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Loosing fuel prime, looking for advice


Brandon.S

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Losing fuel prime, looking for advice

My 94 4.0 loses fuel prime over night. I ran a fuel pressure test and when the engine was turned off the pressure dropped fairly slowly, only about 3 psi in 3 minutes.

Because the fuel pressure dropped slowly, I didn't expect a fuel pressure regulator issue-- in addition there is no fuel in the vacuum line. Also, because the truck started fine when warm, I didn't expect a leaking injector.

However, I replaced the fuel pump assembly and filter and the problem persists.

Looking for advice as what to look for next.
 
Last edited:


RonD

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After engine is warmed up, let it sit for 15minutes or so.
Turn key on, press gas pedal to the floor and hold it down all the way
Try to start, leave gas pedal down all the way.
If engine fires at all then you have a leaky fuel injector.
If it doesn't then replace FPR.


All fuel injection engines have a "Clear Flooded Engine" routine.
This is it:
Key on, computer on, engine off
TPS(throttle position sensor) above 4.5volts, gas pedal to the floor
Computer enters "Clear Flooded Engine" mode
Fuel injector pulses are shut off, spark stays on

As soon as you release gas pedal injectors will start, even while cranking engine.

Assuming there are no fuel line leaks, you don't smell gas, then there are only 3 exits for fuel pressure, fuel pump check valve(in fuel pump), fuel injectors, or FPR(fuel pressure regulator)

We will assume new fuel pump has a good check valve, although that could be wrong these days, lol.
If after letting engine sit for a few minutes which would allow a leaking injector to place some fuel in the intake, if you crank engine in "Clear Flooded Engine" mode and it doesn't fire then an injector is not leaking.

So must be FPR, you can remove Return line from FPR and then cycle key on and off, if fuel comes out of return line hook up then FPR is leaking pressure via Return line.
FPR is set for 40psi so fuel will come out if you get close to that.
 
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