Engine Interesting observation...

Interesting observation...

Using Digimoto to capture some sensor data on my '03 Corolla...short term fuel trim has always been ranging around 0 to -5% (varies with O2 sensor input and fluctuates a lot) at idle. Long-term fuel trim verifies this long-standing *SLIGHTLY* rich mixture, as it is around -5% @ idle - been like this for a few months. The ECU is subtracting fuel to compensate for a slighly rich mixture - it's nothing major...+/- 10% is within normal limits...but still the mixture has been slightly rich for a few months.

I noticed yesterday that my air filter was just horrendously filthy (yeah, that's one maintenance item I don't check as frequently as everything else).

Anyhow...I went, got a new STP air filter from Autozone, replaced it, drove back home and hooked it up to Digimoto....and guess what, short term fuel trims were ranging from +2-7%, indicating it is now ADDING fuel to compensate for a lean mixture. It was like this throughout the rpm range.

Numerical evidence that a dirty air filter can make a difference....the ECU is now adding 2-12% more fuel, depending on conditions.

Trav

And this morning, I watched my long-term fuel trim jump from -5% to 0. I had the EVAP error codes pop up due to a loose fuel cap - so, after clearing the codes I played around for a bit with the Digimoto and while I was idling the engine, long-term fuel trim jumped from -5 to 0. Cool stuff! The only thing that's different is the new air filter.

Trav