Discussion and questions related to the course Understand AFR
This is my first post here on the forum. Glad to be part of it!
I'm a technician that manages a fleet of cars that we track, including a few Mitsubishi Evo X's. We currently have an Evo that is displaying 90-100% injector duty cycle when boost levels reach roughly 18 lbs (24.5 lbs peak). The car was professionally tuned on Cobb and injector duty cycles never reached anything north of 85%. We can't get much info since we currently do not have a wideband/gauge (I know, it's absolutely necessary but we haven't had the time for the install). We did go through some datalogs to monitor what the STFT and LTFT were doing. Nothing looked out of the ordinary when we reviewed them. What else is a cause of this sudden change in injector duty cycle?
Anything that reduces the fuel flow (dirty injectors, clogged fuel filter, failing fuel pump) would lead the closed loop fueling to try an increase the injector pulse width (and thus duty cycle) to compensate. Check the injector wiring to make sure you don't have an intermittent injector connection.
Non-fuel related, a misfire can be detected as a lean condition and more fuel might be requested. So a bad coil or coil wiring.
You say you don't have a wideband fitted, but yet the ECU has Fuel Trims, so it must have a wideband sensor. Perhaps you just need to modify your logging setup to show you the lambda (or the inverse -- equivalence ratio) value.
If the boost has increased or the ambient air temperature has decreased since it was tuned, airflow could be higher, requiring more fuel.