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Negative boost pressure???

36K views 23 replies 14 participants last post by  Calico5 
#1 ·
I have my boost gauge tapped into the map line pre boost fooler and it reads correctly as far as I can tell. I also have my Livewire which has it's crappy gauges too. The livewire reads boost that the MAP sends to the PCM correct? At an idle the livewire shows a negative boost PSI, and always has, usually -2 to -3 PSI. Does this seem right, that the MAP would be seeing a vacuum at idle? Watching both gauges I almost get to 5 PSI on the actual gauge before the Livewire reads 0 PSI. If it isn't normal what does it mean? I've checked for boost leaks and can't find any.
 
#2 ·
I think I would trust the guage
 
#3 ·
I know what ya mean. Just more curious since if the Livewire is working right and the MAP is seeing goofy PSI I'd like to figure it out.
 
#7 ·
Only way you can see negative boost that I can think of is if the turbo was running backwards. If that was happening, the truck would not be running. I would not believe any boost guage reading negative.
 
#11 ·
Could be a bad MAP sensor.

Do you have a way to read the MAP voltage output?

The chart below shows the output voltage vs. pressure. Remember though: The pressures shown below are ABSOLUTE pressure (14.7 PSIA = 0 PSIG). 0 psi on your gauge should be 1.5 vdc. 10 psi on your gauge would be about 2.8 vdc (25 psia).

You could also stick a vacuum gauge on it and see if you actually have a negative pressure at idle.

BTW, even if the turbo was turning backwards, it would still flow a little air and make a little boost. Turning it backwards doesn't make it suck from the pressure end, lol.
 
#17 ·
Could be a bad MAP sensor.

Do you have a way to read the MAP voltage output?

The chart below shows the output voltage vs. pressure. Remember though: The pressures shown below are ABSOLUTE pressure (14.7 PSIA = 0 PSIG). 0 psi on your gauge should be 1.5 vdc. 10 psi on your gauge would be about 2.8 vdc (25 psia).

You could also stick a vacuum gauge on it and see if you actually have a negative pressure at idle.

BTW, even if the turbo was turning backwards, it would still flow a little air and make a little boost. Turning it backwards doesn't make it suck from the pressure end, lol.
I'll give it a test and see what I can come up with. Thanks for the chart.


Well, Negative boost is not really the way to think of it.
at sea level, we have about 14.7 psi just floating around us. (aka "absolute pressure")
if you had a large engine, oh, say, 7.3L, with a lot of restriction in the intake tract, there is potential that there would be a slight vacuum.

Also, and more logical if you're talking 2-3 psi, are you at a decent elevation? (3500 ft +)? because if your electronic gauge is converting a absolute pressure reading into a boost reading, and doesn't know your elevation, then it could very well think it was seeing negative boost even when the truck was shut off. the air is thinner up in the hills, you know. :poke:
Never even thought of this but makes sense that this could be the problem. I'm at about 6300ft.
 
#12 ·
My first set of DPtunes in stock or other tunes was showing 10 psi less on AE than my guage. Reburn and they were both the same. It was the tunes I guess. Don't know for sure but the problem is gone now.
 
#16 ·
Do they match at full boost? you said that one reads finaly reads 0 when the other reads 5ish as boost is building..... if they are consistently off all the way thru the boost, it sounds more like a calibration issue, -2 or -3 is really 0.... kinda like tare weight. The reburn may be the ticket...
 
#18 ·
if a hose becomes disconnected it can read negative also
 
#19 ·
i don't know about the powerstrokes but i know with my buddy's eagle talon it will pull about 5 to 10 pounds vacuum at an idle. here is the type of gauge he has on it. as soon as the motor gets under any load at all the gauge jumps to 0 then goes up from there.

 
#20 ·
Honestly, it would not surprise me if he pulled a 1/10 psi vacuum. 2-3 psi, definately not.


That has alot to do with a throttle body.
 
#23 ·
I have my boost gauge tapped into the map line pre boost fooler and it reads correctly as far as I can tell. I also have my Livewire which has it's crappy gauges too. The livewire reads boost that the MAP sends to the PCM correct? At an idle the livewire shows a negative boost PSI, and always has, usually -2 to -3 PSI. Does this seem right, that the MAP would be seeing a vacuum at idle? Watching both gauges I almost get to 5 PSI on the actual gauge before the Livewire reads 0 PSI. If it isn't normal what does it mean? I've checked for boost leaks and can't find any.

My boost gauge ws a vaccum/press gauge. It never showed negative boost number. "vaccum"
I would not go by what the livewire is telling you.
 
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