Hi everyone
Can someone please explain why the LONG-TERM fuel trim on my 09 fiat 500 Sport 1.2 NO MODS, starts with approx -100% and swings +100% when I increase the reves then when I take my foot off swings -100% so on and so on?
NO FAULT CODES.
kind regards and thanks in advance.
Sid

Is this some gadget like Eco-Drive or something?

If it's an indicator of likely fuel economy then it probably works off a vacuum sensor (the MAP sensor) in the inlet manifold.

If it's suddenly started doing this (what was it doing before?) then that sensor may need a clean. The inlet manifold usually contains an oily mist, especially if the oil level is at, or over, "MAX". Or the wiring connectors to it may be dirty, corroded or loose. The sensor works on small electrical signals, so any increased resistance (or failure of the signal) will be interpreted by the ECU as a reading that something strange is happening, when really it's just a bad signal.

If it's a new to you car and you just don't know what the +/- 100% means.. then I have no idea either..

But vacuum gauges generally.. giving the beast some throttle reduces the vacuum in the manifold (since air can enter unimpeded) so your manifold pressure shows momentary efficiency.. and so the "fuel consumption" meter swings to positive. No throttle coasting on a light load with maybe only a teeny bit of gas on... means the engine is trying to suck in air but the throttle is acting as an obstruction.. so you have greater vacuum (momentary inefficiency) so your fuel economy gauge swings to negative.


Ralf S.

Hi Ralf
Thanks for your help and advice,
check out my other post
Hi everyone, absolute throttle position issues help, please
that is a more in-depth explanation
kind regards
Sid