About Fuel Pump Overflow Valve Knowledge

fuel pump overflow valve blog dieselo

Fuel Pump Return Overflow Valve is an important part of the fuel injection system. The overflow valve is used majorly in the Vp44 fuel pump where it is used to transfer excess fuel back to the fuel tank. 

A return valve can generally last more than 5-7 years. But it can go faulty due to clogging or leak/fail due to excess wear and tear. This can greatly reduce both engine performance and efficiency. Which is why it is important to know
how to check and replace the fuel pump return overflow valve. 

What is a Fuel Pump Return Overflow Valve? 

fuel pump return overflow valve 1

The fuel pump return overflow valve is an important part of the mechanically injected diesel fuel pump. It is located at one side of the pump which works similarly to the modern-day pressure regulator. 

The overflow valve consists of a ball and a spring that holds the pressure inside the injection pump. If the overflow valve fails, the pressure inside the pump would not be maintained. This affects the fuel supply to the injector which in turn affects engine idling, fuel efficiency, power output, and a lot of other things. 

How does the Fuel Overflow Valve Work? 

The working of the Fuel Pump Return Overflow Valve is simple. As the name sounds, its job is to return excess fuel back to the fuel tank. The fuel lift pump is used to transfer the fuel from the tank to the injectors. 

Since the lift pumps are operated by the camshaft, the amount of fuel supplied is determined by the engine speed. But the quantity of the fuel supplied is not done accurately and excess fuel is transferred. It is the Fuel Overflow Valve job to return the excess fuel transferred back to the tank.

The overflow valve or check valve, as mentioned, has a metal ball over a spring. As the pressure increases the metal ball is pushed back and excess fuel is exited through a hole at the end of the valve (as in the image).

fuel pump overflow valve blog dieselo
When the pressure is released, the spring being a counterbalancer pushes the ball back blocking the overflow valve.  

How to know if the Fuel Overflow Valve is bad? 

A faulty Fuel Overflow valve has more than one symptom. Due to regular compression, the metal ball and the spring can both get compressed over time. This makes the valve lose its ability to hold till proper pressure is achieved before opening. 

Due to the valve opening early, the proper amount of fuel is not sent to the injector. Hence the engine itself doesn’t receive enough fuel to operate. At first, this can cause reduced performance at higher RPMs. 

However, if this is prolonged, then:

  • The engine would face trouble staying idle. 
  • The engine would show instability at lower RPM. 
  • Engine power output would be reduced greatly. 

And if the overflow valve is clogged, the excess fuel would not be able to return to the fuel tank. The injector would pump more fuel than required which would cause bad combustion and reduce the fuel efficiency greatly. 

How to Check / Replace Fuel Pump Return Overflow Valve        

fuel pump return overflow valve 3

The Fuel Pump Overflow Valve sits in a banjo bolt housing. This is located either behind the fuel pump or right next to the fuel supply from the fuel line. The valve has no replaceable or serviceable part and is replaced as a whole unit. You can easily buy it for $30- $40 from any store. The valve also has two gaskets to seal it. You should always replace the gaskets even if you don’t replace the Overflow valve.  

To replace the overflow valve all you need is a wrench. A 17 MM wrench is enough to do the trick. 

  • Start by locating the fuel overflow valve and cleaning the area around it. 
  • Then use the wrench to unscrew the banjo bolt. Obviously, ensure that the vehicle is turned off. Things can get a bit oily so keeping a cloth would be helpful.

  • fuel pump return overflow valve 4

  • After removing the banjo bolt out, take out the old overflow valve.  
  • Now in the same fitting, you can install the new valve. Don’t forget to put the gasket on.  
  • Now screw the banjo bolt with the new overflow valve back to the pump. Tighten it to 24 lb. ft of torque.  
  • Start your engine, let it idle for a bit to see for any type of oil leak. If not, drive a bit to take a feel.  

Overflow Valve Diagnosis And Testing

Diagnosing an overflow valve needs a blowgun so proper pressure can be applied to see if it’s functioning. The Overflow valve opens at about 14 Psi. 

  • Start by removing the Overflow valve and cleaning it. 
  • Now set the blowgun pressure to 14 psi and blow the air through the inlet of the valve. 
  • At this pressure, the valve should open letting air out through the hole at the back. Again, reduce the pressure to about 12 psi and see if the air is blocked. Repeat it again at 10 Psi. 
  • If that is the case then the Overflow valve is working perfectly.
  • If the overflow valve passes the air despite the pressure difference, then it has failed. The reason could be the spring or the ball but the reason is wear and tear.
  • If it doesn’t pass the air despite any pressure change, then the valve is clogged. Disassembling and cleaning it should make it work again.
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