Excessive pressure in the fuel rail

The injector fuel pressure regulator is one of the internal combustion engine parts that maintains the required fuel pressure in the fuel injectors depending on the selected engine operating mode. The stability of the engine and the performance of the injectors directly depend on the proper functioning of the fuel pressure regulator.

Since the RTD is classified as a diaphragm valve, signs of a malfunctioning fuel pressure regulator and its failure can have a negative impact on engine performance. Carrying out regular diagnostics of the regulator, troubleshooting it and necessary repairs are procedures that every car owner needs to know about.

Where is the fuel pressure regulator located?

A diaphragm valve-regulator, which is subjected to fuel pressure on one side and spring force on the other, is used to more accurately dose fuel and maintain constant pressure in the system. A VAZ fuel pressure regulator of this type is installed in those systems that have a return line. In them it is located in the fuel rail. Less commonly, the part is installed in the fuel tank, but such systems do not have a return line. How the fuel pressure regulator works depending on its location is described below.

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Location of the regulator in the fuel rail

One of the most common placement schemes: the VAZ fuel pressure regulator is located on the fuel rail. The principle of operation of the element is to pump fuel through the line from the fuel tank using a fuel pump. The regulator is under pressure from the fuel. The RTD itself consists of two separate cavities-chambers, separated by a membrane: a fuel chamber and a spring chamber. Through special inlet holes, fuel enters the regulator, which puts pressure on the membrane on one side; on the other hand, a spring and the pressure of the intake manifold presses on it. If the fuel exerts greater pressure than the spring force and inlet pressure, then the regulator opens, due to which part of the fuel is discharged into the “return”. Excess fuel is sent back to the fuel tank via the return line.

Location of the regulator in the fuel tank

The RTD is placed directly in the fuel tank in systems that do not have a return line. The advantage of such a system is the absence of an additional pipeline. The required volume of fuel is supplied directly from the tank, that is, excess fuel does not accumulate in the engine compartment, and accordingly, there is no need to send it back to the tank. Thanks to this design, the fuel does not overheat and evaporates less, which is an additional advantage.

Recirculation type fuel supply system

1 - fitting for checking fuel pressure, 2 - fuel rail, 3 - pipeline fastening, 4 - fuel pressure regulator, 5 - fuel module, 6 - fuel filter, 7 - return line, 8 - supply line, 9 - injectors
In this system fuel from the tank is supplied by the fuel pump through the filter to the fuel rail, and the excess is discharged through the pressure regulator back to the tank through the return line. The regulator maintains pressure within the range of 2.8 - 3.2 bar (1 bar = 0.98692 atm.). Exactly within the limits ! We'll discuss this further.

Electronic circuit for regulating fuel pressure

The fuel pressure in the system is adjusted using an electronic circuit, the design of which does not imply the presence of a mechanical regulator. The pressure in such systems is controlled by an electric fuel pump. The electronic control system installed in it records the voltage and regulates the volume of supplied fuel. The advantages of using a fuel pressure regulator sensor are maximum savings and reduced fuel heating.

Only a strictly limited amount of fuel is supplied to the engine injectors, necessary for the selected operating mode of the engine and the specific conditions of its operation. Excess pressure does not rise to a critical level due to the presence of a relief valve.

How to check the serviceability of the fuel pressure sensor

There are two ways to check the serviceability of the fuel pressure regulator: with or without dismantling the fuel rail along with the regulator. The first method is more complex, but with its help you can check not only the operation of the pressure regulator, but also other elements of the fuel system. In addition, such a check requires a special stand, which is available only in specialized auto repair shops, in particular, at official representatives of a particular automaker. Although some car enthusiasts assemble similar homemade ones in their garage.

The main symptoms of fuel pressure regulator problems

When diagnosing the engine power system, it is necessary to take into account the signs of a malfunction of the fuel pressure regulator. The main symptoms associated with RTD problems are a decrease in engine power and its inability to reach the required speed. Signs of a malfunction of the fuel pressure regulator and, accordingly, the engine are as follows:

The engine idles unsteadily and stalls.
Fuel consumption increases, power decreases.
The engine responds slowly to the accelerator pedal.
When accelerating the car and when changing the throttle, jerks and dips appear.
The car does not gain speed and does not accelerate.

The symptoms of a problem with the fuel pressure regulator on cars with gasoline internal combustion engines are similar to a failure of the fuel pump or its strainer. For this reason, the fuel pressure regulator is checked if any malfunctions in the engine fuel system are detected.

Simply put, when the above-described symptoms of engine malfunctions appear, the problem may be hidden not only in a breakdown of the relay, the fuel pump motor, or contamination of its grid, but also in the fuel pressure regulator.

Eliminating the error code by replacing the pusher

Next, let's look at what needs to be done to eliminate error p0087 using the example of an Audi A4 car with a direct injection system.

In these cars, the pressure drop is often associated with the injection pump pusher located near the camshaft, which, in turn, acts on the high-pressure pump pusher using a cam.

Before starting work, you should first check the condition of the wiring, lines and filters.

If everything is in order with them, you can start checking the condition of the pusher and replacing it.

To complete the work you will need:

New pusher;
Sealing ring;
A set of keys;
Rags.

Before carrying out work, you need to relieve the pressure in the fuel system. Reset is performed by turning off the fuel pump installed in the tank.

In the Audi A4, this pump starts working immediately after opening the door.

To turn off the pump, just pull out fuse number 28.

After removing the fuse, the car starts to pump out the existing fuel in the system, thereby relieving the pressure.

Then you can start disassembling.

First you need to dismantle all elements that interfere with access to the injection pump. Then the wires going to this pump are disconnected from the injection pump.

The next step is to disconnect the fuel lines leading to the pump.

Before unscrewing the lines, it is advisable to place rags under them, since they still contain fuel.

Next, the injection pump itself is unscrewed. After this, the pump must be pulled away from the block head and the pusher removed from its seat.

Before installing a new tappet, if possible, you should evaluate the condition of the camshaft journal, which interacts with this tappet (cam). If there are traces of heavy wear on it, you will have to change it too.

If the camshaft is normal, a new pusher is installed; before installing the injection pump in place, the sealing ring is changed.

Then everything is put back together.

After assembly, fuel is pumped into the system. If the replacement does not help, the pressure regulator valve in the system may fail.

But since this valve is installed in the injection pump itself, it cannot be changed quickly, and the entire assembly will have to be changed.

RTD defects: spring

The main problem with the regulator is the weakening of the spring, which causes premature draining of fuel into the return line. This leads to starvation of the engine: it does not have enough fuel to operate at high speeds, when the accelerator pedal is pressed, or in transient modes. A faulty regulator spring causes low fuel pressure in the fuel rail, which leads to a decrease in engine power, problems with the ECU and unstable operation of the engine.

How to Troubleshoot or Reset Trouble Code P0088

Some suggested steps to troubleshoot and fix error code P0088:

Connect the scanner to the vehicle's diagnostic port to retrieve the stored codes.
Clear the error codes from memory and test drive the vehicle to see if the P0088 code appears again.
Inspect the fuel tank, including the fuel lines. Repair or replace if you find any defects.
Replace the fuel filter.
Disconnect the fuel pump and check its condition. Replace it if necessary.
Check and replace the fuel rail pressure (FRP) sensor if necessary.

RTD contamination

Another malfunction of the regulator is its contamination and reduced throughput. In this case, the engine stalls regardless of which specific operating mode is selected. If the regulator is heavily contaminated, the pressure in the system increases sharply, and this can lead to fuel escaping through the sealing parts and joints. Vehicle manufacturers always consider the possibility that injector and fuel pump performance will deteriorate over time. This problem is solved by pumping more fuel into the fuel pump. Excess fuel increases pressure if it cannot return back to the return line.

How is pressure regulated in dead-end type fuel supply systems (without return)?

But no way. Another solution has been applied here.

The fuel module inside the fuel tank contains a check valve with a pressure limiter of up to 4 bar

For some reason, repair manuals and auto literature miss this fact, and more often, in general, they write lies, misleading car owners. In a dead-end system, the pressure is always higher than in systems with recirculation and it has no “limits” - it is always constant!

Why is the pressure higher? In recirculating systems, fuel is pumped in a circle and gasoline circulates constantly, cooling the fuel rail. If there is no cooling, then the fuel in the ramp may boil!

And as we know from physics lessons, with increasing pressure, the boiling point of liquids increases. This is why they increase the pressure in fuel supply systems “without return”.

Therefore, if the service center measured the fuel pressure on your car with a dead-end system and it was 3 atm, and they tell you that the pressure is normal and there is enough fuel, then get out of this service as quickly as possible.

It is important to understand that such pressure is necessary not so much for sufficient fuel (the engine will work even at 2.5 atm), but to prevent it from boiling! And if the fuel boils, then you can forget about normal engine operation.

Regulator jammed

The pressure regulator located in the fuel rail may begin to jam at certain intervals. Symptoms of such a problem include pressure drops in the system and jerking of the car. Another sign of a faulty fuel pressure regulator, which is no less common, is the banal wear of all parts: in other words, over time the valve completely exhausts its life. The service life of this engine element depends on the quality of the fuel, the presence of various impurities in it, the operating mode of the vehicle itself and many other factors.

WE REPAIR

Restoring the pump's functionality can only be done by a specialized workshop with qualified personnel and diagnostic equipment. The cost of repairs is from 7 thousand rubles, then it depends on the complexity. In case of some damage, it is wiser to buy a new injection pump. The usual price, about 30 thousand rubles, will shock the tight-fisted diesel operator, which is why repaired or restored products are in use.

A CR diesel with high mileage is often impossible to start due to a malfunction of at least one of the injectors. Fuel leakage through its valve does not allow the pressure in the rail to rise to starting values. There is a special diagnostic kit to check the pressure at start-up. It includes a control pressure gauge, a pressure sensor, connection tubes, plugs instead of actuators and backflow measuring containers.

Fuel pressure regulator diagnostics

In terms of its symptoms, a malfunction of the RTD is in many ways similar to malfunctions of the fuel filter or fuel pump. It is worth noting right away that if, during diagnostics of the fuel system, faults were found in the RTD, then the optimal way out of the situation would be to replace the fuel pressure regulator. This is explained by the fact that replacing individual parts, washing and cleaning the device will not return it to full functionality. Considering that the cost of the RTD is very affordable and acceptable, repairing the fuel pressure regulator is simply impractical.

You can diagnose the RTD yourself in one of the simplest and most effective ways: check the fuel pressure in the fuel system. This is done using a pressure gauge. It is measured with the engine idling; The pressure gauge is connected in parallel with the vacuum hose between the fitting and the fuel hose.

The measurements taken show pressure fluctuations in the system. It should increase, while being in the range from 0.3 to 0.7 Bar. If such indicators were not detected, then it is necessary to reconnect the fuel pressure regulator hose and reuse the pressure gauge. The fitting plug is unscrewed to measure the fuel pressure at the end of the fuel rail. The plug has a special sealing ring: you need to check it for integrity - it should not be elastic. If there are still defects on it, then it is changed either separately or together with the plug.

Features and principle of operation

The operating principle of this type of fuel system is based on the separation of the processes of creating high pressure and direct diesel injection. From the fuel tank, fuel is pumped into the system by a low pressure pump. At the same time, it passes through filters, where it is cleaned of impurities and various contaminants. Through the low pressure circuit, diesel fuel enters the injection pump, which has a mechanical drive. He, in turn, pumps fuel into the ramp, where it accumulates until injection. This allows you to constantly maintain the desired pressure level, regardless of the current operating mode of the engine.

Receiving data from the system sensors, the engine ECU determines how much fuel the injection pump needs to supply to the fuel rail. After this, the fuel dosing valve opens, which enters the battery. The fuel is under a given pressure level maintained by the regulator.

Sectional diagram of a common rail system injector

As soon as the required volume of diesel is pumped into the ramp, the ECU sends a command to open the injectors corresponding to the engine operating cycle. During one cycle of operation of such a system, multiple injections are carried out, consisting of three stages:

Preliminary - necessary to increase the temperature and compression in the combustion chamber, which speeds up the process of self-ignition. At idle, two pre-injections can be performed, while the speed increases - one, but at full power there is no pre-injection.
The main one is directly responsible for the operation of the motor.
Additional - necessary to increase the heating temperature of the exhaust gases, which ensures the combustion of soot and reduces the volume of harmful emissions into the atmosphere.

Modern diesel engines can have from 7 to 9 injection phases.

Fuel pressure regulator repair

After the diagnostics of the system is completed and the sealing ring is inspected, the umbrella is unscrewed from the fitting. You can unscrew it using the metal valve cap. The pressure gauge hose is then connected to the fitting, then the entire structure is secured using clamps. Pressure measurements are carried out only when the engine is running. Standard pressure readings should be 2.9-3.3 kg/cm2. Then the hose is carefully disconnected from the fuel pressure regulator, and the pressure gauge readings are recorded: the pressure should increase to 70 kPa from 20 kPa.

The fuel pressure regulator is changed if the readings remain too low or zero. Replacing this part is not difficult: you can do it yourself in a regular garage. Before starting the procedure, all pressure is released from the engine power system. To do this, unscrew the nut securing the fuel pipe, after which the bolts that secure the regulator itself to the fuel rail on most cars with injection engines are unscrewed.

Subsequently, the regulator fitting is carefully removed from the hole in the fuel rail, after which the RTD is completely dismantled. Before this, the fuel pipe is completely disconnected. The last stage is the installation of a new or working regulator into the ramp, after which the functionality and serviceability of the entire fuel system is checked using a pressure gauge.

Before installing a new fuel pressure regulator, experts recommend lubricating the O-rings with gasoline: this will make installation of the new part easier. A similar procedure is performed when replacing the o-rings themselves.

Today we’ll talk about the fuel pressure regulator, a mechanical device that is undeservedly deprived of attention.

It is popularly called a check valve or bypass valve. Usually, when there are problems with the engine such as: insufficient power, unstable idle, poor starting, engine jerking, they either pay attention to it last or simply ignore it. And in vain, because this is one of the most important elements in engine operation.

So we’ll talk about the “check valve”. The fuel pressure regulator (in everyday life, we all probably call it a “check valve” or “bypass valve”, because it transfers fuel back to the tank, in an amount depending on the engine operating mode), is installed on the fuel “rail” and is designed to maintain constant fuel pressure at the injector inlet at different engine operating modes and at different vacuum in the intake manifold. The regulator is a diaphragm valve. On one side, the membrane is acted upon by fuel pressure, and on the other, by spring force and air pressure from the intake manifold, to which the regulator is connected by a hose. The greater the absolute pressure (i.e., the lower the vacuum) of air in the intake manifold (i.e., the greater the load on the engine), the greater the fuel pressure. When the load on the engine decreases, when the fuel pressure exceeds the total force from the spring and air pressure, the regulator valve opens by a larger amount and excess fuel is returned through the drain line to the fuel tank. Figuratively speaking, the “check valve” serves only to maintain the same difference in pressure applied to the injector from the side of the intake manifold and from the side of the fuel line. ECU. “Does not hold”, which means its complete “openness”, that is, the fuel pumped by the fuel pump passes through the valve and fuel rail freely, almost nowhere and without being delayed by anything, and is quietly drained through the “return” line into the fuel tank. This condition causes low pressure in the fuel system. “Wedges”, “wedges” - in this case the valve works like a “drunk doorman in a restaurant”: “I want to let it in, but if I want to, I won’t let it in!” In this case, the fuel that gets into the fuel rail “sticks” into the valve, and since it has nowhere to go (and the pump at the back continues to create pressure), it begins to look for a way out... And sometimes, when the valve “wants”, it flows briskly into the tank along a completely open highway and no one can predict when all this will happen. “Dead” - of course, means: the valve in this state is like armored doors in a bank - it stands in the way of fuel and does not let it into the tank at all, under any circumstances. As a result, with the described malfunction, the pressure in the fuel system increases significantly. When the valve “does not hold,” the fuel pumped by the fuel pump circulates almost freely throughout the car. Fuel pump - fuel filter - fuel rail - and back to the tank. Let's imagine what is happening: there is low fuel pressure in the fuel rail, despite the fact that the fuel pump is working properly.

Fuel pressure problems

Let's imagine that the fuel pump is worn out and cannot create a pressure of 4 bar, or the pressure limiter is leaky and also does not maintain a pressure of 4 bar. Let's say the pressure does not rise above 2.5 bar. The ECU firmware tables contain a clear algorithm of actions under which conditions to make one or another correction of the injection time. But the ECU does not see that the pressure is not 4 bar, but only 2.5 and continues to do its job according to the algorithms entered in the tables. And due to the reduced pressure in the rail, less fuel passes through the injectors than expected. Accordingly, during injection timing correction, not enough fuel will flow through in the time given by the ECU. This way we will get a dip when pressing the gas pedal.

Let's imagine the opposite situation. The regulator or pressure limiter is stuck open. The pressure increased and became higher than expected. This is also not good. This will lead to jerks in transient conditions and excessive fuel consumption. The same symptoms apply to leaking injectors.

Why is a control valve needed?

The fuel supply system of most passenger cars requires continuous operation of an electric fuel pump. It constantly pumps gasoline into the fuel line and ramp, raising the pressure to the maximum (5–7 Bar depending on the brand of car). But such performance is needed only under increased load on the engine, when it develops high speeds and consumes a lot of fuel mixture. In normal mode, a fuel pressure at the injectors of 3–3.5 bar is sufficient.

The fuel pressure diaphragm valve, installed in the engine power system after the fuel pump, performs 3 main functions:

Limits the fuel pressure in the line at low engine loads, dumping excess fuel back into the tank through a separate tube.
When the gasoline consumption of the power unit increases, the return flow is partially or completely blocked by the regulator. In this way, the valve maintains the minimum pressure required for normal operation of the motor.
Maintaining pressure for a long time after stopping the power unit.

Operating principle of RTD

The valve design and operating principle depend on the type of fuel system of a particular vehicle. There are 3 ways to supply gasoline from the tank to the injectors:

The pump together with the regulator is installed inside the tank; fuel is supplied to the engine through one line.
Gasoline is supplied through one tube and returned through another. The fuel system check valve is located on the distribution rail.
The circuit without a mechanical regulator provides for electronic control of the fuel pump directly. The system contains a special sensor that registers pressure; the pump performance is regulated by the controller.

In the first case, the return flow is very short, since the valve and electric pump are interlocked into a single unit. The RTD, located immediately after the supercharger, dumps excess gasoline into the tank, and the required pressure is maintained throughout the supply line.

Reference. The first scheme with a regulator inside the gas tank has been implemented on all Russian-made VAZ cars.

The second option is used in most foreign cars. A valve built into the fuel rail allows excess fuel to flow into the return line leading to the tank. That is, 2 gasoline pipes are laid to the power unit.

There is no point in considering the third circuit - instead of a regulator, there is a sensor whose functionality is checked using a computer connected to the diagnostic connector.

A simple fuel pressure valve installed in the fuel pump unit consists of the following elements:

cylindrical body with pipes for connecting the supply and return lines;
a membrane connected to a locking rod;
valve seat;
spring.

The amount of pressure in the supply line depends on the elasticity of the spring . While most of the fuel goes into the cylinders (high load on the engine), it keeps the membrane and valve stem closed. When the crankshaft speed and gasoline consumption decrease, the pressure in the network increases, the spring compresses and the membrane opens the valve. The fuel begins to be discharged into the return line, and from there into the gas tank.

The fuel pressure regulator installed in the rail operates on a similar principle, but reacts faster to changes in load and gasoline consumption. This is facilitated by connecting an additional pipe of the element to the intake manifold. The higher the crankshaft speed and the vacuum on the spring side, the stronger the membrane presses the rod and closes the passage of fuel into the return line. When the load decreases and the speed drops, the vacuum decreases and the rod releases - the return flow opens and excess gasoline begins to be discharged into the tank.

Symptoms of element failure

During the operation of the car, a car owner may encounter two types of RTD failure:

The pressure drop in the rail is below the permissible level - the regulator directs most of the fuel through the return line to the gas tank.
Increase in pressure to maximum - the element does not allow fuel to flow into the return line.

Note. As a rule, the first malfunction is accompanied by a rapid drop in pressure in the system after the electric fuel pump is turned off.

It is quite simple to track the signs of the first malfunction - the power unit is sorely lacking fuel for normal operation in all modes. Symptoms appear as follows:

cold starting is difficult, the engine runs extremely unstable until it warms up;
“dips” during acceleration and jerks when moving uphill;
the car often stalls at idle;
Gasoline consumption per 100 km increases.

The increased fuel consumption is explained by the actions of the driver trying to compensate for the lack of fuel mixture by pressing the accelerator pedal. Driving in this mode is quite difficult - it is better to immediately check the fuel pressure regulator for functionality.

When the valve does not allow excess fuel to flow into the tank, the following consequences are observed:

Due to too high pressure from the ramp side, the injectors begin to leak and fill the cylinders with pure gasoline, and not with the working air-fuel mixture.
The engine is bad, it emits black smoke from the exhaust, and sometimes you can hear popping noises in the exhaust manifold. The reason is flashes of unburnt fuel.
Consumption increases noticeably.
Leaks may be observed at the joints of the fuel pipes, and a strong gasoline smell may be felt.

Practical experience shows that a lack of fuel mixture occurs more often than an excess. That is, the most common problem with an RTD is the draining of gasoline into the return pipe and tank.

Injector repair kit.

Injector repair kit.
Injector repair kit.

Already in the mode of cranking the crankshaft with the starter, the injection pump creates a starting pressure of 350–400 bar. At minimum idle speed - up to 500-600 bar, and at maximum load - up to 1300-1500 bar. There are pumps with pressure up to 2000 bar. Its value is set by a regulator located on the fuel injection pump housing or on the ramp and subordinate to the electronic engine control unit. When issuing commands, the ECU relies on signals from the rail pressure sensor.

Fuel is supplied through high-pressure pipes to the injectors, which open under the influence of an electrical signal. There are two design options - electromagnetic or with a piezoelectric element. The first one was not particularly fast at first, which forced the designers to look for an alternative. In a piezo injector, voltage is applied to a piezo crystal, which instantly expands. The spool compresses the spring, the injector needle opens the way for fuel - and it is injected into the combustion chamber. However, designers continue to improve electromagnetic devices - both options work successfully on modern engines.

Causes and Troubleshooting

If the above symptoms are detected, you should check the performance of the RTD using one of the suggested methods:

measure the pressure in the fuel rail, its value should be at least 3 bar;
find the return hose and carefully press it with pliers while the engine is running;
disconnect the vacuum pipe leading from the manifold from the regulator.

The most reliable way is to measure with a pressure gauge. The device is connected to the fitting on the fuel rail, the test is performed with the engine running. If the pressure is below 3 Bar, additionally check the fuel pump - the unit may have lost performance. For diagnostics, you will need a tee with a pressure gauge embedded in the supply line. If the pump produces 3 Bar or more, change the RTD.

The reasons for valve failure look like this:

the spring has lost its elasticity and allows the membrane to bypass fuel at low pressure;
pollution with low-quality gasoline;
rod jamming.

Due to the design features (the element body is rolled), repairing the fuel pressure regulator is impossible in most cases; the part will have to be replaced. The option of washing and blowing helps only with blockages inside the element.

Squeezing the return line is done at idle speed of the engine, preferably “cold”. If engine performance stabilizes, there is a problem with the RTD or pump. To determine the “culprit”, you will still need to measure the supply pressure. Try removing the vacuum tube from the manifold at higher speeds - if the valve has become unusable, the behavior of the power unit will not change.

Possible solutions

A good starting point is always to check the technical service bulletins (TSB) for your specific vehicle. Your problem may be a known problem with a known fix released by the manufacturer and could save you time and money during diagnosis.

A diagnostic scan tool, digital volt/ohm meter (DVOM), a suitable fuel pressure sensor, and the manufacturer's service manual (or equivalent) will be helpful in diagnosing the P0088 code.

NOTE. Use caution when using the fuel pressure gauge under the hood of your vehicle. Fuel is under high pressure, and fuel that comes into contact with hot surfaces or an exposed spark may ignite and cause serious injury.

I like to start with a visual inspection of the system wiring and connectors. Pay special attention to the harnesses and components at the top of the engine. The warmth and ease of access associated with this area make it popular with pests who tend to damage the system's wiring and connectors. If necessary, repair or replace defective or damaged wiring and/or connectors. At this time I would also check the battery voltage, battery cable connections, and alternator output.

If intake manifold vacuum is used to control or monitor fuel rail pressure, the intake manifold vacuum must be sufficient to perform this task. Consult your manufacturer's service manual to find out the acceptable vacuum specifications for your vehicle and make sure your engine meets them.

Check the fuel system pressure using a pressure gauge. Consult your service manual for the exact fuel pressure specifications that apply to your vehicle. Follow the manufacturer's recommendations for using the pressure gauge.

If the actual fuel pressure is higher than that recommended by the manufacturer, a faulty fuel pressure regulator may be suspected. If the fuel pressure is within specification, suspect a faulty fuel rail pressure sensor or fuel rail pressure sensor circuit.

Follow the manufacturer's instructions for testing the fuel rail pressure sensor and circuits with the DVOM. Disconnect controllers from the circuit before testing with DVOM.

Additional diagnostic tips and notes:

The fuel rail and associated components are under high pressure. Use caution when removing the fuel pressure sensor or fuel pressure regulator.
The fuel pressure check should be performed with the ignition key turned off and the engine off (KOEO).
Turn off the ignition to connect/disconnect the fuel pressure sensor.