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Vacuum waveform analogy and ID help
Posted to Technical Tips Forum on 6/25/2013 15 Replies

Here is something that seems to help me in understanding a vacuum waveform. I'm not claiming to invent this and actually didn't even bother searching much to see if this same idea was posted before.

So far I really see this cranking vacuum waveform thing as very useful. I thank my buddy Brian T. for making me revisit the idea. Thanks also goes to the Brandon B at autonerdz for this awesome idea for a $3.00 BLS I have come to love. This is obviously done on a pico but if you have a sync probe for your scope and buy an FLS or make the BLS you can do this with any scope !

Most of you know the vacuum waveform is quite difficult to master. At least it is for the simple minded techs such as myself. The cylinder ID can be helped greatly with a WOT overlay but is still a little cumbersome IMO. This way is sort of crude but I think its down, dirty, and effective it can just be jotted down on a piece of paper if needed.

Most everyone knows about companion cylinders and how to find them right ? For those who don't its really simple take the firing order and break it in half. This one 1342 put the first two numbers 13 on top of the second set 42. So 1 and 4 are companions and 3 and 2 are companions. This means the piston is in the same place as its companion . Knowing this is huge in this simplification process. If we sync #1 to a plug wire or coil we will have this easy to follow map. 1994 Plymouth Sundance Duster, Waveform

I you look at the sync probe in red on #1 wire the first blue trace right under it is the companion cylinder #4 exhaust stroke. If you label the top humps the firing order starting there you get 4213. Since you know that the vacuum event happens right after the exhaust event the first dip in the waveform has to be #4 intake stroke. So you label the bottom 4213.

I don't know why this seems easier to me? Maybe if your struggling with the vacuum waveform this may help you out as-well. I didn't bother posting this in Autonerdz because those guys are way beyond this. This is baby step stuff for most of the guys there .Im quite intimidated there and I feel like it would be more useful here. I really like reading there just don't feel I have much to contribute. I hope nobody has any hard feelings. I am only sharing something I find really cool. If anyone has anything to add please do.

Oh yeah # 3 exhaust follower fell out as that valve is stuck slightly open.

I know this is an easy one and being a 4 cylinder makes it nice with not so many events. The principal still applies to V engines and it seems like cranking waveforms quickly rule out a (major problem) quickly. The running really didn't look much worse I just had to change the scaling a lot and even added some filter.

Mike from Ohio

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