The pressure in the fuel rail drops quickly

The fuel line and the injector ramp that supplies the engine injectors operate under a pressure of about 3 bar. Since gasoline is supplied by an electric pump, the system uses a special valve that limits the fuel pressure. Otherwise, the nozzles will leak and the engine will choke on the over-enriched mixture. To avoid problems with fuel supply, you need to promptly diagnose the signs of a malfunction of the fuel pressure regulator (abbreviated as RTD) and know how to eliminate it.

Measuring pressure: several ways

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There are several ways to determine the pressure in the ramp. So, you can purchase special equipment that diagnoses fuel pressure. This is a kit that, in addition to the pressure gauge, includes an adapter and a drain. The price for such an assortment is only 1.5 thousand rubles, but there are so many benefits.

The second option is to go to a service station and ask the workers to measure the pressure. In Moscow, such diagnostics cost no more than 500 rubles.

The third option is to measure the pressure using a tire air pressure gauge. Its only drawback is that the initial values of the scale cannot be considered as accurate. As a rule, pressure gauges are equipped with a scale of up to 16-20 atm, and the measurement limit for fuel is only 5-7 atm. Measurements in this case may be inaccurate due to the error falling into the initial scale values. For this reason, it would be more correct to use a pressure gauge with a scale no higher than 7 atm.

Pressure is measured as follows:

take a tire pressure gauge;
a fuse is wound onto the nozzle and an oxygen tube with a diameter of 9 mm is inserted (securely secured with clamps);
Be sure to protect the generator with something (a damp cloth) so as not to cause a fire hazard;
provide access to the fuel frame fitting by unscrewing the internal nipple;
To prevent gasoline from splashing out, it is recommended to relieve the pressure in the system in advance by removing the required fuse;
Place the second end of the oxygen tube onto the ramp fitting and secure with a clamp;
Start the engine and take pressure readings.

2.8-3.2 atm is normal.

There are other ways to measure pressure. Here's one of them.

Provide access to the fuel pump module fitting. You will need to insert a tube with an internal diameter of 8 mm into the fitting;
The other end of the hose is put on the pressure gauge.
Take readings.

Take a pressure gauge with a hose from the compressor.
Unscrew the cap from the fitting to check the pressure in the fuel system.
Unscrew the spool valve from the fitting, doing this carefully so that the fuel does not splash too much (you can initially release the pressure).
Pull the pressure gauge hose with a clamp onto the fitting and tighten it with a screwdriver until it stops.
Start the engine and, looking at the pressure gauge scale, take readings.

Preparing for measurements

We string a hose onto the inlet fitting of the pressure gauge and secure it with a clamp - we must avoid pressure losses during measurements.

Then you need to remove the cap from the fuel rail. There will be a nipple under it - remove that too. Be careful - if you recently started the car, there will be residual pressure in the fuel system and gasoline may spill out, so have a cloth ready and try to protect your eyes.

When you have removed the plug and nipple, install the second end of the hose that goes to the pressure gauge. We also fix everything with clamps.

About pressure drop

When it comes to pressure problems, it looks something like this:

when the engine is running in idle mode, 2.8 atm is displayed, stable pressure, builds up quickly;
after turning off the engine, the pressure instantly drops to 2.5 atm;
after a minute or two it drops again to 2 atm;
after 10 minutes – 1.6 atm.

Why does the pressure drop? There are definitely problems in the fuel system, and, as a rule, this is due to the fact that the check valve of the pump does not hold. This is why the pressure drops so quickly after the ignition is turned off.

In extreme cases, it is possible that the DDT pressure sensor fails. Methods for checking both options are given below.

Important to remember

When you check the fuel pressure with the ignition on, note that after the ignition is turned off, the pressure in the rail drops to 0.7-1 atmosphere and remains at this level. If it drops to 0, the problem is in the fuel pressure regulator.

Try increasing the number of revolutions to 3000 - if the pressure gauge needle does not stay constant, but falls, this may mean that it is time to change the fuel pump

If the pressure builds up for a very long time or is lower than required, the fuel filter, fuel pump filter or fuel line may be clogged.

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Useful tips

These tips will come in handy when measuring blood pressure.

If the pump is working, then it makes sense to check it for normal pressure supply. You can do this, as mentioned above, by connecting a pressure gauge, or you can use another method. For example, measure the current at the pump terminals. It must be equal to the voltage of the machine's on-board network. In other words, 12 V for cars and 24 V for trucks.
It is always recommended to relieve the total system pressure before diagnosing pump or rail pressure. This helps release residual pressure, which improves accuracy. As mentioned above, it is reset by turning off the fuse and starting the engine for 3-5 minutes (this is enough for the pressure in the system to drop to 0).
In some cases, the fitting on the fuel frame may be missing and then measuring the pressure becomes more difficult. But there is another way out - connect the pressure gauge to the fuel hose through an adapter.
When measuring pressure, it is recommended to pay attention to the behavior of the pressure gauge needle when the internal combustion engine is operating in idle mode. The arrow should tremble a little, but not oscillate at full strength. The latter indicates a clogged coarse mesh or a malfunction of the DDT.

Video: low fuel pressure

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Replacing a faulty valve

The regulator to be replaced is located in the fuel block, which includes other elements: an electric fuel pump, a coarse filter and a Kalina fuel level sensor.
You will need to remove the entire unit from the tank and then change the regulator. To do this, relieve the pressure, as described above, and then remove the rear seat. The module is located under a hatch covered with a mat and sound insulation, which moves to the side. You need to unscrew the screws and remove the hatch, then disconnect the wires and fuel hoses.

The pressure ring holding the assembly is twisted to the left. You can move it from its place with light blows of a hammer on the protrusion through the adapter. Then the fuel module along with the float is carefully removed from the opening.

You need to remove the wire from the regulator installed in it and pull out the spring retainer with a Phillips screwdriver. After this, the valve can be easily removed and replaced with a new one. Before reinstalling, the O-rings of the new part must be lubricated with fresh engine oil.

When assembling the unit, it is important to monitor the condition of the gaskets and replace them if worn. When installing the fuel module into the opening, you must ensure its orientation; the arrow on the cover should point towards the trunk. At the end, it doesn’t hurt to check the pressure again, and only then you can screw in the spool and start the engine.

To be sure, the Lada Kalina should be tested in motion, making sure that the dynamic properties of the car are restored.

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.

How to measure the pressure in a car’s fuel rail yourself and why you need it

Checking the pressure in the fuel rail is a standard procedure in a long chain of diagnosing engine and fuel system malfunctions. Often this check is carried out in cases where the engine runs unevenly and there are dips during acceleration.

This is one of the mandatory check points if the car jerks during acceleration. I described this in the article “ Why does a car jerk during acceleration and acceleration?”

" Also, this diagnosis is performed in cases where the car has increased fuel consumption.

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.

Topic in the “Audi Repair” section, created by Audi RT, April 25, 2007.

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