Fuel Control Diagnosis

For an engine to run it must have enough compression to heat the air fuel mixture, a healthy spark, and the proper air/fuel ratio.   If we have too much fuel, or not enough air, in the combustion chamber the engine will run poorly, or fail to start.   To maintain the proper air/fuel ratio, vehicles use a computer controlled fuel injection system.  Older vehicles use a carburetor to mechanically meter the air/fuel ratio.

Failures in the air/fuel ratio can be from:
.   .   .  a Lean mixture
.   .   .  a Rich mixture
.   .   .  Contaminated Fuel

If a vehicle fails to start, the engine may be getting too lean of a fuel mixture.   Not enough fuel, or too much air entering the combustion chamber can cause this.   The engine might also get too rich of a mixture.   Too much fuel or not enough air can cause this.   A final possibility for the fuel control system to cause the vehicle to not run is fuel that has become contaminated with water, and is incapable of supporting combustion.

Lean Mixtures

Adding propane to the air intake while the engine is cranking can quickly identify a no-start engine caused by a fuel system that is too lean.   An engine that puts out fewer than 10,000 ppm HC is also too lean to run.   (You must study exhaust gas analysys to understand this last statement)

The most common malfunction is for the fuel system to go too lean due to the engine running out of gasoline.   An empty fuel tank, a plugged fuel filter, or a malfunction in the fuel pump may cause this.   Also the fuel injectors may not be operating.   There are a few easy tests to check for the cause of this type of lean condition.

Fuel Pressure - Volume

The most accurate way to test the fuel supply system is to test the fuel pressure and volume.   On some vehicles a schrader valve makes this test quick and easy.   A schrader valve is just like what you find in a tire valve stem that lets air in, but not out.  If the fuel system uses one you will find it on the fuel line (or fuel rail) that feeds the fuel injectors.   On other types of fuel systems you must use special adapters to hook the pressure tester into the fuel lines.   If you must disconnect any fuel line, or component, to hook up the fuel pressure tester, be sure to relieve the fuel pressure first.  CAUTION: Removing fuel filters, unhooking fuel lines, or fuel injection can allow pressurized fuel to spray out forcefully.   Many service technicians have been badly burned by spraying fuel that catches fire.

FUEL SYSTEM PRECAUTIONS: Always wear safety glasses.  NEVER use an incandescent light bulb.  A common cause of fuel fires in the auto shop is when fuel sprays onto a hot light bulb.   This causes the bulb to burst and the fuel catches fire.   It is best to use a flashlight, or a fluorescent lamp.   A CO2 fire extinguisher can put our many fires without messy clean up afterward.   Know the location of the fire blanket as this may save the life of someone with burning fuel on his or her clothing.

fuel injector

Always relieve the fuel pressure before opening any fuel line.   If there is a Schrader valve on the fuel rail, attach the fuel pressure tester onto that valve and press the volume test button.   Fuel will run out the drain hose so be sure to use a fuel catch bottle.

For systems without a Schrader valve, and as an added precaution any time you open a fuel line, disable the fuel pump.   To disable the fuel pump study an Engine Performance (Shopkey) or Powertrain Management (AllData) wiring diagram.   Look for the fuse that powers the fuel pump, or the fuel pump relay.   If you are working under the vehicle (as you will be to replace many fuel filters) it may be easy to unplug the wires that run to the fuel pump inside the fuel tank.   Once the fuse, relay, or fuel pump wiring has been disconnected, start the engine.   Allow it to run until the engine dies, and then crank the engine for 5 or 10 seconds.   Do not forget to remove the gas cap as fuel tanks will often store pressure.   Even after all these steps, there will still be a small amount of fuel that leaks out. Be prepared to catch this fuel with a shop towel.

After repairs or tests to the fuel system, always double check any fuel connector you have worked with (including Schrader valves).   Pressurize the fuel system and carefully inspect for any signs of leaking fuel before returning the vehicle to the customer or the parking lot.

When you are testing the fuel pressure and volume, or just checking for fuel pump operation, remember that the computer will not allow the pump to run if the engine is not cranking, or running.   To test the fuel pump and fuel supply system it is helpful to manually turn on the fuel pump by jumping the relay, or finding some way to send battery voltage to the pump.   The easiest way to figure out how is by reading the wiring diagram.

To test fuel pressure, attach your pressure gauge and either run the engine, or run the fuel pump.  Compare this reading to the specification.   Many vehicles use a fuel pressure regulator that is vacuum controlled.   Fuel pressure will be the highest with no vacuum applied (engine OFF).  For every 2 inches of vacuum applied to the regulator fuel pressure should decrease 1 psi.   A fuel pressure of 50psi with the engine OFF should drop to 40psi (10 psi less) when 20" of vacuum is applied to the pressure regulator.   Newer vehicles use a returnless fuel supply system that do not use a vacuum pressure regulator.

To test volume, insert the pressure tester's drain hose into a fuel bottle.   With the fuel pump running, simply press the Volume Test button.   Your fuel pump should easily deliver about 1 pint in 30 seconds.  It is quite possible to have a fuel pump that has good pressure, but low volume.   This will cause the engine to run lean and lack power, especially under high fuel demand situations like hard acceleration.   Low fuel volume is most likely caused by a restricted fuel filter, or a worn pump.   Replace the filter first and then retest the volume.

Any fuel that you have left over must be stored and treated as hazardous waste.  The best way to avoid storing fuel collected during a volume test is to pour it back into the vehicles gas tank!

After testing fuel pressure and volume, shut off the fuel pump and observe the gauge.   It will often drop a few PSI but should hold that pressure for hours or even days.   Not all systems have this feature but most rely upon this pressure for quicker starting of the engine.   Systems that slowly lose pressure are leaking past the check-ball inside the fuel pump, the fuel pressure regulator (on the return side), or have an injector that is leaking.

Remember to carefully check for leaks after removing the pressure gauge and turning the fuel pump back ON.   Use a good flashlight or fluorescent light to look at every connection you disturbed including the Schrader valve.   Always replace the cap on the Schrader valve.   If it was missing, use a tire valve stem cap.

Fuel Injectors

fuel injector

Fuel injectors are simple solenoids that allow fuel to spray into the intake manifold when they are turned on.   As they turn On and OFF they will make a clicking noise.   If they do not click, they are either stuck, or they are not getting the proper electrical signal.   The best way to check the electrical signal to fuel injectors is to use a lab scope.

Too much air can also cause a lean mixture.   A leak in the air intake system will allow air into the cylinders that is not recorded by the Powertrain Control Module (PCM).   If this computer cannot accurately measure how much air is getting to the cylinders, it can not give the proper amount of fuel.   Systems that use a Mass Airflow Sensor or a Vane Airflow Sensor rely upon an air intake system that has no leaks so it can directly measure how much air is coming into the engine.   On systems that use an airflow sensor, any air intake leak will cause a lean mixture.   You can check for air intake leaks using a smoke machine.

Rich Mixtures

A fuel system that is too rich can also cause the engine to not start.   A plugged air filter, a fuel pressure regulator that is leaking, leaking injectors, fuel pressure that is too high, or a fuel control sensor sending the wrong information to the PCM can cause a rich air/fuel ratio.

Plugged air filters are easy to check, need frequent replacement, and are often overlooked when trying to figure out why an engine is too rich.   As an air filter becomes clogged the fuel control sensors will sense that less air is entering the system and should reduce the amount of fuel to match.   This will result in a gradual loss of power that may not be noticed by the customer.   Once in a while an air filter will become so dirty that not enough air can get in to start the engine.   Also it is not uncommon for high mileage engines to have excessive oil in the air filter housing.   This can be caused by defects in the PCV (Positive Crankcase Ventilation) system, or by an engine with worn piston rings allowing crankcase pressure to force oil into the air cleaner.   If the air filter is plugged by oil you should diagnose why the oil is in the air filter housing.  Merely replacing the air filter will be a very temporary fix.

Many systems use a fuel pressure regulator that is mounted on the fuel rail.   For Port fuel injection these pressure regulators will have a vacuum line attached to balance the pressure in the fuel line with the intake manifold pressure.   By removing this vacuum hose you can check for gasoline that is leaking into the vacuum line.  Any gasoline in this vacuum hose means the pressure regulator is leaking and should be replaced.  Also if this vacuum line is worn out or disconnected it will cause the fuel pressure to be too high, and a rich mixture will be the result.

When a fuel injector is turned off, it should not allow fuel to flow into the intake.   A fuel pressure test can help you find leaking fuel injectors.

Any one fuel injector can fail however all fuel injectors tend to slowly flow less fuel as they become restricted by varnish and contamination in the fuel.   If you replace just one fuel injector, it will now flow more fuel than the other injectors and a rough running engine can result.   It is recommended that if any one injector is defective, the entire set of injectors should be replaced as a matched set.  New injectors can be very expensive.   There are businesses that specialize in fuel injector service and will recondition fuel injectors for cheaper than replacement.   Another advantage to this service is that they will flow-test the injectors and send you a matched set.  Be sure that the injector reconditioning service uses an ultrasonic cleaning system, and flow-tests each injector to ensure the returned injectors have equal flow rates.

On-Car injector cleaning is an excellent preventative maintenance procedure.   Cleaning the injectors, the fuel rail and the air intake system can improve fuel economy and return an engine to a smoother, cleaner, more efficient performance.  Many inexpensive fuel cleaning systems are available however systems that are not capable of cleaning the fuel rail and "back flushing" the fuel injectors are not effective.

Fuel pressure that is too high can cause an engine to run rich, or not start.  A fuel pressure test is the only way to verify that the fuel pressure is correct.   Be sure to carefully look up this specification, as it will change depending upon the year, make, model, and engine size.

Computer Controls

The PCM must get the correct information from it's sensors to know how long to turn on the fuel injectors.   Checking codes and data to the PCM using a scan tool can help identify which sensor may be sending the wrong information.   Being able to effectively use the scan tool takes careful study and practice.  The more you use it, the easier it gets.