Let's get technical: Tuning the ECU for mpg

[DrivenByNothing]

My approach to better fuel economy has been mainly to increase fuel efficiency.

Enough background, I'll get down to the meat.

Ok, basically your ECU is watching you. Well, not you, but your driving via sensors. It's taking in information and making educated guesses based on it.

I've been researching Nissan ECU's, but I'll try to keep this as broad as I can.

The main things your ECU is looking at to decide how much fuel to inject are a) RPM b) load. Load is calculated from sensor input (ie O2, MAF, MAP, etc.) Depending on engine conditions, the ECU will run in either open or closed loop mode.

Open loop is entered at WOT and at warm-up. Open loop uses fuel and timing values found in 'maps' in the ECU's memory. For a given load and rpm, the ECU decides the pulse width frequency of the injectors. There are only so many programmed rpm and load values, so when the actual values fall somewhere in between, the ECU's processor interpolates to find a happy medium between two known values. Open loop is more or less static (ie the ECU doesn't care how rich/lean the mixture is because it's not taking input from the O2 sensor.)

Closed loop is your friend. This is the mode in which the ECU utilizes the O2 sensors' voltage(s) to make fuel and timing decisions. Similar to the open loop, there are 'maps' for the ECU to read from for given rpm and load values. However, these values are 'difference' values rather than 'absolute' values. Difference values tell the ECU how much to alter the current setting and in which direction (positive or negative).

Closed loop sounds great, right? Well, it has potential. Ok, that's sort of a pun (voltage=potential difference). 99.999% of EFI cars use a narrow-band O2 sensor. The voltage ranges between 0 and 1 volt and it's probably the least accurate sensor on your car (outside of the 'No Oil Pressure' light I had on my Nissan or a 'Your engine is a pile of molten metal, pull over.' light.) In reality, the sensor is there to protect the engine from running lean. The values aren't accurate enough to clearly separate lean and rich. If it runs way rich, the ECU will usually cut back fuel until it's lean again, richen, lean, etc.

So what's the solution? Wideband. Much more accurate, but also more expensive. VW has begun using wideband O2 sensors in their vehicles. I don't believe they've matched the wideband's tech with the ECU's programming just yet, but it's definitely a step in the right direction.