Scanning for misfire - unexplained anomaly occurs

I've been trying to troubleshoot a random misfire. In the process i've been learning about and applying the use of a scanner by autoenginuity. I plugged it in to my 2005 Chevy Suburban and started watching live data on a graph with several sensors being monitored. Everything seemed to be normal and then out of nowhere a little blip occurred and it would occur every so often across all sensors. It would occur intermittently, and sometimes as close as 15 sec apart. All readings at an exact moment would drop to a reading of zero for about a millisecond. This must be the cause of the misfire. Is this perhaps a bad ECU or an ECU that needs to be reprogrammed? It doesn't make any sense.

I doubt an '05 is updating several PIDs once per mS or better. Do these drop outs coincide with noticeable misfires or is it just something you're seeing on the screen?

Hi andyru,

I would scope not only the blip, but its relation with the cause of misfire.

Said this. I would record that blip at the same time is changing injection pulse width or secondary ignition waveform (which usually are causes of misfire).

If you can't notice any change, i'd say is not related.

Here is a snapshot of monitoring intake manifold absolute pressure and coolant temp with engine not running. The same thing occurs with the vehicle running as well. Also, this occurs with 02 sensors and fuel trim readings or any reading at all. For checking injector pulse width and ignition waveforms are you saying to do that with an oscope? I don't have one but will look into getting that test performed.

Yes, i mean a scopemeter.

I don't know the scanner you are using. I my case, i am not able to observe pulse by pulse through my scanner, but by my scopemeter.

You will need at least two channel's. One for any pcm input signal (let's say MAP for instance) and the other for injection pulse width.

andyru wrote: Here is a snapshot of monitoring intake manifold absolute pressure and coolant temp with engine not running.

I've seen the exact same phenomenon when using my Launch Pro Mini to view Global OBD data on similar year GM's. OEM data works perfectly. Do you have another scanner to try?

I don't have another scanner I own personally, but will try to get a hold of another one to try. Thanks for the reply!

When I think I may be dealing with sensor dropout, the scope is the next step.

Meanwhile. There are some tests you can do without scope.

If misfire is intermitent in a single cylinder, you can detect which cylinder is misfiring by unpluging injector by injector and detecting any change inside this new "misfiring frequency".

I mean, if there is no change in frequency when you unplug an specific cylinder, that cylinder is suspicious. On the other hand, if you notice an intermitent misfire when you have a cylinder unpluged, that means the unpluged cylinder is not the cause of misfire.

Does it make sense?

andyru wrote: I don't have another scanner I own personally, but will try to get a hold of another one to try. Thanks for the reply!

If you have the GM enhanced add on for AE you could try looking at the OEM data with it.

Also take a look in Mode $06 to see if it has misfire counters. Random misfires sometimes aren't random.

So I’ve got the enhanced add on for AE and almost all of the misfiring is coming from cylinder 2. But for some reason when a trouble is thrown it usually says random misfire or misfire from cylinder 1. I’ve checked the spark plugs, spark wires, coil packs. The long term fuel trim on bank 1 is about -6% to -9% and bank 2 is closer to 0%. I decided to run a fuel injector test anyway. Everything checked out ok (although I’m not sure how accurate that test is). All the data on the AE doesn’t show anything really suspicious accept that intermittent glitch. But, it doesn’t coincide with the misfiring according to the AE scanner.