Electronic Solutions for Motorsport

High Short Term Fuel Trim – What is it? What does it do?

2 min read
Engine ECU datalog

STFTs stand for “Short Term Fuel Trim”. This is the adjustment the engine ECU is having to make to achieve the AFR (Air Fuel Ratio) targets set in the engine map.

If the numbers are +, the engine ECU is having to add more fuel. If the numbers are negative, the engine ECU is having to take away fuel.

On a standard, unmodified engine we would expect these fuel trims to be close to zero. And on a properly tuned car that was not experiencing issues, we would want the STFTs to be close to zero also.

I would say if we are seeing more than a 5% adjustment on the long term or short term fuel trims (STFT), it could be indicating a problem with the engine. In other words, in my opinion, the short term fuel trim normal range should be less than 5% either way, ideally and I’d say for sure less than 10% either way.

Potential Causes of High Short Term Fuel Trims

  • Vacuum leaks, positive trim
  • boost leaks, negative trim
  • exhaust leaks before lambda sensor, either
  • problem with fuel pressure, either
  • problem with an injector, either
  • problem with intake, either
  • problem with lambda sensor, either
  • problem with MAF sensor, either
  • problem with IAT, either
  • problem with coolant temp sensor, either
  • problem with MAP sensor, either.

That an idea of the causes, there are more. If you have an issue, hopefully it’s something obvious of a fault code gives you a clue where to start.

Basically anything that could lead the engine to believe the amount of air entering the engine is different to the amount of air actually entering the engine. And also anything that could lead the engine to believe the amount of fuel it is putting into the combustion chamber is different from the amount of fuel actually entering the combustion chamber.

This is for a stock engine.

STFTs – Modified Engine

If we are looking at a modified engine then any part we changed that affects fuel or air could be playing a part. Naturally, if we have just changed a part and our fuel trims change, that is a good place to start looking more closely.

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