Form 824262
Page Number - 17
Returnless Fuel Delivery Systems
(Mechanically Regulated)
At this point, it is assumed that the FST has been properly installed
inline with the fuel delivery system as recommended, and that it has
been primed to ensure no leaks are present (See FST Pro Setup and
Installation). The following procedure will ensure the most effective
diagnostic use of the FST:
Test Procedure
Operational Test
1. Check the operating position of the flow control valve on the side of
the flowmeter to ensure the knob is in the OPEN position pointing
down (Fig. 22). This will allow normal fuel system operation.
2. Start the car and allow it to idle.
Cranking or starting the engine should activate the ECM’s electric
fuel pump controls to turn on and run the fuel pump. If the fuel
pump does not operate, refer to the vehicle service information
for electrical diagnosis and repair of the fuel pump and
associated controls.
After connecting the FST, it is normal to have air trapped in the fuel
lines and flowmeter. Cycling the ignition switch (or cranking the
engine briefly) with the flow control valve in the BYPASS
position can help purge air from the system. Once fuel fills the
flowmeter and flows through the bypass hose, return the valve to
the OPEN position. The bypass hose must be routed and secured
into an approved fuel container before operating the flow control
valve in BYPASS mode.
To perform an accurate diagnosis using the FST, the car must be
running in order to provide the correct operating voltage to the
fuel pump. Testing the fuel system by activating the fuel pump
using a scan tool, will cause the fuel pump to severely
under-perform.
3. If the vehicle utilizes a multiple speed fuel pump, use a scanner or
manufacturer’s recommended procedure to operate the pump at
the highest speed.
4. Note the fuel system pressure indicated on the pressure gauge,
and compare it with the vehicle manufacturer’s specification
(Fig. 23). If it is out of spec, this is an indication of a problem with
one or more components of the fuel system. However, performing
a Flow Demand Test and Peak Demand Test as outlined below,
are required to reach an accurate diagnosis of the fuel system and
to pinpoint the cause of the malfunction.
NOTE:
The flow of fuel passing through the tester represents only
what the engine is using at idle. It will be negligible and most likely
not even register on the flowmeter.
Flow Demand Test
5. With the car idling, rotate the flow control valve past the CLOSED
position to the BYPASS position pointing up (Fig. 24).
With the valve in the BYPASS position, the flow of fuel is routed
through the bypass port located above the valve, through the
bypass hose, and into the reservoir. All restriction to the flow of fuel
is removed. This allows the pump to output its maximum flow, the
value of which can be read on the flowmeter. The free flow output
of a typical fuel pump is between 2.5 and 4.0 LPM.
NOTE:
Turning the valve to the CLOSED or BYPASS position will
prevent fuel from flowing to the engine. If left in either of these posi-
tions for too long, the engine will stall. If this happens, simply return
the valve to the OPEN position and restart the vehicle.
Fig. 26
Fig. 25
Maximum
fuel volume
requirement
(Appendix A)
