Checking for misfires... P0300

[ferocity02]

Ever since the cam swap I have been getting a P0300 code at idle only. If I idle for long enough it will set the MIL, but most of the time it stays pending because I don't idle for long periods of time.

I have also swapped PCM's but that was years ago and never got a P0300 until the cam swap. I have tried to do the CASE learn but it never works. I think it might be because my speedo will climb even when revving in neutral... And with the T56 swap it thinks it's in gear all the time.

I have changed the plugs, wires, coils, and checked for vacuum leaks.

The misfire PIDs show that it's misfiring about evenly on all cylinders at idle only. Off idle it does not misfire at all according to the misfire PIDs.

How can I use the scan tool and wideband to check for actual misfiring? Would it be an obvious drop in AFR? My idle AFR jumps around quite a bit. I'm idling at 700 RPM and 21 deg timing.

Since it's cammed, do I need to adjust the misfire tables? The cam is pretty small, 212/212 0.561/0.561 114LSA.

Thanks!

[Chevy366]

Mine did the same, just bumped up the idle, if it idles to low it will throw P0300.

[ferocity02]

Mine did the same, just bumped up the idle, if it idles to low it will throw P0300.

Stock idle is around 550 RPM, it didn't like that with the cam. It will idle fine at 650, but I bumped it up to 700 for the manual trans.

I guess I want to know for sure that it's not actually misfiring. And if it's not, should I adjust the misfire tables to prevent P0300?

[joecar]

...

I guess I want to know for sure that it's not actually misfiring. And if it's not, should I adjust the misfire tables to prevent P0300?Yes, correct.

[gmh308]

As Joecar mentions, you can adjust the misfire tables, or alternately increase the minimum rpm for the misfire test to run. The misfire tables are calibrated by GM with the stock engine in the stock car with the stock trans, so any changes to anything and they wont be right again.

[ferocity02]

As Joecar mentions, you can adjust the misfire tables, or alternately increase the minimum rpm for the misfire test to run. The misfire tables are calibrated by GM with the stock engine in the stock car with the stock trans, so any changes to anything and they wont be right again.

That makes sense. Is there a good way to use the scan tool to check for misfires besides the misfire PIDs? I want to make sure its not actaully misfiring before I adjust the tables.

[gmh308]

That makes sense. Is there a good way to use the scan tool to check for misfires besides the misfire PIDs? I want to make sure its not actaully misfiring before I adjust the tables.

From one perspective, it technically is misfiring. Especially at idle. Thats why in the carb days plugs got fouled when idled for too long with a strong cam. Misfires.

Not sure whether in the freeze frame data it actually gives the event time that the misfire was defined and logged. Worth a look.

Misfire testing is there to protect cats from cooking mainly. EPA mandated IIRC. Once you have dropped a strong cam in, the emissions statement has mostly been made. :)

[joecar]

+1 as gmh308/Ian said, the misfire tables/algorithms take into account all the "crankshaft" rotating mass (from pulley/damper to flexplate/converter/clutch)...


i.e. a CASE relearn is required when components are mix/matched/replaced.

[ferocity02]

+1 as gmh308/Ian said, the misfire tables/algorithms take into account all the "crankshaft" rotating mass (from pulley/damper to flexplate/converter/clutch)...

i.e. a CASE relearn is required when components are mix/matched/replaced.

Makes sense. A CASE relearn is out of the question for my setup at this time. For some reason VSS will increase when I rev in neutral, it gets up to ~5mph or so. Also my clutch switch isn't hooked into the PCM so it thinks I'm in gear. One or both of these is making it so the CASE learn won't run.

[joecar]

You might be getting noise into the VSS circuit.

You will need clutch switch (and various other inputs) to get the CASE relearn to run.

[Chevy366]

Every time you flash the misfire will return for a duration until idle is relearned, at least that has been my experience, usually takes about 4 or 5 drive cycles.
You do realize Top Fuelers idle at 2,500 to 3,500RPM, not saying to idle that fast, just a tidbit.