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Old 02-22-2019, 06:35 PM   #1
lowbee
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Default Boost Gauge reads funny - too negative

My boost gauge has been acting funny lately... it reads correct values when I am in boost but reads too negative when I am not in boost.

My boost gauge used to read -15 psi when idling, now it is reading -23 psi while Cobb Accessport is reading -10 psi. The accessport and my boost gauge reads about the same value when I am in boost.

Anyone seen this before ? Insight ? Do I need to get a new sending unit ?

Edit:
The above description of the problem is not accurate, please see update in post #7
https://forums.nasioc.com/forums/sho...35&postcount=7

Added a pic.
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Last edited by lowbee; 02-23-2019 at 12:19 PM. Reason: updated problem description
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Old 02-22-2019, 10:28 PM   #2
avikm289
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-15 on your gauge is kinda high. i read -20/-22 at idle and i believe cobb is normally -10 on the access port. -23 should be fine, any lower you may have a vacuum leak.
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Old 02-23-2019, 08:27 AM   #3
Turpid Porpoise
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What type of gauge and how is it hooked up?
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Old 02-23-2019, 08:57 AM   #4
MortonCharbo
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Quote:
Originally Posted by avikm289 View Post
-15 on your gauge is kinda high. i read -20/-22 at idle and i believe cobb is normally -10 on the access port. -23 should be fine, any lower you may have a vacuum leak.
I hate to be pedantic, but any lower than -23 would indicate that you have a slightly more powerful vacuum. Any higher than I'd say about -20 indicates you may have a vacuum leak.

It could be your sending unit. If you had a vacuum leak, the gauge would read closer to 0. If anything, it sounds like you had a leak when it read -15. You either fixed it, or it fixed itself. -23 is a pretty acceptable number in my opinion.

Did you do any work on your car before it started doing this?

Last edited by MortonCharbo; 02-23-2019 at 09:12 AM.
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Old 02-23-2019, 09:51 AM   #5
lowbee
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Quote:
Originally Posted by MortonCharbo View Post
....Did you do any work on your car before it started doing this?
A new EBCS was installed when I got dyno-tuned recently but the "more negative" boost gauge reading didn't show up until at least 4 weeks later (when it was really really cold one day -4F) so I am not sure the dyno-tuned/EBCS install is related to the more negative boost gauge reading. I also notice the tuner pulling the vacuum host from the BOV teeing it off to plug it in the dyno thing but he obviously put it back after the dyno-tune or my car would drove very differently.
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Old 02-23-2019, 11:28 AM   #6
Dreadsix
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Often times boost guages use inches of mercury for vacuum instead of psig. In this case, 23 inches of Mercury is about -11.3 psig. Pretty close to your Access Port reading. Verify your guage's units of measure for vacuum (could be listed as inHg).
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Old 02-23-2019, 12:00 PM   #7
lowbee
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Now that I am paying more attention to the boost gauge when I am driving around, I have actually described the problem incorrectly in the first place.... (I will go back to the top post and correct it later).

What is actually happening is the boost gauge would read "normal" most of the time. Normal to me is:
1. it reports boost pressure when in boost accurately (same as accessport)
2. it reports vacuum around -22 at idle and -15 on light throttle

and then all of a sudden, it drops to -27 at idle and the gauge is flashing as if the power going on and off (initializing like when you start the car)....so I am thinking if I have a leak somewhere, the gauge would consistently reads -27 at idle and not -22 most of the time and -27 once in a while.

The fact that it flashes on and off when it reads -27 leads me to believe I have a bad sending unit.... does this make sense ?
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Old 02-23-2019, 12:23 PM   #8
lowbee
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Quote:
Originally Posted by Dreadsix View Post
Often times boost guages use inches of mercury for vacuum instead of psig. In this case, 23 inches of Mercury is about -11.3 psig. Pretty close to your Access Port reading. Verify your guage's units of measure for vacuum (could be listed as inHg).
You are right about this. Good call
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Old 02-23-2019, 11:50 PM   #9
Turpid Porpoise
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Quote:
Originally Posted by Turpid Porpoise View Post
What type of gauge and how is it hooked up?
This makes a difference. All gauges are different, what gauge are we talking about here?
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Old 02-24-2019, 12:13 AM   #10
lowbee
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Quote:
Originally Posted by Turpid Porpoise View Post
...... what gauge are we talking about here?
See pic in the first post, it's a Depo
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Old 02-24-2019, 02:33 PM   #11
CarmelValleyWRX
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sounds like a bad electrical connection if your gauge is flashin and resetting and reading wrong sometimes. 20inhg is normal for idle. 15-10inhg cruising on highway. 0-5 boost up slight hills on highway. 20inhg is about 5psiA. 10inhg is about 10psiA. 0 is 14.5-14.7psiA normally depending on weather and altitude that the gauge was turned on at.
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Old 09-10-2020, 05:37 PM   #12
djharlock
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Not to necro this thread, but looking for an unrelated topic and I came across this discussion. Kind of surprised there is this many people talking about boost negative boost, I think the missing piece of information some people here didn't have was that most gauges do not read boost in the same measurement while negative, or rather vacuum is measured in HG, not PSI.

Example: if you are cruising down the road with an accessport plugged in and have a gauge setup to display boost, the accessport is displaying the state of vacuum as -11 and and then if you give it gas and go wide open throttle around 3,500 RPM or higher, you'll then see the gauge climb up to roughly 16 for positive boost, both readings display as a PSI measurement.

Now let's say you have an analog 30 psi boost gauge also installed. If you were to check the physical gauge at the same time you checked the acceasport while cruising, you would see the gauge give a reading of -23 or so. That same gauge would then read 16 while wide open throttle and creating positive boost. The accessport only reads out in PSI so the device converts the vaccum-state measurement from Hg to PSI for simplicity.

Tl;dr most physical boost gauges measure vacuum, or what you call negative boost in Hg, and positive in PSI. Roughly 2Hg = what the equivalent would be of 1 psi, that's super-simplifying things but the boost for dummies takeaway would be 2hg=1psi.

So if your reading of -22/23 boost is from a digital or analog gauge that's installed on your dash, the accessport equivalent is more like -11 PSI. Again apologies for the necro here, but since google is pulling this up as the top result when you search "wrx negative vacuum..." I figure I would chime in with hopefully some helpful info so future readers know not to freak out.

Again apologies
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