Fuel Trims – Is It A Necessary Element For Getting The Right Tune For Startup Fuel

Startup Fuel Trimming For Cars That Are Modified

This video is all about getting a basic fuel trim down on a car. In this example, we’re using a 2002 WRX, with an STI 2.5L block, 20G turbo, front mount intercooler and 1000cc injectors.

Originally, the car’s owner was sent a base map just to get going, since 1000cc injectors were unknown at the time. Now, the EFI readings show that the car actually runs lean on idle (higher than the ideal 14.7 air-to-fuel ratio). Checking the Air Fuel Learning, the system is already adding lots of fuel, but can’t even keep it idling.

Usually, this means we might be off with injector latency, but since we’re off across the board, it’s probably not just the injector latency, but also the fuel scale. Mass air flow (MAF) calibration may also be off.

To solve the problem in this example, we’ll add some fuel by changing the injector scale. We know we’re off by about 15% across the board, so we’ll increase the real-time fuel injector scale table by at least 15%. In this case, we’ll increase it by 20%. Do that by hitting the “m” key (for “multiply”) and enter “1.2” (without the quotes). The idle immediately gets smoother, but the screen shows we’re still running lean.

Next step, put in some load and see if it’s still running lean at part-throttle as we are on idle, to see if we need to worry about our injectors more, or the latency. We get the following results:

• Up to 3,000rpm: A correction of -10% at mass air flow value of 47g/sec, which is at the higher end of our A/F learning.
• Down to 2,500rpm: Correction of -6 to -10% at mass air flow of 33g/sec. So the 20% we added was probably a little bit too much for the fuel injector scale.
• Bring addition down to 10%: When we hit 40g/sec of air flow, we’re close. We’re just on the lean side. But on 35g/sec, we’re REALLY lean, which is unusual (it shouldn’t go down that much).
• Down to 2,000rpm: Still 15-16% lean.
• At 20g/sec air flow, still 20% lean

This information tells us that we need some latency tweaks. We need to flash the ECU for this:

• Bump latency up by 15% – brings up to 25% enrichment, but still higher than 14.7 A/F ratio on idle. Bring mass air flow down a little bit.
• Save map
• Disconnect ECU. Turn car off. Connect test connectors and flash connectors
• Flash ECU
• Start car back up after 5-10 seconds after reflash
• Connect to ECU. You’ll be loading the stock map, so you’ll have to re-upload the map you just saved.

Now, the re-flashed ECU is running very close to the ideal 14.7 air-to-fuel ratio. To hit the ideal, make further adjustments to the injector size and MAF values.


Now as you see fuel trims can be necessary for everyday tuning.

Now if you are new to tuning engine management systems and discover the 5 steps to planning a tuning strategy for your vehicle.

If you have done some tuning but liked to know more get the training that is showing you how to tune from start-up to finish.

1 Comment

  • Faiza

    Reply Reply February 20, 2016

    Thank you for your reply. I’ve checked the ineoctjr using Noid Light, unfortunately there’s no sign of pulsation. Do you think there might be something wrong with the wire or? electronic parts that goes to the ineoctjr? I’ve been reading about Relay and Transistor but I don’t know where to locate them in my car. By the way, I have Hyundai Coupe 1993. Thank you so much for helping. Take care.

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