Been cleaning up some exhaust and boost leaks recently. Last night after I was finished and took the car for a drive I noticed that when coming to a stop my AFRs will read really rich (10.6) and then creep back up to stoich (14.7) at idle. Cursing doesn’t seem to be an issue as it’s within the 13.8-15 range, and idle seems perfect.
its only just before completely stopping that the wideband all of a sudden starts to read really rich then slowly creeps up to stoich. This will happen even without throttle, in example if I’m rolling down the street in neutral getting no reading (—) and then only just before coming to a complete stop does it start to read rich (10.6) and then after completely stopping it creeps back to normal.

Is this normal? I never noticed this prior to the leaks, but the car seems to be running fine 🤷

 

So I’ve also noticed that I’m running rich when shifting, going to start looking into the BPV, maybe there’s a leak there

 

I would say thats normal. If anything, its relearning the Fuel Trims, especially if you recently disconnected the battery. The afrs should never go full lean while coasting in neutral. The only time you should see that is when youre in decel, like when your completely off the throttle, coasting, but the car is still in gear (engine braking). That happens because the wheels are turning the transmission which is connected to the crankshaft, thus turning the engine over. When that is happening, the injectors will automatically shut off, hence the max lean reading. Just before coming to a stop, or upon going into neutral, the injectors will instantly re-activate and resume injecting fuel. Since the injectors are going from being completely off, to instantly on, there will be a period of time where too much fuel is being injected, mostly to keep the engine running without any disruption. Then the short term fuel trim will begin to add or pull fuel, based on what its seeing from the O2 and other sensors, in order to hit the 14.7afr idle target. The STFT doesnt work super fast because any major changes to the AFR will cause a noticeable change in engine smoothness, so they are designed to slowly walk the fuel ratio towards the target. Thats why youre seeing the AFRs go rich at first, then creep down to the proper mixture. You have three separate long term fuel trims that the ecu uses as reference when its trimming fuel, idle, cruise, wot. The ecu sets those values based on a cycle of time. So there can be times where your idle afr is more inconsistent than normal. Idle fuel trims specifically, are most sensitive to any alterations from the ecu. Thats why you arent noticing much difference during cruise. Also, the evo x uses a fly by wire throttle system, so instead of using a idle air control valve to regulate idle, it uses the throttle body solenoid and ignition timing to regulate idle. That tends to make the afr values you see on your gauge fluctuate without any real consistency. I hope my explanation makes sense. I probably couldve explained it more accurately, but i think i covered the main points. Peace.
-WILL