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05 SS/SC not boosting

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05 SS/SC not boosting

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Old 08-11-2018, 07:05 PM
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05 SS/SC not boosting

I bought an 05' Cobalt SS a few months ago and the boost gauge has never read anything over 0lbs of boost. It will go from -10 vac up to 0 on WOT, and then return back when I let off the gas.

I've tried diagnosing the supercharger's bypass valve and it seems to be working as expected. The valve will open and close freely when throttle is applied, so I dont think thats the issue.

The car also is throwing a few codes due to me having an aftermarket downpipe and cat, (P0455, P0420, P0140) but again, I dont think thats causing it either.

Does anybody have any ideas as to what could be causing the car not to boost?
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Old 08-11-2018, 10:26 PM
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First off, 10" of vacuum isn't good unless that's at part throttle. See if you have 14"-16" at idle (altitude dependent) and around 20"-21" under deceleration. If it doesn't run poorly, you may have a bad leak on the gauge hose.

Disconnect the hose that goes from the SC bypass valve (lower fitting) to the solenoid and test drive it. That will allow the bypass valve to close and stay closed as vacuum drops.



Hopefully the bypass valve plate isn't missing.

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Old 08-11-2018, 10:35 PM
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I did try disconnecting the hose from the bypass to the solenoid and it did not seem to have any effect on it, like I said the valve opens/closes as expected when throttle is applied. How would I check if the bypass valve plate is missing?
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Old 08-11-2018, 10:47 PM
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First, is your vacuum as I listed above?
You have to pull the SC to get to the valve and that's a long shot that it is missing.

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Old 08-11-2018, 11:05 PM
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Looking at it again the vacuum does sit at -15" on idle and acts as you mentioned upon de-acceleration
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Old 08-11-2018, 11:12 PM
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If the bypass plate was missing you would be able to pull the shaft out.....
There is an adjustment so it shuts completely. The 2 bolts that hold the valve on.
Does it run that poor?
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Old 08-11-2018, 11:19 PM
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Ill check that tomorrow. The supercharger whines like it is making boost, but the gauge never indicates that it is. The car doesnt feel nearly as fast as it should (compared to my buddies n/a Cobalt it feels almost identical)
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Old 08-13-2018, 11:00 AM
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Thumbs up

It turns out that my boost gauge was not working properly.
I got a bluetooth OBD2 adapter and the torque pro app on my phone said that the car was boosting to 8psi on WOT, so im just going to replace the gauge.

Thanks for the help
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Old 08-13-2018, 12:22 PM
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That's what I was going to guess. If your car was making zero PSI your car would be a complete turd and should be slower than an N/A SS Cobalt.
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Old 08-13-2018, 02:00 PM
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Originally Posted by Manerd05 View Post
It turns out that my boost gauge was not working properly.
I got a bluetooth OBD2 adapter and the torque pro app on my phone said that the car was boosting to 8psi on WOT, so im just going to replace the gauge.

Thanks for the help
Once you get a properly working gauge, hopefully that 8psi will also go up as that ism't that good.
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