Mityvac MV5545 User Manual download pdf (Page 14)

Fig. 21

Fig. 22

Page Number - 14 Form 824127

Types of Fuel Delivery Systems

For the purpose of accurately applying and testing with the FST Pro,

today’s fuel delivery systems can be divided into three typical

configurations:

• Return (Bypass) Fuel Systems

• Returnless Fuel Systems – Mechanically Regulated

•

Returnless Fuel Systems – Electronically Regulated

The fundamental differences between return and returnless fuel

systems affect the values for pressure and flow displayed by the FST,

and influence its diagnostic capabilities. Even the differences between

mechanically and electronically regulated returnless systems produce

ubstantially varying results. The key to achieving the most accurate

fuel system diagnosis begins with a basic understanding of the differ-

ences between fuel systems, and how the FST responds to each.

Return Fuel Systems

In a return type of fuel system, a continuous volume of fuel is pumped

to the engine via the fuel supply line. The engine uses what it needs,

and the rest is returned back to the tank via the fuel return line

(Fig. 21). The fuel pump receives power directly from the electrical

system. Pump speed is not controlled by any external source, so

when the system is functioning properly, it outputs a constant flow.

Fuel pressure in a return system is created by a pressure regulator

mounted at the exit point of the fuel rail or fuel return line. As the

unused fuel leaves the fuel rail, it passes through the regulator, which

restricts the volume of the returning fuel (see Fuel System

Components/Pressure Regulator). This causes backpressure to build

up in the fuel rail and fuel supply line, all the way back to the fuel

pump. This “backpressure” is the pressure at which fuel is delivered

to the injectors, and what is referred to by the vehicle manufacturer’s

specification for proper fuel pressure.

An inline filter is installed in the fuel supply line between the fuel pump

and the fuel rail to filter out impurities before they reach the injectors.

It’s typically mounted under the frame or in the engine bay, making it

relatively easy to replace.

A properly functioning return type system will always deliver a much

greater volume of fuel to the engine than what it will require, even

under a heavy load or a wide-open throttle (WOT) condition. A

normally functioning return fuel system will continuously circulate

around .5 gallon (2 liters) of gas per minute at normal operating

pressure. Ultimately, the fuel system designer determines what the

flow will be based on the engine requirements, but it will typically

vary between the .3 and .7 GPM depending on engine size.

Due to its layout, and the location of critical components, return

systems are the most easily and accurately diagnosed by the FST.

When installed and operated as recommended, the FST can measure

four (4) critical values that combine to precisely diagnose the fuel

system and pinpoint any malfunctions.

Returnless Fuel Systems

The term “returnless” describes fuel delivery systems that do not

return unused fuel back to the tank once it has entered the fuel rail.

The fuel rail becomes the end of the line, where fuel pumped from the

tank remains under pressure until it’s used by the engine (Fig. 22).

Pressure Test Port

Pressure

Regulator

Fuel Return

Line

Fuel Rail

Fuel Filter

Fuel

Tank

Fuel Supply Line

Inlet Strainer

Pump Module

Pressure Regulator

Fuel Rail

Fuel Filter

Fuel

Tank

Fuel Supply Line

Inlet Strainer

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Mityvac MV5545 User Manual Page 14

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