× A place to discuss hardware/software and diagnostic procedures

New to wave form diagnostics

  • KC8OYE
  • Topic Author
  • Offline
  • New Member
  • New Member
More
11 Jul 2021 17:11 #49956 by KC8OYE
New to wave form diagnostics was created by KC8OYE
I've watched Eric O. on YouTube use wave form diagnostics to pinpoint a problem.. I was hoping I could too.  I just bought a brand new digital storage o'scope (really for electronics work, not automotive) but since I have two inductive pickup probes.. i figured why not.  I've looked at these screen shots, but with no idea what I'm looking at.. the only thing that jumps out at me, is the #4 pulse is much shorter than the rest...
background: this is a 1994 CPI 4.3L.  I changed the spider injector, it ran like a swiss watch.  I drove it 10 minutes to lunch, shut it off.. had lunch.. came back to a crazy hard start, and when it did run, a very noticeable hard miss.  I would suspect it's the same cylinder every time because it's a very steady, rythmic miss.. it's not spitting and sputtering.
in the images, purple trace is the #1 cylinder.  yellow is from the ignition coil.  firing order is 1-6-5-4-3-2
 
Attachments:

Please Log in or Create an account to join the conversation.

  • Tyler
  • Tyler's Avatar
  • Offline
  • Moderator
  • Moderator
  • You don't scare me, I fix CHRYSLERS
More
11 Jul 2021 18:51 #49957 by Tyler
Replied by Tyler on topic New to wave form diagnostics
For our reference, was the miss taking place during your capture?

I'm not sure if your scope has an invert function or not, so I took one of your images and rotated/inverted for ease of viewing:

 

I would agree with your suspicions about #4. It's not the burn time that bothers me, but the sharp downward slope of the burn line. I usually see that kind of slope related to a short in the secondary circuit. Carbon track, coolant in the cylinder, like that.
 
Attachments:

Please Log in or Create an account to join the conversation.

  • KC8OYE
  • Topic Author
  • Offline
  • New Member
  • New Member
More
11 Jul 2021 18:52 - 11 Jul 2021 19:04 #49958 by KC8OYE
Replied by KC8OYE on topic New to wave form diagnostics
yes, the miss was present during these captures.

I'm not sure what you mean by the sharp downward slope..  as far as I can tell the only difference I see with #4 is short peak.

edit:
i went back and looked at the pics I have... i think i understand what you are referring to now..
i marked on this pic what I think you are talking about.
edit 2: well, I was going to.. but the website keeps saying "Mime Type %s not allowed"

 
Last edit: 11 Jul 2021 19:04 by KC8OYE.

Please Log in or Create an account to join the conversation.

  • Tyler
  • Tyler's Avatar
  • Offline
  • Moderator
  • Moderator
  • You don't scare me, I fix CHRYSLERS
More
11 Jul 2021 19:07 #49959 by Tyler
Replied by Tyler on topic New to wave form diagnostics
I'm probably way too far zoomed in at this point. :silly: But you can kinda see the downward slope of #4 compared to the others:

 

#4's burn line starts much higher up the spark line than the other cylinders. The burn lines for #3 and #2 are generally flat after the spark line.
Attachments:

Please Log in or Create an account to join the conversation.

  • Tyler
  • Tyler's Avatar
  • Offline
  • Moderator
  • Moderator
  • You don't scare me, I fix CHRYSLERS
More
11 Jul 2021 19:15 #49960 by Tyler
Replied by Tyler on topic New to wave form diagnostics
Mime Type % sounds like it doesn't like the file format? :huh: Haven't seen that error yet!

As an idea for your next round of testing, you could use one pickup on the #4 and the other on another cylinder. Preferably #5 or #3, as they're adjacent in the firing order, and make for easier captures. You'll lose some detail not being on the coil wire, but you'll be able to get very comparable readings from a suspected bad and a suspected good cylinder.

Please Log in or Create an account to join the conversation.

  • KC8OYE
  • Topic Author
  • Offline
  • New Member
  • New Member
More
11 Jul 2021 19:38 #49961 by KC8OYE
Replied by KC8OYE on topic New to wave form diagnostics
well, I did part of this.. I did put the coil pickup directly on the #4 wire.  for some reason it didn't save everything.. not sure why.
but I did get this one shot.
 
Attachments:

Please Log in or Create an account to join the conversation.

  • Not Allowed: to create new topic.
  • Not Allowed: to reply.
  • Not Allowed: to edit your message.
Time to create page: 0.791 seconds