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Discussion Starter · #1 · (Edited)
*The wiring may be different at the pump and the ASD Relay will be different year to year(84,85-87,88-90,91-up) but the test is basically the same.
*MPFI (multi-port fuel injection) works on pressure differential across the injectors.
*Since the injectors are exposed to manifold vacuum total fuel pressure is a combination of push(pressure) and pull(vacuum).
*When checking fuel pressure you need a fuel pressure gauge and a vacuum/pressure gauge.

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1)Static Pressure is max regulated rail pressure.
Key on/engine off with the ASD actuated fuel pressure should read 53-57 PSI.
If you do not have a scan tool you can manually actuate the ASD Relay on 84-90 vehicles.
A)Locate the underhood diagnostic connector.
B)With the key on/engine off ground the DB/YL wire in the diagnostic connector.
C)The ASD Relay will now be actuated and the fuel pump should run.
(Performing this will set a Code 42 in memory)
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D)"Deadhead the Pump" by momentarily pinching off the return line, this will show you the pumps maximum output.
Maximum output needs to be a minimum of 80 PSI and pressure needs to rise to max instantly.

2)Check for Fuel Rail Pressure Leakdown.
A)De-Actuate the ASD Relay by turning off the ignition key and removing the jumper wire from the diagnostic connector.
B)Fuel Pressure should remain at Static Pressure.(Normal Leakdown happens over a 15-20 minute period)
If pressure drops quickly either the fuel pump, fuel pressure regulator or injector(s) are at fault.
C)Re-Pressurize the fuel rail and as you deactivate the pump pinch off the return line, if pressure now holds replace the FPR.
(Always remove and check the FPR vacuum line for raw fuel, if fuel is found replace the FPR)
D)If pressure does not hold repeat the fuel rail pressurization and this time as you deactivate the pump pinch off the fuel inlet line.
If pressure now holds replace the fuel pump.
E)If pressure still does not hold with both lines pinched immediately after deactivation find and replace the leaking injector(s).

3)Key on/engine running.(connect fuel pressure gauge and tee the vacuum pressure gauge inline with the FPR(fuel pressure regulator)
With the engine running the injectors are subject to engine vacuum which is pulling on the injector so rail pressure does not need to be as high to maintain the static pressure.
If static pressure was 55 PSI and you start the car and have 18"HG(vacuum is measured in inches of mercury,HG) rail pressure will drop to 46 PSI.
2" HG = 1 PSI so 46 PSI rail pressure + 18"HG( 9 PSI) = 55 PSI.

4)Pressure under boost
Fuel pressure will increase 1 PSI over static pressure for every 1 lb of boost.
To check this the fuel pressure gauge needs to be routed to and taped to the windshield so you can drive the car.
If static pressure was 55 PSI @ 12 lbs of boost rail pressure will be 67 PSI.
*It is advisable to tape the fuel pressure gauge and vacuum/pressure gauge side by side on the windshield so you can watch rail pressure and vacuum/pressure rise and fall together.
*Never attempt to do this alone, always take along somebody to either drive or watch the gauges.

MANUALLY ACTUATING THE ASD RELAY
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