I checked the fuel trim of my car the short term is negative like -3% and when the rpm is about 2000 it will be negative - 13%. The long term does not move is positive +0.78%. What could cause this condition
The car has an issue. Each first start up in the morning the car has long crank time and after that it catch but it goes slowly to fast idle
December, 9, 2013 AT 10:13 PM
That's typical of the fuel pressure bleeding down over time. That can be caused by a leaking pressure regulator, a leaking check valve in the fuel pump, or, most commonly, a leaking injector. Regardless of the cause, it isn't a serious problem as much as it is aggravating. To work around that, most cars run the fuel pump for about one or two seconds when you turn on the ignition switch to get the pressure up for starting in case it did bleed down. Then the pump shuts off as a fire safety measure, and will only start up again once you're cranking the engine. If the pressure bled down completely, that might not be long enough to get the pressure up. It will slowly build while you're cranking the engine until it reaches the pressure where the fuel will spray from the injectors.
There's two things you can try. One is to pause for a couple of seconds in the "run" position with the ignition switch before you crank the engine. The other is to switch it to "run, then back off, then back to "run, and crank the engine. That will give the pump twice as long to build fuel pressure. You can also verify if this is what's happening by connecting a fuel pressure gauge so you can see what the pressure is after a few hours.
The fuel trim numbers you mentioned show that the system is working properly. The Engine Computer starts out with factory-programmed fuel metering values called "look-up tables", then, as it sees the exhaust is too rich or too lean, it makes adjustments called the "short-term fuel trims", (STFT). The computer can modify the fuel metering by about plus or minus ten percent. Yours is subtracting three percent.
When the computer sees that it is always adding or subtracting the same amount of fuel during the same set of conditions, like air temperature, rate of throttle change, engine load, etc, it moves those corrected numbers into the "long-term fuel trims", (LTFT) and runs off those numbers instead of what was programmed in as a starting point at the factory. All of those fuel trim numbers are lost when you disconnect the battery, but the tables start to be rebuilt as soon as you start to drive the car again, and they're so close that you rarely notice a performance issue. In your case it's adding less than one percent fuel, so the mixture is real close to what was designed-in at the factory. That number changes very little over time. Also, what you're seeing is an average. Fuel is being added only under some conditions and may be being subtracted under others.