redtan

iTrader: (5)

So using a predator handheld I saw that cylinder 4 was misfiring every once in a while, even tho there are no CEL codes at all. I checked the sparkplug and it was in great shape, but I changed it anyway.

So what else should I look for, and why is it that one scanner says I have a misfire but the CEL is not showing any codes?

Btw this is an LS2 gto.

NoGo

iTrader: (2)

Sometimes the cylinder-specific misfire gets the wrong cylinder on the misfire count. You may want to check the cylinder immediately prior to #4 in the firing order as well.

The SES may not set if the misfire count doesn't get high enough prior to the regular reset.

Other items that will cause a cylinder specific misfire: bad coil, bad plug wire, bad injector, broken valve spring, broken rocker, bent pushrod, boofed cylinder/piston.

Try swapping coils and wires, injectors around and see if the misfire follows the hardware.

redtan

iTrader: (5)

I did swap out the coil pack and wire to another cylinder and the problem remains on #4. I guess I will go ahead and see if I can swap out injectors.

Can you elaborate a little more on the SES light and how it tracks misfires (and why it's not showing up as a code)? Also, I dunno if I was clear enough in my first post, but these misfires happen very very randomly. There is not specific time they take place (high load, certain RPM, idle only etc) so I cannot replicate the problem. Even when taking the car at WOT there are no counts of misfire, mainly when under light throttle (which might be a coincidence since I drive lightly most of the time).

Wouldn't a mechanical problem (injector, rocker, pushrod) misfire all the time, not very randomly (and not at WOT)? Why would such a problem (besides a misfiring coil/plug) run smoothly 95% of the time and only misfire every once in a while?

NoGo

iTrader: (2)

Misfires are tracked by the PCM watching the rate of change of the reluctor wheel on the crank. When a misfire happens the rate of the reluctor wheel doesn't match what is anticipated. The PCM also has a cam sensor. So given those two sensors, it can give a pretty good estimation as to what cylinder the misfire is coming from.

The misfire code is only set if you get a certain amount of misfire counts.

It isn't a hard and fast rule, but typically when a misfire is intermittent like that it is ignition related.

You are correct, a mechanical problem usually creates a persistant misfire condition.

Are you actually feeling these misfires, or are they just showing up on the scanner? How modified is the car?

redtan

iTrader: (5)

I *think* I can feel/hear a misfire when idle, but I am not sure exactly and could be all in my head. A couple weeks ago I had a bad 02 sensor and had a P0300 code and I could tell that the car was misfiring, the idle was awful. Right now the idle is smooth with only a little bit of shaking, but again that is just how the LS2 is anyway.

I scan using a predator for misfire counts, and #4 sees random counts of 2, maybe 4 and then goes away after a few seconds. Minutes later I see 2 or more then goes away. I've seen these counts go as high as 8 for a split second, but does this mean that the counts are not 'high' enough for the pcm to sense a misfire and pop a code?

My car has LT headers, no cats, h-pipe and magnaflow mufflers. I did replace the spark plug and moved the coil pack/wire to another cylinder and the problem remained on #4.

Thanks for your help btw!

NoGo

iTrader: (2)

4 to 8 is not high enough to set a code

Although not ideal, it is not uncommon to have occassional low misfire counts on a modified car. The only odd thing about your situation is that it consistently shows up on #4.

The last thing easy thing you may want try is moving the injector. The second to last thing is check the rubber intake gasket around the #4 cylinder intake port....you may have a small leak there (long shot) that is only coming into play in high vacuum situations.

If you've checked everything for that cylinder (ignition, fuel, air) and you still can't find anything, then you may be chasing a ghost.

redtan

iTrader: (5)

Yeah that's what I plan on doing in the next day or two. I'm having some work done on my car by Don at Slowhawk Performance in a couple weeks, so I might have him look at it too if it doesn't go away. Is it harmful to the engine in the long run to have these low counts of misfire, especially if I go WOT and all that?

NoGo

iTrader: (2)

If it truly is misfiring, then it isn't great for the motor long-term and it is something that's you'll want to get fixed.

Don's been playing with these things for a while. He should be able to give you a better idea of what is going on when looking at the car in person.