Case study...GM Tech 2 misfire graph

[Logan]

Some of this is for Fisher...as I know he wants to buy one. A GM Tech 2.

Anyway....victim is 2009 Chevrolet Traverse....getting a misfire at just the right load at highway speed. 115k miles. 

So far...and to my surprise...no P0300 code.

Likely a coil going bad...or maybe a bad plug...but where to look?

Picture says it all. GM Tech 2 misfire graph. Better go check cylinder #2....Intake has to come off to get to #2....pretty easy to do....but not as easy as fixing 1,3,5...but now I know.

 

[BodybyFisher]

Coil on plug. You are right about the coil.  I did a search and coil and coils getting wet is a potential issue.  It is interesting how a couple of these owners complained of misfires but no check engine light similar to your experience. The misfire CEL threshhold must be high.  The dealer wouldn't proceed without a check engine light. That has to be frustrating for owners who experience the problem.  It should set a P302. See this link,

http://m.carcomplaints.com/Chevrolet/Traverse/2009/engine/spark_plug_coil_failure.shtml

Given the "Cycles of Misfire Data" that the Tech2 shows of 2013 counts, I have surmised that each hashmark equals 400 counts and  the total misfire count shown for all 6 cylinders at this moment is 2500?, with #2 showing 1700?   I took 32 and divided it by 8 graph segments to net 4 and given the Misfire data of 2013, assumed the graph is x100.  Am I interpreting that correct?

I am surprised to see misfire counts showing in #1, #5 and #3.  Plugs?  At 115k have they been changed?  

The screen provides a lot of useful data.

Thanks @Logan

PS, there is another P300 2001 coil thread I wanted you to look at, I used the @ sign here to alert you, if you don't see it Ill tag you again, thx

 

 

[Logan]

Original plugs.

It's normal to see one-sees and two-sees misfires on the other cylinders.....#2 is the obvious problem.

The graph is only 32 across. So each box is only 4 misses.

Car has to be going over 65....in lockup....going up a slight grade to get the momentary misfire/chuggle.

Had not heard of the water drip coil problem. These coils are dry.

 

[BodybyFisher]

Thanks

[Cadillac Jim]

The miss counter uses the expected crankshaft speed so a consistent miss on one cylinder will show occasional other cylinders as having a miss or two.  But, if the car has over 100,000 miles on it with the original plugs, there is likely a real miss here and there on any given cylinder.  They will likely all have a miss count of zero after new plugs are installed.

[Logan]

Didn't touch the plugs.

New coil on #2 solved the skipping problem. New coil was labeled ACDelco...original labeled Bosch.

Anyway....15 mile highway drive...only one misfire registered....nothing to worry about.  Ignore one-sees and two-sees..

[BodybyFisher]

Very cool Logan, thanks

So the misfire is reported by the diagnostic system  as small misfires other cylinders?  This feels like a harmonic of the actual misfire being reported but not actual misfires on the other cylinders.  Is that whats happening?

[Logan]

#2 had a obvious problem. Was it causing the other small reported misfires? I doubt it.

Every engine naturally will have some misfires. You just don't notice them. Think lawn mower...you hear them all the time.

Over 10-15 miles....ones...twos...three misfires register on other cylinders you just ignore.

 

[BodybyFisher]

I had not noticed that slight misfires occurred,  although you are correct, if you listen at the tail pipes you do hear random anomalies, an occasional puff or slight miss. I used to blame that on ignition wire crosstalk, but it could be anything.    It seems that the Tech2 has terrific resolution to be able to pick up such small misfires, compared to my Autotap that does not seem to be as sensitive. 

I just noticed that the graph after replacing the #2 coil showed that the other cylinders that were experiencing a small level of misfire seemed to clear up also.