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Scope captures and questions

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7 years 6 months ago #1633 by JoesAutoElectric
Ok so I had a 1980 Corvette towed in today. Car is a basketcase and the guy wanted to get it running. 5.7 liter and once I fixed the fuel delivery problem and got the timing set it was missing. I decided to use the pico to attempt to find the misfire. Well, I found it. I included a pic of the screen capture from the secondary. I also decided to try doing a relative compression test on the pico more to get familiar with the tool than anything else. Cranking sounds like it may have an issue so why not try the pico? Anyway. I disabled the distributor and had my buddy crank it. This is what I got time and time again. I would love for someone else to take a look at it and get opinions.
Also I have another thing to run by you guys. If I wanted to determine firing order on this engine while doing this test, how would you go about it? I mean, you have to disable the ignition to get the relative compression waveform, so no way to sync with a secondary probe. Thanks guys.

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File Name: 1980Corvette.pdf
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File Name: 1980Corvette.pdf
File Size:471 KB

"The man who says it cannot be done should not interrupt the man doing it."
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7 years 6 months ago #1634 by mech1965
looks like a lean mis to me

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7 years 6 months ago #1637 by JoesAutoElectric
I agree that it appears to be a lean misfire...

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7 years 6 months ago - 7 years 6 months ago #1638 by Andy.MacFadyen
To identify low compression cylinders you can re-connect the ignition but disable the injection another trick is to remove 1 spark plug.
Generally where something shows up on a cranking compression test your going to have to get out the compression gauge or leak down gauge and test multiple cylinders anyway. With such prounced (apparent) lean mis-fire identified I iwould go straight for a leakdown test on identified cylinder and would expect to find a major exhust valve sealing issue.

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Last edit: 7 years 6 months ago by Andy.MacFadyen.

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7 years 6 months ago #1641 by JoesAutoElectric
Thank you, but keep in mind this is a carbureted application not injection. I suppose I could disconnect the fuel line and allow it to run into a container to test...

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7 years 6 months ago #1642 by JoesAutoElectric
So this morning I ran compression using a standard gauge on this thing. Only did the one bank but had consistent reading of 110 psi. I did running as well and found it to be around 75psi. I then wanted to try my fluke PV350 with the pico. I set it as a 500psi transducer on range 1. The result I got was consistent again on all cylinders, though the pattern is not what I was expecting to see. Anyone able to explain this?

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7 years 6 months ago #1644 by Andy.MacFadyen
The only time I have heard of 110psi compression test reading across the board was when the normal cranking compression gauge test had been done without holding the throttle wide open. With the throttle wide open twice the mass of air fills the cylinder and compressed so the peak pressure is nearly doubled.
With engine running the pressure will be even lower because the pressure in the inlet manifold will be even lower than it is cranking the engine with the throttle closed.
Another thing to allow for is the length of extension tube between gauge and the spark plug hole has a major effect on the peak pressure reading, a long extension reduces the compression raito.

Now turning to your scope compression test I think something is wrong with your setup what I don't know,

" We're trying to plug a hole in the universe, what are you doing ?. "
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7 years 6 months ago - 7 years 6 months ago #1646 by Doc
Replied by Doc on topic Scope captures and questions
Something not right with your setup there I think - did you have a schrader valve fitted to the hose?
The pressure is not dropping to zero so either you have a plugged exhaust, valves not opening or a hardware problem with your scope.
HTH

PS just looked at your cranking amps and secondary captures - there is something amiss with the way this engine is breathing - sticking valves, valve timing, blocked intake or exhaust, what I'm seeing does not add up at all.

Youtuber specialising in Diagnostic techniques, sharing skills and helping technicians be the best they can be #Picoflu
Last edit: 7 years 6 months ago by Doc. Reason: added post script

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7 years 6 months ago #1649 by JoesAutoElectric
Ok rookie mistake guys. I had a Schrader valve in the damn hose. Got mixed up and grabbed the wrong one. Anyway, I have some new patterns if you are not sick of me already.

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7 years 6 months ago #1663 by Doc
Replied by Doc on topic Scope captures and questions
That looks better :)
Can you upload the psdata file?
the noise on your top capture, does this coincide with an ignition event? If so, I would say the coil is firing way before it should be.
My own preference is not to filter out noise as it can be a great source of information as to what else is going on in the engine bay without having to dedicate a scope channel to it.

Youtuber specialising in Diagnostic techniques, sharing skills and helping technicians be the best they can be #Picoflu

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7 years 6 months ago #1668 by JoesAutoElectric
Thanks Doc. I am unsure how to upload the file. I have been having trouble with the pico files when I try to email them because it always seems to say they are too large. Still learning to use this software. I will try though. In any event, the noise is just that, noise. I only had the pressure transducer set up on that capture. I'm making a strong attempt to learn about in cylinder testing, but the more I am learning the more variables seem to pop up. I figured this engine would be perfect to learn a few things because it is an old school small block chevy with an HEI in it and no frills. But looking at it I cannot figure how I would sync ignition to this pattern for a timing test. Only way is with a secondary probe to the spark plug wire which is disconnected and attached to an HEI test plug.

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7 years 6 months ago #1676 by Doc
Replied by Doc on topic Scope captures and questions
Hi Joe, you can make your files smaller by opening the capture, save AS, then select "current waveform only"
Very useful for known goods etc.
Re: noise, looking at the noise on that capture it is possible for the transducer, lead or scope to pick up noise from an ignition or injection event and to draw it. The spacing of the noise makes me think it may be Ignition and the timing of it looks too far away from TDC.
To add a ignition sync just select secondary ignition probe from ChB drop down and put it on any known ign lead, you can work out cyl ID using the firing order from there.
Paul does this in a lot of his videos featuring relative compression tests.
Hope this helps (HTH)
Steve...

Youtuber specialising in Diagnostic techniques, sharing skills and helping technicians be the best they can be #Picoflu

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7 years 6 months ago #1678 by Tyler
Replied by Tyler on topic Scope captures and questions
Like Doc was thinking, it appears that spark timing is pretty far advanced of peak compression. I think the Pico software will allow you to calculate this?

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7 years 6 months ago #1684 by JoesAutoElectric
In order to do a sync on this application the engine would have to be running as it is not a fuel injected engine. I suppose that would work but to see where the timing is in relation to tdc on that cylinder, I have to connect channel B to the spark plug wire on the same cylinder.

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7 years 6 months ago #1703 by Doc
Replied by Doc on topic Scope captures and questions
Hi Joe,
You can look at ignition when cranking, just disable the fuel system i.e. if mechanical lift pump just pull or clamp the suction line from tank to pump or if electric pump then pull the fuse.

Your sync does not have to be on the same cylinder - it is easily worked out using firing order and you will inevitably have enough noise on one of the channels to be able to work out where the firing event is. In any case just earth the plug lead using a test light to GND and you can use the same cylinder as the transducer is fitted to.

HTH

Steve...

Really recommend you watch Pauls videos on relative compression and cylinder ID / sync

Youtuber specialising in Diagnostic techniques, sharing skills and helping technicians be the best they can be #Picoflu

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