Mityvac MV5545 User Manual download pdf (Page 12)

In regards to return fuel systems, knowing the pressure and flow at

idle is a very good indication as to whether the fuel system is

functioning properly. However, while pressure can be compared to

a manufacturer’s spec, idle flow is not typically documented. Most

return fuel systems will flow around .5 gallons (2 liters) per minute at

dle. However, the idle flow may range from .3 to .7 GPM (1.1 to 2.6

LPM) depending on the vehicle make, model, year and engine. Under

a heavy load, a 5.0 liter engine may require as much .5 GPM (2 LPM),

while a 2.0 liter engine may only require .2 GPM (.75 LPM). Any return

fuel system flowing less than .3 GPM at idle should be considered

suspect for a malfunction. Due to flow variances, additional testing

described below will provide a more accurate diagnosis of fuel

system performance.

Using the FST to test a returnless system at idle will provide a

measurement for idle pressure. However flow at idle will provide no

additional insight as to its peak performance capability. Using the

patented flow control valve on the FST to execute simple pressure

and flow demand tests is required for an accurate diagnosis.

Simulating Engine Demand

At idle, an engine requires very little fuel, and places very little

demand on the fuel delivery system. A typical fuel pump is capable of

providing up to 30X more fuel than an engine requires at idle. Testing

a fuel delivery system at idle may be acceptable if the driver has no

intention of actually driving the car, but what happens when a driver

needs to accelerate or commit to a driving condition that places a

load on the engine?

The key to determining if a fuel delivery system can meet maximum

engine requirements is to test it under the engine’s most demanding

conditions. Using patented technology, the FST Pro is the only tester

capable of simulating demand on a vehicle’s fuel delivery system to

determine if it can meet the peak requirements of the engine.

The FST has a built-in flow control valve that enables an operator to

manually vary the flow restriction such that it simulates a car’s throttle

position from idle, all the way to wide-open. The flow control valve is

located on the side of the FST flowmeter assembly. Arrows on the

faceplate and rubber boot illustrate the three positions of the valve

as OPEN, CLOSED, and BYPASS.

When the valve is in the OPEN position pointing down, fuel flows

normally through the tester without affecting the normal operation of

the fuel delivery system (Fig. 18). On a return system,

.3 to .5 GPM of flow is typical, while a returnless system will indicated

little to no flow. Fuel pressure should read within the manufacture’s

specification regardless of the type of fuel system.

When the valve is rotated 90° to the CLOSED position, flow through

the FST is completed restricted (Fig. 19). On a return system or elec-

tronically regulated returnless system, this is referred to as “dead-

heading” the fuel pump. The flow will drop to zero, and the pressure

gauge will indicate the peak pressure output of the pump. Peak pres-

sure will typically be 50% to 100% higher than the normal operating

specification. This is termed the Peak Pressure Test, and

is effective only on return or electronically regulated returnless fuel

systems. Closing the valve on a mechanically regulated returnless

system will have no effect on fuel pressure.

CAUTION: A pump should never be dead-headed for longer than a

brief instant. Doing so can cause serious damage to the fuel system

and/or pump.

Page Number - 12 Form 824127

Fig. 18

Fig. 19

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Mityvac MV5545 User Manual Page 12

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