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Discussion Starter #1
So recently I noticed that when driving in decelerating in gear my vacuum readings as reported by my LCDBC are super weird. So as I'm driving and let off, vacuum spikes to ~-11psi, BOV goes off like normal, as I continue to decelerate in gear with no throttle input the vacuum reading will suddenly go to -0.9psi with no real change in how the engine is running. After talking with Blackstealth the max vacuum reading is -14.5psi so I'm not over-running the sensor. Blackstealth suggested that the hose might be collapsing under vacuum.

When looking at logs this phenomena almost always occurs exactly with decel fuel cut, which I find kind of weird. I went back through logs going to last year and this was present on pretty much all of them. From when I had a basically stock car with a LCDBC, to now having a FMIC and vacuum reduction mod, it appears on both. I guess this kinda helps the collapsing vacuum line theory, but since I tend to overthink things I figured I'd reach out and see if any of you had any ideas. No one really makes "reinforced" vacuum hose, so i guess the only option would be silicone vacuum hose but I'm not sure how much better that would do against collapsing compared to normal vacuum hose.

Any ideas?


Since the new forum won't allow log files as attachments for some reason here are google drive links:


 

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Discussion Starter #3

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Have you seen these?


Not saying its better or worse than what you have, just offering it up....

Bob.
 

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What are you currently using? Off the shelf (OTS) auto parts store hose? Vibrant is another good choice for silicone hoses - just secure the ends well.

-sent from my Galaxy Note 9
 

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Discussion Starter #8
Yea I bought a whole roll of Gates(same stuff you get from AutoZone/Advance) vacuum hose a while back and replaced most of the vacuum lines. Guess I might have to bite the bullet and get some silicone hose from vibrant and see what happens.

Was basically just curious if anyone had any idea why it lined up perfectly with fuel cut. Granted I realize that the engine just being an enertial air compressor would be the highest vacuum the engine sees.
 

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I dunno man, I’m using the same hose I always did and I feel like before my vac reduction and arc2 install I was getting higher vac readings. Its hell getting old though because I also really don’t remember because all I cared about was the positive number lol.

I’m very curious to see how this plays out for you!
 

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What are you using to log the boost? A elecontric boost gauge signal wire or a sensor like the 420AP?

Also what is it at warm idle? Dont really see that in the log. I idle at about -10psi or -22 inhg or so. This is both before and after vacc reduction, no change from that really.

In the logs it looks like you only drop to -0.9psi when you are under decel. and it recovers when you are back down close to idle speed even when you still have no TPS input and 0 injector DC.
 

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Discussion Starter #11
The log is via the LCDBC, using the internal sensor on the newer version, which is the 4250AP. Those logs were started while move and in gear in attempt to keep them short and make the -0.9 part easier to find. At idle it stays right around -9.1 psi. I still have a few boost leaks at the throttle body shaft seals and BISS but they aren't terrible.

And yea the -0.9 only under decel fuel cut is kinda why I made the post. I even purposefully got the log long enough to catch it going back to a "normal" without any input from me. It is just kinda....weird. I bit the bullet and ordered some silicone vacuum hose from vibrant which should be here Friday hopefully, we'll see if it is the hose collapsing.
 
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