Adapted from an article written by Larry Carley for Underhood Service magazine
Fuel injection problems encompass everything from hard starting, stalling and misfiring to hesitation, surging and no-starts. Dirty injectors, for example, will restrict the amount of fuel that is sprayed into the engine with every pulse of the injector resulting in a leaner-than-normal fuel mixture. This, in turn, can cause lean misfire, hesitation, poor performance and an increase in hydrocarbon (HC) emissions.
Because EFI is part of the powertrain control module's feedback control loop, problems with the coolant sensor or oxygen sensor also can affect fuel delivery. A coolant sensor that always reads cold will prevent the engine from going into closed loop resulting in a rich fuel mixture and poor fuel economy. A dead oxygen sensor can have the same effect. So too, can a contaminated or sluggish O2 sensor.
The PCM also relies on inputs from the throttle position sensor, airflow sensor (if one is used), manifold absolute pressure (MAP) sensor and intake air temperature sensors to adjust the fuel mixture. Most sequential injection systems use the signal from the camshaft or crankshaft position sensor to trigger and sync the injector pulses. Problems in any of these sensor circuits also can affect fuel delivery.
Most PCMs have an internal driver circuit for grounding (or in some cases energizing) the injectors. Problems here can disable one or more injectors depending on how the system is wired. In some cases, a shorted injector will kill the driver circuit in the PCM. And don't forget the power relay that supplies voltage to the injectors. If this relay dies, it will disable all of the injectors.
There's also the components in the fuel system itself: the fuel pump, pump relay, fuel filter, fuel lines, pressure regulator and injectors. Problems with any of these components may prevent fuel from reaching the engine or reaching it at the correct pressure.
The point here is a lot of things can affect the operation of the fuel delivery system. The challenge is to figure out what is causing the problem without wasting a lot of time chasing dead ends. We don't have the space to explore every possibility, so we'll focus on the main components in the fuel system itself.
Cranks But Won't Start
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 has not 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 is wrong with the fuel delivery system?
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 is not 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 is 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.
Fuel Pressure Checks
Depending on the application, the fuel system may require anywhere from 30 to 80 psi of fuel pressure to start and run. Pressure specifications will vary according to the type of fuel injection system on the engine as well as the performance, fuel economy and emission requirements of that particular model year vehicle. There are no rules of thumb. Every application is different, so always look up the pressure specs when troubleshooting fuel-related performance problems.
When there is too much fuel pressure, the engine runs rich. This causes an increase in fuel consumption and carbon monoxide (CO) emissions. An engine that is running really rich also may experience a rough idle, surging and possibly even carbon-fouled spark plugs.
When there is not enough fuel pressure, the engine may not start. Or if it does, it may idle roughly and run poorly. Low fuel pressure creates a lean fuel condition that can cause lean misfire, hesitation, rough idle, hesitation and misfire on acceleration.
To check fuel pressure, you need a gauge and a place to attach it. There are a number of different checks that can be made, including static or rest pressure (key on, engine off), residual fuel pressure, running pressure, maximum or "dead head" pressure and volume of fuel delivered. The fuel pressure regulator also should be tested, and a fuel pressure drop test performed to check for dirty fuel injectors.
Different vehicle manufacturers recommend different test procedures. On many European EFI systems, the OEMs recommend using a static pressure test with the engine and ignition off. This is done by bypassing the fuel pump relay and energizing the pump directly. Most domestic and Asian vehicle manufacturers, on the other hand, provide a test fitting on the fuel rail so pressure can be checked with the engine running.
If you are working on a vehicle that does not have a pressure test fitting, you will have to tee a pressure gauge into the fuel line just ahead of the injector fuel rail.
Caution: Before hooking up your pressure gauge, relieve all pressure in the fuel system.
Static Fuel Pressure Test
With the key on, engine off (or with the fuel pump energized), fuel pressure should come up quickly and hold steady at a fixed value. Compare the pressure reading to specifications. If you get no pressure reading, check for voltage at the pump. If there is voltage but the pump is not running, you have found the problem: a bad fuel pump.
If you do get a pressure reading but the reading is lower than normal, the cause may be a weak pump, a blockage in the fuel line, filter or tank inlet sock, or a faulty pressure regulator. Also, low voltage at the pump may prevent it from spinning fast enough to build up normal pressure. Check the voltage at the pump. If OK, check the fuel filter and lines for obstructions and the operation of the fuel pressure regulator before you condemn the pump.
Residual Fuel Pressure Test
When the pump is turned off or stops running, the system should hold residual pressure for several minutes (look up the specs to see how much pressure drop is allowed over a given period of time). If pressure drops quickly, the vehicle may have a leaky fuel line, a leaky fuel pump check valve, a leaky fuel pressure regulator or one or more leaky fuel injectors. Low residual fuel pressure can cause hard starting and vapor lock during hot weather.
Running Fuel Pressure Test
With the engine idling, compare the gauge reading to specifications. Fuel pressure should be within the acceptable range given by the vehicle manufacturer. If low, the problem may be a weak pump, low voltage to the pump, a clogged fuel filter, line or inlet sock inside the fuel tank, a bad pressure regulator, or nearly empty fuel tank.
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 3/4 to 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.
Fuel Pressure Regulator Test
This test checks the operation of the fuel pressure regulator to make sure it changes line pressure in response to changes in engine vacuum. This is necessary to maintain the proper operating pressure behind the injectors and to compensate for changes in engine load.
With the engine running, disconnect the vacuum hose from the pressure regulator. As a rule, fuel system pressure should increase 8 to 10 psi with the line disconnected. No change would indicate a faulty pressure regulator, or a leaky or plugged vacuum line.
If the diaphragm inside the regulator is leaking, engine vacuum will suck raw fuel into the intake manifold through the vacuum hose (look for fuel inside the hose).
Fuel Pressure Drop Test
This test measures the drop in static system fuel pressure when each injector is energized. The amount of pressure drop for each injector is then compared to see if the injectors are dirty and need to be cleaned or replaced. This test requires an "injector pulser" tool to energize the injectors.
To perform this test, turn the key on or energize the fuel pump for a few seconds to build up static pressure in the fuel system. Then turn the key off, pulse one injector for the specified time and note the pressure drop for that injector. Turn the key back on to rebuild static pressure and repeat the test for each of the remaining injectors.
An injector that is pulsed 100 times for five milliseconds should produce a minimum pressure drop of about 1 to 3 psi, and no more than 5 to 7 psi, depending on the application.
The difference in pressure drop between all the injectors should be 2 psi or less. If you see more than 3 psi difference between the highest and lowest readings, the injectors are dirty and need to be cleaned or replaced.
If you see no pressure drop when an injector is energized, the injector is clogged or defective and needs to be replaced. If the pressure gauge needle bounces, the injector is sticking.
After cleaning, repeat the test to see if cleaning did the trick. All injectors should show about the same amount of pressure drop (less than 2 psi difference) and no more than 7 psi drop at 100 pulses for 5 milliseconds. If there is no change in the readings or the drop exceeds these limits, the injector(s) need to be replaced.
By connecting a Low Amps probe to the fuel pump's voltage supply wire, you can create a scope waveform that will reveal internal wear in the brushes and commutator that may not show up in a traditional pressure or volume test.
Observing the waveform will tell you if the pump's amp draw is normal for the application or is high or low, and if the pump is operating at normal speed or is running slow. Problems such as a bad spot on a commutator or a short or open in the armature also will be obvious in the waveform.
A "good" electric fuel pump waveform will generally seesaw back and forth with relative consistency and minimal variation between the highs and lows. A "bad" waveform will show large or irregular drops in the pattern, with large differences between the highs and lows.
If you have 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.
Normal range for both STFT and LTFT is typically plus or minus 8. If the numbers are +10 or higher for STFT and LTFT, the engine is running LEAN (not enough fuel). If the values for both STFT and LTFT are both minus 10 or more, the engine is running RICH (too much fuel).
For more information on this subject, see What Is Fuel Trim?
Vacuum leaks can create a lean-running engine that requires extra fuel to balance the fuel mixture. If you rev the engine to 1500 to 2000 rpm and hold it for a minute or so, and the STFT value drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. If the STFT value does not change much, the lean fuel condition is more likely a fuel delivery problem (weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum leak.
For more information about using fuel trim to diagnose a lean fuel condition, read this article on Fuel Trim by Wells Manufacturing (PDF file, requires Adobe Acrobat to read).