Intermittent dip in fuel pressure

2007 Nissan Versa DOHC

To help diagnose an intermittent prob with a weak idle as well as a high fuel trim readings, I attached a fuel pressure meter to the fuel line, and mounted the meter on my windshield. The car has no leakdown pressure problem and generally runs at a normal 50psi.

However, while driving, the pressure occasionally dips to approx 47psi for, say, less than 10 seconds, then recovers. During a 30-minute drive, I observed this once while stopped at a red light (and it seemed to cause rough idle) and once while driving at approx 25mph, with no performance issue. I never have a problem starting the car.

(1) Are small dips like this enough to explain my very high fuel trim (short + long >20 recently) as well as the occasional rough idle at stoplights?

(2) Would such small dips be a pump issue, pressure regulator issue, or perhaps something else?

If fuel rail pressure is too low, that could result in a positive fuel trim. Computer would determine it had to pulse injectors with longer intervals (that it otherwise would for the given intake airflow) to satisfy O2 sensor. So maybe.

Another theory, when the engine’s intake manifold vacuum changes, the fuel rail pressure has to change accordingly, to maintain the same injector flow. Injector flow is proportional to fuel rail pressure - intake manifold pressure. Maybe that is what you are seeing to explain the fuel-rail pressure changes. . The problem w/this theory is that the fuel rail pressure would more likely be briefly going up rather than briefly going down; for example when you were accelerating the intake manifold pressure raises, which would cause the fuel rail pressure to rise.

Do you know how the fuel rail pressure vs intake vacuum is determined on your car? On my Corolla there’s a fuel pressure regulator at the end of the fuel rail, and it has a vacuum hose input. Newer cars, this is often done by the fuel pump itself. Not sure how the fuel pump determines the intake manifold vacuum on that configuration.

1 Like

The only fuel pressure regulator I know of on this car is attached to the pump in the gas tank, and it doesn’t seem to have any “smart” operation (though I don’t know that definitively). I have the pressure meter attached right before the fuel rail. The pressure seems to stay a steady 50, even when accelerating, other than the occasional seemingly random dips to 47. The dips don’t happen during acceleration, though. They seem to happen when driving a steady speed or when idling at a stoplight.

I’m guessing my best bet is to replace the pump/regulator assembly in case something’s wonky with either.

This might be helpful

1 Like

Hi George–thanks for article. It’s a bit hard for me to understand it, but it seems that the function of the fuel pressure regulator is to guard against overpressure. If I’m experiencing occasional underpressure, then perhaps that indicates a pump weakness problem?

Concur, article doesn’t clearly address the fuel rail pressure vs intake manifold pressure requirement for configurations (returnless, etc) where the fuel pump is the only gadget that regulates the fuel pressure. Maybe somebody here knows how that is done. Or maybe on those configurations the fuel rail pressure is constant all the time, independent of intake manifold vacuum, & the powertrain computer adjusts the injector pulse width as a function of intake manifold pressure (which it knows via the MAP sensor).