Negative boost PSI
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Senior User
Joined: Oct 2014
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Negative boost PSI
So i have a scangauge 2, and when at idle it shows .2 psi, when i go into high idle it is at 2.0 lbs...
Also when getting on the highway it shows a negative amount past 20 or 25... like at near or at WOT, it reads -14.5lbs..
Today i was passing a bigrig going up a steep grade like 4-7% not 100% sure... as i passed him i was at -17 psi, then the CAC tube blew off.... i was able to limp it to the top of the hill and re-tighten it and drive another 100 miles or so and get home... I think the hose blew off because i removed it about 400 miles ago to add my coolant filter and must have not tightened it 100% but seems to be fine now... ill keep yall posted on CAC issue, if it is an issue...
But i would like to know what my PSI is above 20 or 25... its a stock turbo, and stock engine... so i know its not off this world high...
Once i figure out what the numbers are... what PSI should i stay under for a stock engine.... no studs, and can i surpase that psi with a stock turbo?
Also happy new years forums!!!
Also when getting on the highway it shows a negative amount past 20 or 25... like at near or at WOT, it reads -14.5lbs..
Today i was passing a bigrig going up a steep grade like 4-7% not 100% sure... as i passed him i was at -17 psi, then the CAC tube blew off.... i was able to limp it to the top of the hill and re-tighten it and drive another 100 miles or so and get home... I think the hose blew off because i removed it about 400 miles ago to add my coolant filter and must have not tightened it 100% but seems to be fine now... ill keep yall posted on CAC issue, if it is an issue...
But i would like to know what my PSI is above 20 or 25... its a stock turbo, and stock engine... so i know its not off this world high...
Once i figure out what the numbers are... what PSI should i stay under for a stock engine.... no studs, and can i surpase that psi with a stock turbo?
Also happy new years forums!!!
Cargo Master
Joined: Oct 2003
Posts: 2,833
Likes: 4
From: Jefferson, Iowa
28 psi is the magic number, and yes you can go past the 28 psi if there is a problem with the map sensor or turbo vains are stuck. With your Scan gauge they will do that with the stock boost gauge. There is a x gauge for boost, someone will chime in with the codes.
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how would i know if the vains are stuck? When it sits over night and its 50 or below it makes a little squeek or bird chirp sound then the jet engine sound from tail pipe, no cat or muffler. But there is not performance issues with the turbo other than blowing the CAC off which i think was my fault... im just wondering why the scangauge will go -10 - -15LBS usually after 25 lbs. idk if i were to add -10 to the 25, it would mean i am actually at 35 lbs, or it doesnt work like that? should i just ignore the bst? i never inputted a code for boost i went into settings and switched MAP to boost...
But again the truck runs fine, no boosting issues, again other than CAC. No codes, accelerates great.. i just dont want to be boosting like 45+PSI and blow my heads
But again the truck runs fine, no boosting issues, again other than CAC. No codes, accelerates great.. i just dont want to be boosting like 45+PSI and blow my heads
Strokin Okie
Joined: Apr 2004
Posts: 1,883
Likes: 21
From: Oklahoma
Do you have a tuner by chance? I noticed the same behavior on my Dashboss, but I assumed it had something to do with the tune I was running. I have since returned it back to stock, but haven't checked the boost since returning it to the stock tune.
Lead Driver
Joined: Feb 2011
Posts: 8,241
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From: L.A. (Lower Alabama)
Lead Driver
Joined: Oct 2009
Posts: 5,406
Likes: 229
From: Pa
My truck does this too. I'm thinking a couple of the sensors are dirty. (ebp, map, baro....etc)
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Cargo Master
Joined: Oct 2003
Posts: 2,833
Likes: 4
From: Jefferson, Iowa
Put in the x gauge for boost. The one that comes on a Scan gauge will do this. use rusty link above.
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Joined: Dec 2014
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From: South Carolina
On the dashboss there is 3 separate selection I believe. One is map pressure and 2 are low and high boost. The low and high boost max at about 25# and show negative while the map pressure will show actual boost. I have a banks iq and they picked the boost pressure to show which does the same thing. They have a lot more money for r&d and I would have thought they would picked the pid that would show high boost but they didn't.
Lead Driver
Joined: Oct 2010
Posts: 5,420
Likes: 16
From: Ouray, CO
The negative boost value on the SG 2 is (probably) just a case of the math logic inside the SG2 being "simple". The SG2 infers from the MAP reading, so if you calibrate your MAP at 10k feet and then drop to a few thousand feet, the SG2 will report you have 5-10 PSI at idle. Clearly you do NOT have that but the SG2 thinks you do (because the MAP reading is so much higher at sea level vs. 10k feet).
Ambient air temp, humidity, and barometer all effect the MAP value.
Ambient air temp, humidity, and barometer all effect the MAP value.
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