test the fuel pressure. If necessary, leave the pressure gauge on the vehicle and test it both cold and hot. The manufacturers list minimum fuel pressure specifications. Be accurate. If the pressure is even one or two PSI too low, it can cause problems. It could be a faulty fuel pressure regulator, but more often than not, it is the fuel pump that causes the problem.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net