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My vehicle (Ford Fiesta 2014 Petrol) always have a positive long term fuel trim, which means its running lean mostly. The log data that I recorded using torque pro can be downloaded here : Torque Pro Log

LTFT always quite unstable, and swing from around 5% to 15% on lower rpm (<2000 rpm). However on higher rpm ( > 2000 rpm) it tends to stable around 5%.

First O2 sensor looks to be working normally, fluctuating from 0 - 0.9v, and second O2 sensor tends to stable around 0.65v, and occationally drop to low number for a while when I let go of the pedal before recover to normal voltage.

So I am a bit worried on the constant lean. Is it a vacuum leak?

No error code has been generated, but I couldn't trust the error code as well, as my vehicle had run without thermostat before , and coolant temp staying <= 70 C max all the time for a year, and ECU never gives any error code.

Many thanks

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    Can you provide a hot idle log, a constant speed log and a WOT 2nd to 3rd gear log? You don’t need to log the gps stuff limit it to iat, tps, stft, ltft, b1s1, maf, load and rpm please. What units are you logging in? kpa? G/s? c?
    – Ben
    Commented Feb 6, 2018 at 2:45

2 Answers 2

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+- 5% for LTFT is ordinary. +- 15% is more concerning but still not detrimental.

If you see +-30% then you should worry.

That said, the smaller the problem the harder it is going to be finding it. Since the long-term fuel trim seems to go high at idle, a vacuum leak is the thing I would suspect.

You have to do this carefully. Using either brake parts cleaner (the flammable kind) or propane (put a tube on a small torch) spray around the engine and engine compartment where you have any vacuum lines or the intake manifold. Watch the STFT while you do this. If you can graph the STFT even better. As soon as you hit the leak the STFT will go negative and in a hurry. Then replace what you find is wrong.

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The mechanic which works on my car happens to find it when cleaning the throttle body. He said the nuts of the ducting of throttle body not being tighten on last throttle body cleaning!. After the nuts were tighten, the LTFT now 0 or very very close to 0.

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