Fuel Pressure drops to 0 after engine dies

[RustyDusty]

I recently checked the fuel pressure on my 1987 ford ranger and had a question for y'all. First I checked if it held pressure with engine off after priming it. After 10 minutes the needle hadn't moved so I started it up and it held steady at 32 PSI while idling. I unplugged the fuel pressure regulator and pressure went up to 40 PSI. I turned off the engine and fuel pressure stayed around 32 PSI. I started it again and this time gave it some gas and the engine died (I'm hunting down some issues) but after the engine died I watched the needle go all the way down to 0 in about 5 or so seconds. Is this normal or is this indicative of an issue? Might my stalling issue be a fuel pressure related issue?

Thanks!

 

[Uncle Gump]

What you described sounds like maybe an intermittent fuel pump operation.

 

[RonD]

The computer runs the Fuel Pump Relay in the engine bay, its the GREEN one, check its pins and slots for corrosion

When you turn on the key the computer will activate this relay for 2 seconds, and 2 seconds only, but it will do this EACH TIME you cycle the key off and on again

The computer will not turn on the relay full time until RPMs are above 400, cranking speed is 200rpms, just FYI
So that how it works

IF..............your engine is stalling with key on then when RPMs drop low the computer WILL shut off the relay, but injectors stay on, so pressure will drop as engine stalls out

When you turn off the key, relay and injectors go off at the same time so pressure holds

To test if pump might be at fault, hold engine at say 2,000rpms steady, and watch the pressure, should hold steady as well, shouldn't be slowly dropping, that would mean a dirty filter or weak pump

IAC Valve is what the computer uses to set idle RPMs, and computer reacts very fast to keep idle RPMs at its target level
So it could be the IAC Valve is sticking, so computer can't control idle levels
Easy to clean

You can ONLY replace Ranger IAC Valve with a motorcraft or hitachi brand IAC Valve, other brands will cause what you described, also wandering idle
Ford computer needs a true solenoid IAC Valve, other brands use a stepper motor valve
Nothing wrong with stepper motor valves they just don't work well with Ford's PWM solenoid control
Many car makers use stepper type, they just don't crossover too well, lol

 

[RustyDusty]

The computer runs the Fuel Pump Relay in the engine bay, its the GREEN one, check its pins and slots for corrosion

When you turn on the key the computer will activate this relay for 2 seconds, and 2 seconds only, but it will do this EACH TIME you cycle the key off and on again

The computer will not turn on the relay full time until RPMs are above 400, cranking speed is 200rpms, just FYI
So that how it works

IF..............your engine is stalling with key on then when RPMs drop low the computer WILL shut off the relay, but injectors stay on, so pressure will drop as engine stalls out

When you turn off the key, relay and injectors go off at the same time so pressure holds

To test if pump might be at fault, hold engine at say 2,000rpms steady, and watch the pressure, should hold steady as well, shouldn't be slowly dropping, that would mean a dirty filter or weak pump

IAC Valve is what the computer uses to set idle RPMs, and computer reacts very fast to keep idle RPMs at its target level
So it could be the IAC Valve is sticking, so computer can't control idle levels
Easy to clean

You can ONLY replace Ranger IAC Valve with a motorcraft or hitachi brand IAC Valve, other brands will cause what you described, also wandering idle
Ford computer needs a true solenoid IAC Valve, other brands use a stepper motor valve
Nothing wrong with stepper motor valves they just don't work well with Ford's PWM solenoid control
Many car makers use stepper type, they just don't crossover too well, lol

That's really great info! I'll do some further testing with that in mind.