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

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19 Feb 2021 17:52 #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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19 Feb 2021 17:56 #46886 by Mike T
Here you go
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19 Feb 2021 18:28 #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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19 Feb 2021 19:02 #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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19 Feb 2021 21:26 #46893 by Mike T
This is all 8 cylinders. Firing order is
1-8-7-2-6-5-4-3
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19 Feb 2021 21:59 #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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20 Feb 2021 08:15 #46899 by Tyler
Did you happen to save the .VSM file for the scope capture? Would you mind sending it to me? This email address is being protected from spambots. You need JavaScript enabled to view it.

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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20 Feb 2021 09:15 #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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20 Feb 2021 09:55 #46902 by Matt 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


Were you using AC coupling? Looks like the current trace drops below zero.
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20 Feb 2021 11:10 #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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20 Feb 2021 11:20 #46905 by Mike T
Yes I was using AC coupling

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20 Feb 2021 11:26 #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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20 Feb 2021 11:44 #46907 by Mike T
Here’s another pic
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20 Feb 2021 11:46 #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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20 Feb 2021 11:51 #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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20 Feb 2021 11:53 - 20 Feb 2021 11:57 #46910 by Tyler

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.
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20 Feb 2021 12:06 #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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