What causes the FPR to fail?

[The Fred]

I wonder how the FPR works,

Does it clog up over time or does it have some sort of pressurized seal in there ... or..

[Ranger]

There is a rubber diaphram in it with manifold vacuum on the top half and fuel pressure on the bottom. When the diaphram ruptures, it leaks fuel into the vacuum side which goes straight to the intake manifold unmetered and causes a rich fuel condition.

[Cadillac Jim]

Ranger tells us how it fails. My uninformed guess as to how it works is this:

• The rubber diaphragm separates manifold vacuum and fuel rail gas to the injectors.
• The rubber diaphragm moves a spring-loaded piston.
• At a preset pressure between manifold vacuum and injector fuel pressure, the piston is just opening up an orifice to the fuel pump line that lets gas into the injector side of the fuel rail.

This keeps a constant pressure differential between the fuel at the injectors and the vacuum, so that the same pulse widths to the injector solenoids always give the same amount of fuel.

The diaphragm and piston are constantly moving small amounts as the vacuum changes and the fuel flows. When the car is shut down or you coast down a slope, the piston moves farther, opening the aperture on shutdown or closing the aperture to coast. In time -- a lot of time -- the diaphragm will eventually crack.

[BodybyFisher]

I also believe that while it cracks it can also simply leak past the seal, leaking the fuel rail pressure down, causing long cranking. It can leak past the seal in either direction as the seal wears.

[The Fred]

It ought to screw down into there so you could replace it without dismantling things. I'm probably wrong though.