Fuel pressure leak

[Speedwagon]

2002 4.0
After replacing plugs and wires, I've been getting idle misfires, but no codes. Decided to check fuel pressure, especially since it's been getting harder to start lately. If I turn on/off multiple times, it will start right up.

Hooked up the gauge today, and I get instant fuel pressure drop. If I turn it on(not running) about 4 times, the pressure builds up to around 45/50psi. But starts to drop instantly. Once I crank it over, the pressure is holding around 65psi, and only slightly dips when I throttle it.

1: Where is the fuel pressure regulator on these?

2: Does it seem like the pump or FPR at this point?

 

[Dirtman]

Short answer is, there is no fuel pressure regulator. Its is integrated into the fuel pump assembly so it is not an individually replaceable part and must be replaced as a unit.

Before you do that however check for a leaking fuel injector. Prime the fuel system by turning the key to RUN but do not start the truck, wait a few seconds and turn back to the OFF position. Wait a minute and do that again. Then hold the throttle pedal to the floor and attempt to start the truck. If it starts, or even stumbles, you have a leaking fuel injector. Holding the pedal to the floor disables the fuel injectors so the only way the engine could fire would be for the injector(s) to be leaking.

I've also seen many fuel pressure guages that simply never hold pressure so don't rule out a bad guage until you check it on another vehicle and verify the guage is not whats leaking.

However, In the case of a leaking injector you would expect to see a CEL with RICH codes, and in the case of a weak fuel pump or regulator you would expect to see a CEL with lean codes. If this started directly after replacing the plugs and wires I would carefully inspect the plugs, ensure proper gap, and test the new wires. New doesn't mean "Good" these days.

And finally anytime you mess with the fuel system it's basically worth just throwing a new fuel filter at it first. They are cheap and take 2 minutes to change so its one of those things thats simply worth replacing just "because".

 

[Speedwagon]

Short answer is, there is no fuel pressure regulator. Its is integrated into the fuel pump assembly so it is not an individually replaceable part and must be replaced as a unit.

Before you do that however check for a leaking fuel injector. Prime the fuel system by turning the key to RUN but do not start the truck, wait a few seconds and turn back to the OFF position. Wait a minute and do that again. Then hold the throttle pedal to the floor and attempt to start the truck. If it starts, or even stumbles, you have a leaking fuel injector. Holding the pedal to the floor disables the fuel injectors so the only way the engine could fire would be for the injector(s) to be leaking.

I've also seen many fuel pressure guages that simply never hold pressure so don't rule out a bad guage until you check it on another vehicle and verify the guage is not whats leaking.

However, In the case of a leaking injector you would expect to see a CEL with RICH codes, and in the case of a weak fuel pump or regulator you would expect to see a CEL with lean codes. If this started directly after replacing the plugs and wires I would carefully inspect the plugs, ensure proper gap, and test the new wires. New doesn't mean "Good" these days.

And finally anytime you mess with the fuel system it's basically worth just throwing a new fuel filter at it first. They are cheap and take 2 minutes to change so its one of those things thats simply worth replacing just "because".

I do have a second gauge (I own one, but no ford schrader adapter). I got the parts store rental one, because it has the right adapter. I can swap the two and check again.

I already replaced the fuel filter. It's a new to me truck, so that was on the short list to get done, along with plugs and wires. I messed up the threads on the plugs at first, so I ended up going back in and being extremely meticulous with re-doing them. I also tested against the cheap motorcraft copper plugs, with no change. One of the wires that I replaced came apart, so that went back for a return, and purchased a brand new Motorcraft wire set. No changes in the random misifres. But no CELs either.

And the plugs looked lean every time I pulled them out.

The drop in fuel pressure is insanely quick now. After running it for a few seconds and showing 65psi, it dropped back down to 0 within a minute or two. And does not restart right away, without several primes of the pump. I can't imagine an injector dropping pressure that quickly, but maybe?

edit to add: One time I went WOT to get onto the highway, and as I approached a high rpm, the truck stumbled and threw flashing CEL misfire codes. I don't normally go WOT, so this was the only time it did it.