2009 Chevy 1500 5.3L did i miss something???

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3 years 1 month ago #46885 by Mike T
Hey guys i just wanted to talk about what i found today and what yall think about it. I got in a job today it was a misfire. They already shot it with a parts gun. they said when they went to autozone the scanner said #4 cylinder miss fire so he did plugs wires. Then a new coil on #4. When the truck came in it had P0121- TP sensor, P0300-Misfire Detected, P0573-Brake Switch codes. On the scan tool it showed 48,750 misfires on Number #4, 200 on #8. So I got out the scope and started testing everything.. I need to get better at scoping anyway. Well I did a relative compression test and as far as I can tell its good. I tested injectors and spark on all the coils with my probe that i have to hold on there. And everything looked good. So then I went back to my other tools ... Fuel Pressure, injector pressure drop and then compression test. Turns out #4 compression was a little low 95 psi, still I didnt see the problem. So i checked the next easiest one to get to and it was #2 Turns out it only has 65psi. Of course I didnt trust it so I went back and forth and cleaned out my test hose and gauge. But it wasnt lying. So I got the scope back out! went back to doing a relative compression test I new what i was looking for and still couldnt find it with the scope lol.. did i miss something??? I will post a picture of the test in a minute off my phone. Has anyone else had a problem like this???

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3 years 1 month ago #46886 by Mike T
Here you go
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3 years 1 month ago #46887 by Matt T
I'm really not sure what settings you used to capture that waveform??

Anyways there do appear to be some repeating high and low points which suggest there is a compression problem.

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3 years 1 month ago #46889 by Mike T
I used my amp clamp inverted to make it go up set at 2v on 2s screen and my probe to mark a cylinder. The cylinder with the mark is #2 the problem child

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3 years 1 month ago #46893 by Mike T
This is all 8 cylinders. Firing order is
1-8-7-2-6-5-4-3
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3 years 1 month ago #46894 by Mike T
Here is a little better pic the one on the end does look low but I checked #4 and #2 on compression I didn’t check anything else.
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3 years 1 month ago #46899 by Tyler
Did you happen to save the .VSM file for the scope capture? Would you mind sending it to me? wrenchturnsyou@gmail.com

There's something too symmetrical about those compression humps, IMO. You're positive you were clamped around the correct battery cable?

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3 years 1 month ago #46901 by Mike T
Yeah I’m sure I was clamped around the right on I did positive and ground trying to figure it out lol.. I’m pretty sure I did save the capture but not sure how I saved it or how to send it lol. But I will try to send it to you in a little while

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3 years 1 month ago #46902 by Matt T

Mike T wrote: I used my amp clamp inverted to make it go up set at 2v on 2s screen and my probe to mark a cylinder. The cylinder with the mark is #2 the problem child


Were you using AC coupling? Looks like the current trace drops below zero.
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3 years 1 month ago #46903 by Mike T
I tried to send them let me know if you get them I sent you all the ones I did while trying to get it set up right

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3 years 1 month ago #46905 by Mike T
Yes I was using AC coupling

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3 years 1 month ago #46906 by Tyler
Thanks for the email, Mike T. B)

Like Matt guessed, I believe the problem is AC Coupling on both channels:



If you were using an amp probe on channel #4, then that'd be why the waveform missed the obvious compression issue. :blush: Do you still have the truck? Try retaking the same waveform with coupling off on both channels. Should generate a much clearer picture.
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3 years 1 month ago #46907 by Mike T
Here’s another pic
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3 years 1 month ago #46908 by Mike T
Dang, No I don’t have the truck anymore he just wanted Diagnostics when I found the issue was compression.

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3 years 1 month ago #46909 by Mike T
So I should leave all the voltage the same Just No ac? Here is a pic of the probe I was using for ignition mark
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3 years 1 month ago - 3 years 1 month ago #46910 by Tyler

Mike T wrote: Dang, No I don’t have the truck anymore he just wanted Diagnostics when I found the issue was compression.


I wouldn't sweat it, sir. You're learning a testing method. You made the right call with no parts changed. :cheer:

So I should leave all the voltage the same Just No ac? Here is a pic of the probe I was using for ignition mark.


Probably? If you were using a low amp probe on the 10 mV/A setting, then 2V will work for the relative compression trace. With the Pico wand, I'd probably turn on Peak Detect to make sure you get a solid ignition event.
Last edit: 3 years 1 month ago by Tyler.
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3 years 1 month ago #46911 by Mike T
I like to think I’m pretty good at working on vehicles.I take pride in not being a parts changer. I added this Zeus ( scope) to my tool collection in hopes to be faster at it and some things it has already really helped me on. But with others it’s kicks my butt lol! If I didn’t have you guys to help me I might have lost my mind already lol thanks a lot for all the help!!!

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3 years 1 month ago #46913 by Matt T

Mike T wrote: Dang, No I don’t have the truck anymore.


Experiment on another vehicle. You can pull a spark plug to create an obvious compression problem. Then screw in a compression tester hose to make it less obvious.

Once you get the voltage settings figured out then experiment with filtering to clean the waveform up.
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