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.
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.
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.
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.
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.
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.
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.
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These problems result mainly from poor combustion, which stifles the engine.
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