Mityvac MV5545 User Manual download pdf (Page 31)

ty, causing the float in the FST flowmeter to bounce up and down

during the peak flow and/or capability test. It can also cause the nee-

dle on the pressure gauge to bounce. In addition, as the pump tries to

pull gas through the clogged strainer, it creates a pressure drop that

may cause air bubbles to form and become visible.

hese indications of a clogged inlet strainer may or may not be evi-

dent during testing. If the results are inconclusive, and the fuel system

has an accessible inline filter, follow the procedure above to rule out a

clogged filter. If it is determined that the inline filter is not the problem,

replace the strainer or the pump module, and retest.

Fuel Pump/Electronic Control Failure

If testing with the FST indicates the engine is not receiving the proper

fuel pressure and volume, and a clogged inline filter or fuel line are

determined not to be the cause, then a failure of the electronic control

system or the fuel pump is the most likely cause. Follow the vehicle

manufacturer’s recommended procedure for troubleshooting these

components to determine the malfunction. Once repairs are com-

plete, retest the system for proper fuel delivery system performance.

Fuel Visualization

Visualizing the fuel as it passes through the flowmeter can provide

hints of fuel system problems or fuel contamination. A rapid drop in

pressure possibly caused by a clogged inlet strainer can cause the

fuel to boil, even at normal temperatures. Air released by the fuel

forms bubbles that are visible as they pass through the flowmeter. Air

bubbles can also be caused by a bad swirl pot or leaking fuel pickup

line in the tank, which allow air to be sucked into the fuel pump.

Performing flow demand tests can sometimes cause air to form in the

fuel lines, but air bubbles should never be present once the FST is

connected, air is bled off, and fuel is flowing normally to the engine.

Fuel contaminated with water may appear discolored or cloudy. Rust

or scale will typically be filtered out before reaching the tester, but

additives can cause discoloration. Don’t be fooled by gas that

appears normal. Many chemical impurities are not visible, but can

cause the engine to run poorly.

Pressure Leakdown Test

To facilitate hot starts, modern fuel delivery systems should maintain

some pressure for several hours after the engine is turned off. Once

the fuel pump is no longer operating, the pressure is trapped in the

fuel system by a check valve in the fuel pump, and closing of the

pressure regulator and fuel injectors.

With the FST connected inline, before the fuel rail, turn off the engine

and watch the pressure gauge. If the pressure drops more than 5 psi

in 5 minutes, a leak exists somewhere in the system.

Return Fuel Systems

1. Start the engine to build pressure.

2. Turn off the engine and immediately clamp off the FST outlet hose

running from the tester to the fuel rail. This will trap the pressure

gauge between the clamp and the fuel pump.

3. If the pressure gauge indicates a leak, check the fuel supply line

from the tank to the tester. If no leaks are evident, replace the fuel

pump due to a faulty check valve.

4. Remove the clamp from the fuel line, and restart the engine to

build pressure.

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