Fuel Pressure Question

JustinHammonds · Oct 31, 2014

Okay so i have a 1999 4.0 ohv, lately it has been hard to start only when cold and has a rough idle, constantly between 700-1000, i did a fuel pressure test and i have 25psi ignition on and 60 running, would this be a bad pump problem?

I have already done every other test possible, no vacuum leaks, good plugs clean MAF etc.

Any help would be greatly appreciated

RonD · Oct 31, 2014

You should have 65-70psi on a '99

It shouldn't drop more than 10psi with key off, and it shouldn't continue to drop.

'99 only has one check valve that holds pressure after key is off, that is in the fuel pump in the tank.

You could have a leaking fuel injector, test for it this way:
warm engine
engine off
key on count to 3, key off, key on again(this primes fuel system twice)
press gas pedal to the floor and hold it down
Crank engine, it should NOT start, it should NOT fire at all, it should just crank
If it fires, tries to start, then you have a leaking injector and that's where fuel pressure is going.
If it doesn't fire then injectors are fine, check valve in fuel pump is going bad, it will get worse.

The above test is called Clear Flooded Engine routine, all fuel injected engines have this routine, not just Ford.
Key on
Gas pedal to the floor
Crank engine

Fuel injectors are shut off but spark stays on

When you release the gas pedal injectors will start again, even while cranking.

Until resolved turn key on count to 3, turn key off, repeat 3 times, this will build up pressure in the system for starting.
The computer only runs the fuel pump for 2 seconds when key is turned on, fuel pump won't come on again until rpms are above 500, so not until after startup, turning key off and on again gets another 2 seconds

JustinHammonds · Oct 31, 2014

okay i will try this out tomorrow at the shop, if it is the check valve or pump, should i just replace the entire fuel pump assembly?, not really worried about the cost, more worried about potential future problems

RonD · Oct 31, 2014

Check valve is in the pump, so yes, whole assembly needs to be replaced.
Kinda like the seal on a water pump, when it starts to leak out the hole you have to replace the whole pump.

JustinHammonds · Nov 9, 2014

did the gas pedal test and it didnt start, so the injectors are fine,
i replaced the fuel pump and now the truck runs like a champ, its crazy that just a few psi low caused that much of a problem.

RonD · Nov 10, 2014

Good work

Simkins12 · Jan 28, 2015

RonD,

I just did the clear flooded engine test and mine started. I'm guessing I have a leaking injector(s). If this is the case, is the best way to tell the leaking injector by a spark plug(s) that has a heavy carbon build-up?

If not, then what is the best way to find the leaking injector. Is there anything you would recommend, in addition to replacing the injector, when I do this or should just look at replacing all the injectors so I know I have new ones all of the same age?

The following are new: plugs (6 months); wires; fuel filter, fuel pump assembly (4 mos), fuel pressure regulator, fuel pump relay.

Thanks,

Fuel Pressure Question

JustinHammonds

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RonD

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