'15 Civic Si P0300
All this started after PCM replacement. Will add more at the end.
This civic runs awesome until something happens and it starts to misfire as if its being thrown into limp mode. Its all of a sudden you can restart vehicle while in motion and its fine until it happens again. I found that if i hold vehicle in the service bay a 3000 rpms it will start to misfire. What I have found is that the #1 injector is not firing when this happens. Im not sure if this is the cause of the result. Input on this would be awesome. Ive ran a relative compression test numerous times, ive checked the injectors, coils, and plugs. Ive performed multiple crank clear and relearns. It seems odd honda would only turn off one injector when all cylinders are flagging misfires. My brain is fried on this car and im in need of help to figure this one out.
The vehicle come in from the auction with no cats and no rear 02sensor with NO CEL. Customer (carlot) install a new manifold converter and drove 800 Miles the vehicle wouldn't ready the O2Sensor or the Cat monitors. We are in a inspection state. He leaves vehicle with me over night so i can inspect ecu data to figure out why it will not run monitors. Everything is in spec according to the service information, So a 40 mile test drive was performed, which confirmed it will not run monitors. I determined this was caused within the pcm software, i attempted to force flash over the files, but honda j2534 doesn't allow same calibration update. So a used computer was ordered, installed, and programmed to car. Vehicle was drove in a manor to set readiness monitors which resulted in the O2sensor readying up. But on the way back to the shop within a 10 mile test drive. The dash lit up like a christmas tree with various warning messages, a flashing CEL, and a felt misfire. This was VERY odd to me as it did not do this before, so the old ecu was installed and the problem went away confirmed by two 15 mile test drives. So a NEW ecu was ordered, because I know we have issues with used modules on newer cars. Just not enough documentation to confirm if it applies to this car or not. NEW ecu is installed and programmed according to SI. Which included a crank clear and relearn with Multiple tools. So i drove vehicle lightly which caused the vehicle to READY up ALL monitors. But when driven normal the vehicle will go into a limp type state.
It seems to me this car was on a modified ECU calibration. Your allowed to delete things like the rear o2 monitoring among other things. very Common. Another thing the tuner can do is turn off misfire detection, most people turn it off when they upgrade the flywheel with a lighter one. Light flywheels on these cars produce p0300 codes. Might be worth checking.
If the PCM was modified,then other modifications were probably done. Especially since it is an SI. One popular mod is to replace the OE flywheel with a lighter version. This will trigger misfire codes because the profile is different from what the PCM expects. The way around this is to disable the misfire monitor via aftermarket flashing. Check the flywheel.
Interesting! Out here they just lower it and add fart pipes. The kids are hi-tech in L.A. :-)
Will disabling the misfire detection affect the IM readiness monitors?
I dont think so, but am not 100% certain.
Inspect the engine harness, most likely was modified to aftermarket set up RYWIRE. sells complete harness that look like factory. does The orginal ecu has any sticker like, HONDATA, Neptune, Ktuner?Check new parts that are OE and not aftermarket crap, check connections at rear o2 and follow the wires
Thats old school. These newer Hondas do not need the harness changed and the ECU is flashed right through the OBD2 port. There is no longer a need to swap out the ECU, the factory ECU can be flashed to a performance ECU on the fly nowdays. The only way you can tell is by looking at the monitors status.
Car being from auction anything is possible engine swapped, harness swap, specially being the Si model. I know all Honda and acuras with K series engine all aftermarket performance programming could be done at obd port without swapping ecu, but they also offer complete ecus already programmed for racing application and engine swaps new and or old
Michael and Rusty both nailed this one. As soon as i read the post I knew this was the cause. It's the only thing that made sense at this point. A lightweight flywheel has been installed and with honda monitoring the crankshaft speed for misfires. The lightened flywheel changes the characteristics of the crankshaft speed.