No Fuel Pressure... What I'm going to do now?

No fuel pressure is a fairly common symptom on any gasoline engine, but it takes a bit of knowledge to decipher how to approach this problem. See how this knowledge may be able to help you, right here...

 

THEORY OF OPERATION

The following vehicle repair strategy is to be used when encountering a NO FUEL PRESSURE/VOLUME symptom. The fact that fuel pressure is not present does not automatically mean that the fuel pump is bad. The knowledge gained here will help in present and future system specially on newer variable speed fuel pumps that are controlled by the ECM.

FUEL SYSTEM OPERATION

Modern automotive fuel systems are composed of the following components.

• FUEL PUMP

• DEDICATED FUEL LINES

• FUEL FILTER

• FUEL PUMP RELAY

• FUEL PRESSURE REGULATOR

• RELATED ELECTRICAL WIRING

At present time there are 3 basic fuel delivery systems employed in automobiles, RETURNED FUEL SYSTEM, VARIABLE SPEED FUEL SYSTEM, and RETURNLESS FUEL SYSTEM. It is important to understand that fuel pressure and fuel volume are NOT one and the same. The ability of a fuel delivery system to maintain a specific pressure (in PSI) is an important factor when analyzing a fuel system. It should not be confused with volumetric or ability to deliver a specific amount (in Gal. per minute or Litters per minute) of fuel. As a rule of thumb, when a system can not maintain a specific pressure, the vehicle will probably run very bad or not run at all. On the other hand, it is quite common to see vehicles with good fuel pressure and have performance problems due to a lack of fuel volume. Good fuel volume is the system’s ability to maintain a specific quantity or volume regardless of vehicle operation. Such quantity or volume is measured in GALLONS or LITTERS per minute. The use of a fuel volume gauge is highly recommended to diagnose modern fuel systems. These units have a small specific gravity scale that tells the technician the amount of fuel flow going through it in gallons or Litters per minute.

• The RETURNED FUEL SYSTEM is the average system used on most older and late model cars. These systems use a fuel pump, filter, dedicated fuel lines, fuel pressure regulator, and return fuel lines. In this system excess fuel that is not needed by the engine is dumped back to the tank. The main drawback of this system is the heating of the returned fuel by the engine; causing excessive fuel vapor inside the tank. This is the main reason why this system is being phased out in favor of the returnless fuel system.

• The RETURNLESS fuel system is being employed in late model cars superceding the older RETURNED system. It consists of a fuel pump, in-tank fuel pressure regulator, filter, fuel lines and NO fuel return lines. Given the ever stricter emission control laws, the advantage of this system is that no heated fuel is returned back to the fuel tank. This system greatly reduces fuel vapors inside the fuel tank, making for an easier and cheaper to design EVAP system, thereby, reducing the amount of fuel vapors being released to the atmosphere. Sometimes the fuel regulation is accomplished by the use of electronics as explained next.

• The VARIABLE SPEED SYSTEM is basically an adaptation to either the RETURNED or the RETURN-LESS system. Fuel delivery is normally accomplished with a regular fuel pump that is being controlled by a fuel pump module or the ECM itself. The fuel pump module actually controls the available current going to the pump, therefore, varying the pump’s speed.

Two basic variations of this system are available the dual speed and the duty cycle controlled systems. The dual speed system has two voltage settings feeding the fuel pump. The system supplies full battery voltage (maximum current) to the fuel pump during high fuel demand conditions (as in cranking, warm up, and acceleration) and a lesser voltage (low current) being applied during less demanding engine conditions (as in cruising and deceleration). Duty cycle controlled means that the fuel pump itself is constantly being controlled by the fuel pump module or ECM, by way of a pulsating DC current. This is accomplished by applying a variable width square wave (duty cycle) to the fuel pump. In other words, by varying the ON-TIME pulse being applied to the fuel pump the pump’s rotational speed can be regulated. The higher the demand for fuel the longer the on-time and the lower the demand the lesser the on-time. Again, this system can be found with either the returned or the return-less fuel system. Some of the new systems actually have the fuel control module, fuel pump and fuel sending unit housed in the tank, thereby, doing away with the fuel pressure regulator all together. Not only is this system good for the environment, but also extends the life of the fuel pump itself.

FUEL SYSTEM TESTING

The following steps should be used to properly diagnose fuel system problems. These steps are arranged in a logical sequential order. Connect a fuel pressure/volume gauge to determine if there is a problem. Fuel problems are categorized into two possibilities, fuel pressure and fuel volume problems. Proceed to the correct testing procedures bellow according to which of the two specifications is at fault.

FUEL PRESSURE is the ability of a fuel system to maintain an adequate PSI value or pressure regardless of engine conditions. Fuel pressure specifications are widely available and fairly easy to find. It is important to remember that a specific amount of pressure can be maintained regardless of volume. In fact it is possible to have adequate pressure and no fuel going onto the engine. A good example of that is when the fuel pump is primed and dead-headed, by crimping the return line. (It is not recommended to dead-head the pump for a long period of time). In this case the fuel gauge will read maximum pressure without any fuel flow at all. Fuel pressure problems could be caused by the following:

1. Defective or weak fuel pump. (Fuel pumps can be checked by using current ramping techniques while they are in operation, as well as a fuel pressure/volumetric gauge).

2. Defective fuel pressure regulator. (Regulators can either fail shut causing excessive pressure or open up letting too much fuel go back to the tank through the return line. A punctured regulator will cause a rich condition).

3. Contact resistance in the fuel pump relay thereby, reducing available current going to the pump. (It is important to always measure the available voltage and current at the pump. Performing a voltage drop reading across the fuel pump relay is a good indication of the fuel pump relay’s general condition. placing a headlight in place of the fuel pump, while activating the fuel pump relay, will also reveal a lack of current problem. The brightly lit headlight will draw an adequate amount of current of 3 to 6 amps, pointing to good circuit conditions).

4. Kinked fuel lines. (This will cause either very low or very high fuel pressure, depending on where the lines are kinked).

NOTE: Always check fuel flow with the engine running. A running engine will have 14.2 volts available to the fuel pump as opposed to 12.6 volts. The difference in voltage between KOEO & KOER will skew off the readings making you assume that the fuel pump is actually defective.

NOTEIt is important to understand that a clogged fuel filter, in real life, may not cause fuel pressure problems unless it is totally shut. On the other hand, it will cause very low fuel volume or almost none at all.

FUEL VOLUME delivery is the ability of a fuel system to maintain an adequate amount or quantity of fuel going to the engine regardless of engine operating conditions. Fuel volume specifications are very hard to find and they are usually acquired by trial and testing the vehicles that the technician actually works on. A fuel volume gauge is a highly recommended tool when analyzing fuel systems. The gauge works by passing the fuel going into the injector rail through a glass tube with a small metal ball inside. That ball has a specific gravity (exact weight) and as the fuel moves up the glass tube so does the small ball. The exact amount of fuel flow being delivered by the fuel pump can be ascertained with such a gauge. Fuel volume problems could be caused by the following:

1. Clogged fuel filter. (A clogged filter will cause a low fuel flow condition. A reading of under 0.3 Gal. per minute is usually considered inadequate to run an engine).

2. Weak fuel pump. (Will cause low volume). Contact resistance in the fuel pump relay reduces the available voltage going to the pump. (Low voltage at the pump will cause low fuel volume, since the pump is rotating at a lower RPM than normal).

3. Defective fuel pressure regulator if stuck shut. (Will cause low or no volume, while the pressure will be very high).

4. Kinked fuel lines after the fuel rail. (Low volume and high pressure since fuel can not get through. If the kink is before the fuel inlet side, the result will be low volume with low or normal pressure). These steps should point you in the right direction and prevent you from replacing unnecessary parts.

NOTE: Some variable speed fuel pumps will have fuel flow reading lower than 0.3 gal. per minutes at idle. This is acceptable, since the demand for fuel at idle is quite low.

 

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