Pressure Waveform Interpretation

Mervin Owner/Technician Indiana Posted   Latest  
Discussion
Driveability
2002 Chevrolet Express 2500 5.7L (R L31 Gas) 4-spd (4L80-E) — 1GCGG29R821234620
Misfire

Hello to all,

Am opening myself here to all, so i have been very interested in pressure waveforms...... have been trying to capture know good waveforms on various engines over the last few yrs..... my hang up seems to be ..... to be able to look at captures that are known bad ( primarily intake and exhaust waveforms) and be able to quickly pick out the fault...... i know it may take much time and learning, of which i am planning to do this yr hopefully in PA Rosedale Tech......... with Scanner Danner and Brandon Steckler, but for the time being is there a few proven steps and guidelines to go by each time...? Maybe a few of you that have this perfected this art have some direction for me......???

My questions are;

  1.  Is there certain tips tricks to setting up the Firstlook sensors to always get relativley the same results?
  2. Is there certain rules, tips, or guidlines when interpreting these waveforms,( Intake, exhaust, and even in Cylinder?

On this 5.7 engine it has a dead miss on Cy 3 via scanner, via relative compression, and actual compression with the WPS 500..... finally at the end of the testing just to verify we did a cylinder leakdown test, the first gauge being at 70psi, the second at 15-20 psi, air is escaping the exhaust valve, we can feel it at the tailpipe.

By looking at the waveforms below, how can this be quickly and accurately detected? or do theses waveforms not look clear enough? Sorry for all the questions...... i really want to get this testing method to where i can use it in a efficient, clear and accurate way, the same every time!

Thanks in advance to you all...... you may reach out to me personally if any of you want at …, shop phone is …

Thanks Mervin

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Helpful
JJ Technician
Massachusetts
JJ Default
 

The Piston Position overlay from Driveability Guys is a huge help in interpreting these waveforms. driveabilityguys​.​com/software-and-t… Plug in the firing order and cylinder you're synced on, then place the chart right over the wave form. It really helps when you can see companion cylinders at a glance and you can start to connect the dots of what is going on where, especially on…

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Mervin Owner/Technician
Indiana
Mervin Default
 

Thank you JJ, i will most likely do that!

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Jim Mobile Technician
Pennsylvania
Jim Default
 

Mervin, First of all, MUCH RESPECT for you to be asking for some help in understanding this Great Diagnostic procedure. I personally have started a lot of techs in understanding this testing, but the VERY first thing I tell EVERYONE, go back over your understanding of the 4 strokes of the ICE. My first suggestion would be to search youtube, there are some GREAT videos by people like Mario Rojas…

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Mervin Owner/Technician
Indiana
Mervin Default
 

Thank you Jim, I appreciate the reminder to go over the 4 strokes of the ICE again.... it seems that in the wake of all the info on the screen its easy to lose sight of the basics....... Will also check out the Video from John, i have had the opportunity to be many of Johns classes over the last few yrs, a great teacher...!

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Jonathan Educator
California
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Mervin; Good morning Sir, I hope this message finds you doing well. I'm attaching a PDF that includes a Piston Overlay to assist with analyzing your waveform. Cylinder #3's Intake Stoke appears to be similar in shape as six of the other cylinders. I would think if Cylinder #3 had an Exhaust Leak as mentioned above Cylinder #3 wouldn't be able to create an equal vacuum signature. Cylinder #2's…

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Mervin Owner/Technician
Indiana
Mervin Default
 

Hello Jonathan thank you for your input it is greatly appreciated. We did get the head off today I will attach a couple pictures it was the exhaust valve on number three it must’ve stuck some time and hit the piston therefore it was bent. However I still would like to know how to interpret this by looking at those waveforms. See pics below Note I am shining my flashlight in the exhaust port on…

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Dmitriy Analyst
Ontario
Dmitriy Default
   

Hi Mervin, this video by Mario R. (diag​.​net/u/u318hvyhbvmb…) answers quite a few of your questions: youtu​.​be/PPL4UFCg-vQ It talks about how changing crankshaft speed (due to compression loss in one cylinder) shows up on the intake pulse waveform, with analysis confirmed by... you guessed it, leak down test! And another one: youtu​.​be/KgAJG2mkJYw

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Ray Diagnostician
Ontario
Ray Default
 

I'd like to add some tips to the videos. When turning the engine over by hand trying to find the compression stroke, I attach a balloon to a regular compression hose. When the balloon begins to swell, it means your on the compression stroke. On some engines, you can turn the alternator nut, to rotate the crank. Attach a rubber glove tightly over the tailpipe. Another glove over the throttle…

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Mervin Owner/Technician
Indiana
Mervin Default
 

Hello Ray, Thanks so much for the great tips, Have never tried the balloon or rubber glove trick but will employ it!

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Mervin Owner/Technician
Indiana
Mervin Default
 

Hello Dmitriy Thank you very much for the links..... i will most certainly check them out!

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Ray Diagnostician
Ontario
Ray Default
 

The first capture is from your 02 Express with the leaking exhaust valve. The second capture is from an 06 Escape 3.0 with some fuel flooded cylinders, including cylinder 2. CH B is a FLS on the dipstick tube and some of the compression pressure in cylinder 2 is leaking by the piston rings into the crankcase CH C is the starter amps CH D is a WPS in cylinder 2 and the waveform looks similar…

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Mervin Owner/Technician
Indiana
Mervin Default
 

Thanks Ray!

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Jonathan Educator
California
Jonathan Default
   

Mervin; Attached is another PDF made from the waveforms you posted. Thank you or the response and the pictures you supplied of the cylinder head and valves. I'm sorry I didn't see the waveform that included the In-Cylinder trace before my previous post. If a cylinder looses pressure faster than it builds pressure the Compression Tower will lean (see attached file). This along with No Defined…

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Mervin Owner/Technician
Indiana
Mervin Default
 

Thank you Jonathon! I am humbled at how little i know or rather knew.......... i now can clearly see the leaning towers and understand the theory more clearly!

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