ASE A8 Fuel Pressure:
Fuel injection systems depend on sensor inputs to monitor whats going on in the physical world. These sensors provide an input to the Electronic Control Module or Power Train Control Module (ECM/PCM).
The ECM/PCM process the input signals information to determine how much fuel the engine needs at a particular time.
The fuel injectors are held close by spring pressure, and deliver fuel to the engine, based off controlled commands from the ECM/PCM.
This way the engine will always receive fuel to the engine that is close to the ideal air/fuel mixture. The injected fuel is sprayed in a very fine mist that vaporizes easily.
There are two basic fuel injection designs, Port Fuel Injection and Throttle Body Fuel Injection.
Port Fuel Injection systems are more efficient of the two, mainly because the fuel is injected near the intake valve on each cylinder. This eliminates the problems associated with fuel distribution through the intake runners, some manufactures also call this set up multiport fuel injection or Sequential Electronic Fuel Injection.
Throttle body fuel injection systems deliver fuel at a throttle body. A controlled spray of fuel enters the air stream above the throttle plate. Although the TBI is a better design than carburetors it still falls to some the problems with the intake design (fuel pudding) and fuel delivery through the runners on the intake manifold.
ASE A8 FUEL PRESSURE
When troubleshooting a fuel injection system one of the most important checks would be the fuel pressure. When the fuel pressure is incorrrect the whole fuel injection system does not function as design.
High fuel pressure the vehicle will run rich
Low fuel pressure the vehicle will run lean
Maintaining the fuel pressure is critical for the operation and performance of the system
Fuel injection fuel pumps will pressurerize the fuel system with more fuel than the vehicle actually neeeds, the fuel pressure regulator will return the unused fuel back to the fuel tank, while maintaing a constant pressure across the injectors.
Most fuel pressure regulators use intake manifold vacuum to control the fuel pressure regulator. When the vacuum drops or the intake pressure increases the fuel pressure will usually increase to maintain constant fuel supply across the injectors for different loads.
If the fuel pressure regulator sticks close the fuel pressure will be high and the vehicle will run rich.
If the fuel pressure sticks open the fuel pressure will be low and the vehicle could run lean.
Both problems can cause an emission failure, hard start and driveability problems
ASE A8 FUEL PRESSURE TROUBLESHOOTING
The fuel system on some cars should hold fuel pressure, especially domestic vehicles but not all imports will do this.
But let’s say this is one of these vehicles that holds fuel presssure and it doesn’t.
For an example when the engine is turned off, the fuel system loses pressure immediately, this could indicate a leaking;
fuel pressure regulator
or even the fuel line
Each one will cause a driveabilty or starting problem, and this pressure check should be performed to rule out a defective component.
ASE A8 FUEL PRESSURE: NOID LIGHT VERUS LOGIC PROBE
The logic probe has three different settings it’s like having three different test lights in one unit.
One detects no voltage or ground
One detects the present of voltage
One detects a switching or pulsing voltage
The green LED indicates the voltage is below .8 of a volt
The yellow LED indicates that the probe tip is touching a Direct Current (DC) pulse is occurring
When a DC digital pulse is detected, the Red and Green LED’s will blink alternately with the Yellow LED (Light Emitting Diode) .
Noid lights are a quick check for an injector pulse.
A noid light will not stress the injector circuit because it only loads the circuit with less than 600 milli amps while some injectors will peak at 6 amps under normal driving conditions.
As you can see that is a big difference, so keep this in mind while troubleshooting a problem.
Also, keep in mind the the light bulb inside the noid light can blow or break.
So, do not condemn an injector circuit based solely on an non blinking light.