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Fuel-Related Problems Explained


4/13/2010
By Larry Carley

If the fuel pump stops working for any reason, the flow of fuel to the engine stops and the engine dies.
 
The fuel pump is the heart of the fuel system. Most late model vehicles have an electric pump mounted inside the fuel tank. If the fuel pump stops working for any reason, the flow of fuel to the engine stops and the engine dies. Fuel pump failures tend to be sudden and unpredictable, with few symptoms to warn the motorist that trouble is brewing. And the higher the mileage on the vehicle, the greater the risk of a fuel pump failure.

If an engine cranks normally, has spark and compression but won’t start because it is not getting any fuel — or it lacks adequate fuel pressure to start — the tendency is to blame the fuel pump for the no-start problem. Unfortunately, replacing the fuel pump doesn’t always fix the problem. Why? Because many times, the problem is not the fuel pump but something else.

Nearly 10 percent of all the fuel pumps sold by auto parts stores are returned because the pump did not work when it was installed or it failed to start the vehicle.

The Fuel Pump Manufacturer’s Council (FPMC) says that up to 80 percent or more of the fuel pumps that are returned for being “defective” work normally when tested by the manufacturer. The problem was not the fuel pump, but not diagnosing the no-start condition accurately.

Any number of things can prevent the engine from getting enough fuel to start and run:
• The fuel strainer or inlet sock inside the fuel tank could be gummed up with dirt or rust.
• The fuel filter could be plugged.
• The fuel line could be pinched or blocked.
• The fuel pressure regulator could be leaking.
• The wiring connector or ground connection at the fuel pump could be loose or corroded.
• The fuel pump relay could be bad.
• The fuel pump fuse could be blown.
• The fuel tank might be empty, or contain water, diesel fuel or other contaminants.

DIAGNOSING THE PROBLEM
Accurate diagnosis is important because it eliminates the replacement of good pumps, and the unnecessary return of new pumps. Warranty returns waste everybody’s time, especially the technician’s time because a tank-mounted pump often takes a couple of hours to replace. Warranty returns also mean extra paperwork for the parts store, warehouse distributor and pump manufacturer, not to mention the shipping costs.

Currently, the only way to test a fuel pump is on the vehicle. This requires a certain level of know-how and some special tools. If an engine does not seem to be getting any fuel (cranks and has spark, but won’t start), one of the easiest checks to make is to listen for the pump to run for a couple of seconds when the ignition is first turned on. No buzz from the pump means the fuel pump is not running. It may be a bad pump or it may be an electrical problem (no voltage to the pump).

Not enough pressure?
If an engine starts, but runs poorly (no power), it may not have enough fuel pressure. This can be checked by attaching a fuel pressure gauge to the service fitting on the engine fuel rail, or teeing the gauge into the fuel supply line. If fuel pressure is less than specifications, the next step would be to check the fuel pressure regulator, fuel lines and fuel filter for possible problems.

A fuel pump check should also include testing how much fuel the pump flows. A weak pump may develop adequate pressure at idle, but cannot keep up with the engine’s fuel demands at higher speeds, causing a loss of power.
Fuel volume can be checked with a flow meter teed into the fuel supply line or return line, or the fuel supply line can be disconnected from the fuel rail and placed in a container to see how much fuel the pump delivers in 30 seconds. A good pump will usually deliver about a quart of fuel.















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