How does the PCM know

[ViperGMC]

I am getting a P0304 & P0309 (#4 & #9 cylinder misfire). So how does the PCM know there is a misfire and which cylinder??? There is no feedback loop that at I am aware of. What is telling the PCM of a misfire? If the injector is not connected, you get an injector code, if the coil isn’t firing, you get an ignition coil code. There isn’t an O2 in each exhaust pipe, so it can’t tell. A lean condition can trigger it but how does is know that a particular cylinder is lean. In trouble shooting, it sure would help to know how the code is being triggered. If it is a random condition why does it always return 4 and 9 only?

 

[Tom F&L GoR]

It knows when TDC of cylinder 1 happens and measures the time until the next TDC of cylinder 1. Then it calculates which cylinder is misfiring by correlating the momentary change in rotational speed with where it is in the firing sequence.

 

[ViperGMC]

Hmmm . . . very interesting. Thanks for that insight. So what the PCM is saying is that it is not detecting any appreciable combustion contribution at the time that #4 cylinder should be firing. It doesn’t know why, just that there is no power at that time. Given the rings/valves are OK and there is spark, the only thing left is no gas. So if it reports #4 then 9# and then P0300 random cylinder misfire, then it could mean that the entire engine is too lean. The only thing contrary to that is the LTFT’s are way negative. I guess I will reset the adaptives and dump more fuel and see what happens. Thanks Tom

 

[Tom F&L GoR]

You are reminding me that some cars with newer (more sensitive) diagnostic systems had CELs with lightweight flywheels? Sound familiar?

Injectors can form deposits which simply affect spray pattern and not flow volume.

You can move the injectors around and see if the codes chase the injector or stay with the cylinder.

 

[ViperGMC]

Changed injectors, wires and plugs and it did not move, so that eliminates the easy stuff.