Boost Problem New SC owner HELP

I bought an 89 sc 5 speed with the posi rear end a few weeks ago.

I have put new motorcraft plugs and new wires on it. I have put all new belts on. I have taken my supercharger top off and checked that my coupler in the snout has no play and spins freely. I have also resealed the IC and supercharger top with copper based rtv silicone. My bypass valve is working as it should.
When i am just sitting in the car not moving, i will rev it up and my boost gauge does not move. It stays at 18 in. of vac. In 1st and 2nd the gauge barely moves, but in 3rd 4th and 5th if i hold it to the floor going up a hill or on the highway i can make up to 12 psi of boost. It seems like it takes forever for the boost to build.
I saw some of the videos on the internet of guys sitting still and their boost gauge moving a ton. Mine doesnt move at all. Could this be a bad supercharger, bad gauge, the car runs good and doesnt have the check engine light on.

You might want to check the connection between the gauge cluster and the fitting on the IC tube

Boost Issue

I checked the connection, all seems well. I don't understand how I can have 12 psi boost and have it build SOOOO slow, yet show good vaccum on my gauge. Thats why i am not sure if it is my gauge or something more serious. The gauge shows 18 in. of vac at idle and 12 psi of boost at WOT in 3rd gear wound out, but nothing in 1st 2nd or while sitting still, it might take about 15 seconds of WOT on the highway, but it registers. Could my gauge function in vac and boost and still be bad?

My bypass valve is working as it should

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how did you test this?

89 & 90 used a eec controlled valve to close the bypass valve. have you checked this?

Boost Issure

Car not running the valve is on the set screw. Car running and producing vac. it is far away from the set screw, rev it up and it closes toward the set screw. I took the vac line off and stuck my finger over the fitting of the diaphragm, and moved it and it did not leak any air until i removed my finger from the end, so i know its not torn. When i rev the engine it moves closed. I also removed the boost controller solenoid and piped the bypass valve diaphragm straight to the intake plenum to see if this would fix my problem, but it did not. Any advice would be greatly appreciated. How else could i check this?

Your bypass is working fine then. Does the boost gauge swing up to just over 0 and slightly into boost?

Fraser

Wrangler97 said:

Car not running the valve is on the set screw. Car running and producing vac. it is far away from the set screw, rev it up and it closes toward the set screw. I took the vac line off and stuck my finger over the fitting of the diaphragm, and moved it and it did not leak any air until i removed my finger from the end, so i know its not torn. When i rev the engine it moves closed. I also removed the boost controller solenoid and piped the bypass valve diaphragm straight to the intake plenum to see if this would fix my problem, but it did not. Any advice would be greatly appreciated. How else could i check this?

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the valve might be working but did you check to make sure the plate didn't fall off inside?

you can do a search hear and find out how to check for VAC/Boost leaks with an air compressor and soapy water. That would be a good thing to rule out and I would bet you have a leak anyways.

How does the power feel? Is there a change in power that matches when you see the boost gauge move? If not, and power feels fine, it might be the boost gauge is bad.

Otherwise you have a leak somewhere between the outlet of the blower and the intake manifold.
Some possible causes:
I/C tubes (cracked?)
Intercooler (cracks in the fins)
intake plenum (distortion at the intake manifold not allowing seal)
Bypass Valve seals (The seals fail allowing air to escape around the shaft)

Make sure the bypass valve is getting a vacuum source that is in front of the blower and not plumbed in after the blower.

Boost Issue

I finally figured out what my problem was, the stock in dash boost gauge is bad. I put a manual vac/boost gauge on it and it boosts to 12 psi with a touch of the throttle. Thanks for all the help.