What it Is, What It Does and What’s Going on When it Won’t.
The fuel pump creates pressure to push or pull fuel from the fuel tank to the engine. All fuel-injected engines have a high-pressure electric pump (35 to 90 psi), which is often located inside the fuel tank. If a pump fails, it will prevent the engine from running (no fuel). Replacement pumps must have the same pressure rating and flow characteristics as the original, as operating pressure is critical with most fuel injection systems. Electric pumps are energized by a relay, which is controlled by the engine computer. A relay failure can also cause a no-start.
Cranky But Won’t Go
Where do you begin your diagnosis if you have an engine that cranks, but won’t start? One of the first things you should do is check for spark. Got spark? What about compression? If the engine has a belt-driven cam, make sure the belt hasn’t failed. Also, check for any loose hoses that might be creating a huge vacuum leak.
If ignition and compression are both OK, that leaves fuel as the obvious culprit. Now the question is, what’s wrong with the fuel delivery system? Well, the most likely causes are:
1. A dead fuel pump (could be the pump, pump relay or wiring circuit);
2. A plugged fuel filter;
3. Low fuel pressure (weak pump or restricted line); or
4. No pulse signal to injectors (bad injector relay or PCM driver circuit).
One of the first things to check is the fuel pump. Does the pump run when the engine is cranking? The pump should make a little noise. No noise would tell you the pump isn’t spinning.
On most vehicles, the pump is energized by the PCM via a relay. The pump circuit also may be wired though an oil pressure switch and/or an inertia safety switch that kills the pump in case of an accident. Refer to the wiring diagram to find out what’s involved before jumping to any conclusions.
Other electrical problems that can affect the pump include low voltage in the pump’s power supply circuit or high resistance in the pump’s ground connection. Either may prevent the pump from running or spinning fast enough to generate normal fuel pressure.
There are a couple of good tests to help you diagnose a faulty fuel pump. They are:
Dead Head Pressure: This checks the maximum output pressure of the fuel pump. With the return line pinched shut, the pump should produce two times its normal operating pressure at idle. If the pressure rating does not go up with the return line blocked, the pump may not be able to deliver enough fuel at higher engine speeds. Possible causes include a worn pump, low voltage at the pump, a plugged fuel filter or inlet sock in the tank, an obstructed fuel line or almost empty fuel tank.
Fuel Volume Test: A fuel pump that delivers normal pressure may still cause driveability problems if it can’t deliver enough fuel volume to meet the engine’s needs. A fuel volume test may therefore be the best way to evaluate the pump’s condition.
A fuel volume test measures the volume of fuel delivered over a specified interval. This test can be done by connecting a fuel flow gauge into the fuel supply line, or by disconnecting the fuel return line from the fuel pressure regulator and connecting a hose from the regulator to a large container. Caution: Make sure there are no open sparks or flames nearby while doing this test!
With the engine off, energize the pump and measure the volume of fuel delivered during the specified interval of time. As a rule, a good pump should deliver about one quart of fuel in 30 seconds.
Causes of low fuel volume delivered include a worn fuel pump, a plugged fuel filter or inlet sock in the tank, obstructed fuel line or nearly empty tank. Don’t forget that low voltage at the pump can also prevent it from running fast enough to generate adequate fuel flow. The pump’s supply voltage should be within half a volt of normal system voltage. If it is low, check the wiring connectors, relay and ground.
Putting It Back Together
If you’ve diagnosed a bad fuel pump and replacement is needed, be sure to inspect the inside of the fuel tank. The presence of rust or debris in the pump’s pickup screen would tell you the tank needs to be cleaned or replaced.
When replacing an in-tank pump, always disconnect the battery to prevent any unwanted sparks. Then drain the tank before removing the tank straps and opening the pump’s retaining collar. When installing the new pump, always replace the in-tank pickup screen and use a new O-ring for the sealing collar.
