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6 votes

P0171 and Fuel Trims

Great question, you supplied enough data to give direction and this is a common problem. This pattern of fuel trim behavior, higher positive at idle than at cruise speeds, is commonly seen when the ...
Fred Wilson's user avatar
  • 11.6k
2 votes

What does the ECU use the long term fuel trim for?

This isn't a 100% verified answer yet. I will search for more direct sources when I have time. Pᴀᴜʟsᴛᴇʀ2's comment got me thinking about the obvious question: If it didn't need it for something, ...
Jason C's user avatar
  • 4,890
2 votes

Very high short term fuel trim

The post o2 generally does not effect performance. It's rather more a gauge of how well your catalytic converter is functioning. In a perfect world Stoichiometry is what the secondary is trying to ...
cloudnyn3's user avatar
  • 3,343
1 vote

Should the PCV breather hose affect fuel trims?

With the pipe disconnected and open to the air (not plugged), as you say, unmetered air will be getting in, bypassing the air flow meter. Therefore the mixture calculated by the engine ECU using ...
HandyHowie's user avatar
  • 25.4k
1 vote

Cleaned MAF sensor and long term fuel trim dropped to -10%

Your engine is not running rich, a negative fuel trim means that the ECU has reduced the fuel in the fuel-air mix to keep it from being rich. If your MAF was dirty and under-reporting air flow before ...
GdD's user avatar
  • 17k
1 vote

Are mineral oil smoke test machines sensor safe?

If you apply the smoke to the pipes and sensors for extended periods of time (weeks or months) then most likely yes. However, for a 1 off short period to find a fault probably not - this does assume ...
Solar Mike's user avatar
  • 34.6k
1 vote

Why does my LTFT1 make wilder, faster changes than STFT1?

Long term fuel trims are stored values in this model. These are stored for various engine load ranges. They are changed when the short term trims exceed predetermined limits, then the new values are ...
Fred Wilson's user avatar
  • 11.6k
1 vote

Can fuel trim (LTFT) be too high or too low? Not using stock injectors

By using this fuel injector calculator, your new injectors are flowing 24.2 lb/hr @ 49psi, which is a bit more than stock (by 1.0 lb/hr). This could definitely be why you're seeing LTFT levels being ...
Pᴀᴜʟsᴛᴇʀ2's user avatar
1 vote

Help on diagnosing positive Long term fuel trim

+- 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. ...
vini_i's user avatar
  • 16k
1 vote

High O2 voltage paired with high LTFT reading

The O2 reading is odd given the LTFT, but if it were me I would start with the obvious basics... The KISS rule is and always will be in full effect. Check for vaccuum leaks. Even small ones. That ...
SteveRacer's user avatar
  • 12.4k
1 vote

P0172 System Too Rich, while fuel consumption is too low

The reason was a leaky nozzle. It went well after changing the nozzle on the misfiring cylinder. I think that the fuel enhance actually worsened or caused this situation. I used it because I drove ...
Aus's user avatar
  • 276
1 vote

What is the the base fuel calculation in a fuel injected vehicle based on?

This is for the 80's-90's Dodge ECU. A bit out of date, and for a MAP (not MAF) system, but has some good info. It will vary by manufacturer, but ...
rpmerf's user avatar
  • 8,403

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