Fuel Rail Pressure (FRP) Sensor
Direct Injection & Air Induction Systems Diagnostics Fuel Rail Pressure (FRP) Sensor
The high pressure system develops pressure mechanically, so closed loop control is even more critical than on other feedback systems. Therefore, there will always be a high pressure sensor mounted to a fuel rail (only one sensor is used on V-engines). It works like any 5-Volt, 3-wire sensor, but with a much wider range. There will always be a high pressure PID on the Scan Tool. You can also backprobe the signal circuit to match the voltage to the PID, but that’s not a very helpful comparison for most faults. In any case, all FRP sensors output a low voltage for low pressure, and high voltage for high pressure. However, the normal range varies quite a bit by application.
ATG TIP: Most FRP sensor faults involve miscalibration, and since nobody makes a practical tool to directly measure the rail pressure, you have to get sneaky. Relieve the high pressure by disconnecting the FPR solenoid (most applications), or by using a Scan Tool control to disable the low pressure or high pressure systems (controls vary in both name and effectiveness). Then cycle the key so that the low pressure pump primes the system and pushes through the high pressure pump check valve into the rail. Compare FRP sensor PID to the low pressure sensor PID (install a low pressure gauge if PIDs aren’t available). The FRP sensor has a much higher range, so it won’t be as precise at the lower end of its range. Additionally, the rail pressure will always be lower because of the spring tension in the high pressure pump check valve. A 10 PSI drop is common, but we’ve seen over 30 PSI on some known good applications. On other applications, the low and high pressure match, but it’s likely that this is because the PCM adds to the FRP value to make them match. Either way, it’s still a great ‘ballpark’ test.
Note: Many FRP sensors can only be used once, so check repair information. You’ll probably get away with reusing one, but if it EVER leaks, it’s your liability. With up to 3,000 PSI of flammable liquid in a hot engine compartment, it’s best to follow manufacturers’ instructions.
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