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The fuel rail should always have pressure. Even when the engine is not running. A leaking fuel injector can cause hard or rough starts, rough idle, and plug fouling, fuel in the oil and reduced gas mileage.

A fuel pressure gauge is needed to test for pressure drop. If no drop is found, then a leaking injector is most likely not your issue. If you do see a pressure drop, then leakage should be suspected. Leaks can be internal like injectors, or external like o-rings, fittings, hoses or the fuel pressure regulator. Clamping the fuel return line might be diagnostic in determining if the regulator is leaking down.

The fuel pressure should remain constant for several minutes.
If the pressure drops, try cleaning the injectors with a fuel additive and/or injector cleaner that attaches right to the fuel rail. If an injector is suspected of leaking, and cleaning does not help, it should be replaced.

Some large auto parts stores may have a tool rental program available if you do not own a fuel pressure gauge.



The gauge will thread onto the fuel rail, and allow fuel pressure to be read by the gauge. Be sure you know the pressure specification for you vehicle.

http://www.fordforumsonline.com/forum/engine-drivetrain/308-howto-check-fuel-injector.html
http://www.fordforumsonline.com/forum/engine-drivetrain/269-fuel-injection-service-not-just-cleaning.html
 

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Just a little bit to add here Electro. You can have a pressure drop off through the fuel pump as well if the valve in it goes bad. You need to be able to distinguish which is which. It is usually the timing of the leak down. If it is fast then you most likely are looking at injector leaks. If it is very slow you might look at the pump. A gauge in the line that can isolate the pump would be good for that.
 
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