Fuel economy gone down

[00pathyse]

Thanks for the reply. I went for a drive with nds3 app. The 07 must have the 2.2v ones because that's what I was seeing most of the time while driving. AF alpha b1 and b2 were both showing 100 most of the time as well. They would bounce up and down a little while accelerating and decelerating but quickly went back to 100 (90-100, 100-120). I did notice wb sns 1 and 2 would both jump to 5v on occasion I think when I let off the gas and was coasting. Does all that sound reasonable? Thanks.

[VStar650CL]

Yep, the voltage will always spike low when you give it sudden pedal and then high when you drop pedal. The ECM is doing both on purpose, opening the injectors wide to act like an accelerator pump on a sudden opening of the butterfly, then closing them off to prevent backfire when the butterfly closes suddenly. You won't see either effect if you work the pedal smoothly. Here's a waveform of normal A/F behavior I swiped off the net (credit Samarins):

[VStar650CL]

PS - A/F-alpha is Nissan's name for STFT (Short Term Fuel Trim). The 100% is perfect. Past 125% (+25%) you get lean (P0171/P0174) codes or past 75% (-25%) you get rich (P0172/P0175) codes. Anything within +/-10% is considered normal by Nissan.

[00pathyse]

Awesome, thanks very much. That helps a lot. If the injectors were messed up or the fuel pump would that show up on then a/f data? The car runs great.

[VStar650CL]

Well, actually, I didn't explain that very well. On most scanners STFT will show up as a +/- percentage, but on a Nissan scanner it shows up as a percentage applied to 100%, i.e., 75% (-25%) to 125% (+25%).

There, that's better.

[VStar650CL]

The fuel pump might not show up much unless the pressure was way off, but you'd definitely see some bad alpha if the injectors were misbehaving.

[00pathyse]

[00pathyse]

I'm stumped. I'm starting to think my mileage is just what I get.

[VStar650CL]

It may be something mechanical in the engine, like a slight compression loss in old age, or even something unrelated to the engine like drag in the drivetrain. When the engine parameters all show up healthy, you're pretty much left with external factors. As Sherlock Holmes once posited, "When you've eliminated the impossible, whatever remains, no matter how improbable, must be the truth."

[00pathyse]

Lol, I like the comparison!

[00pathyse]

Is the reading of 54 for the Afr heater correct. Should that value be 100 or does that fluctuate also depending on the conditions?

[VStar650CL]

Yep, that's a PWM percentage which the ECM drives according to the resistance of the heater element. The ECM has a measuring circuit (dropping resistor) for each heater, so it knows exactly how much current (and therefore wattage) is passing through each heating element. It would be throwing codes if anything had an issue.

[00pathyse]

Cool. What would the value of 100 mean for the htr?

[VStar650CL]

PWM is basically an on-off percentage, i.e., 75 would mean three quarters of the time "on" and one quarter "off". Depending on the firmware, some heaters will go 100% for a short time at startup to warm the sensors in a hurry.

[00pathyse]

Happy to say highway mileage seems to be much improved! Along with your advice, I also made the switch to Chevron gas, and ran a tank of 94 octane. No idea of that would help. And Italian tune up.

[VStar650CL]