94 Bonne SSEi Stops boosting

[Chris A]

Anyone have any Ideas on what could cause a 3800SCI to stop boosting? After doing some routine maintenece on my moms 94 bonne, it wouldn't build any boost. I tried to isolate the bypass valve, but am not familiar enough with its function to see if it was accidentally opening up when it should have been closed. I am still suspecting this to be the problem but any insight would be appreciated.

Chris

 

[terryk]

Check for a split in the blower to intake gasket just below the nose drive. There is a seal there that splits.

 

[99GS]

Maybe the boost guage became disconnected. It happened to me on my GN.

 

[terryk]

I was assuming that you knew you had no boost. Good point. check the line to the map sensor.

 

[Chris A]

Your assumption was correct. The car was not only indicating 0 boost, it was making 0 boost.

It turns out to be a problem with the bypass valve. Kinda like a wastegate but not. It equalizes the pressure in the SC under light load conditions, reducing the amount of drag it creates. Its a dual port actuator. In the normal state it goes to the boost (closed) position. The top port is connected to the throttle body and when there is vacuum (under light throttle) the valve opens (bypass). As vacuum drops the spring in the actuator pulls to the closed position.

There is a second port on the actuator that is connected to the side of the supercharger. I am assuming that this port will show boost. When connected the bypass valve will open under boost, when disconnected the car will build boost normally (bypass valve closes). There is an electric solenoid in line with this port, that I assume is bad since it will pass boost to the bypass actuator.I think this is a way of controlling boost, but haven't found any information yet. Also, at idle there is vacuum on the port that will counteract the first port and pull the actuator (and bypass valve)closed. If I tap on the solenoid, the actuator will return to its proper position.

Chris

 

[SubZero350]

The solenoid you refer to is the boost solenoid. Durning normal driving conditions, the PCM will put 100% duty cycle to the solenoid allowing full supercharger operation. During reverse gear selection, the PCM will put 0% duty cycle to the solenoid allowing no boost buildup in reverse. Effectively, the solenoid should be "on" all the time allowing no boost pressure to accumulate on the boost side of the diaphram. The vacuum side will operate normally during all driving conditions without the boost line hooked up. Sounds like you either have a stuck boost solenoid, burned-out solenoid, or bad PCM quad driver on the boost solenoid circuit which should turn on the check engine light.

In a nutshell, you can unhook and plug the boost line leading to the solenoid. This will not cause harm or effect boost output under WOT. But beware, this will allow the engine to build boost with the trans in reverse which GM never intended.

 

[Chris A]

Thanks for confirming my theories. Now to get GM to find me one. I called last week and one dealer didn't know what it was.

Chris

 

[SubZero350]

Goto a local junk yard and take the old one with you. I think that an EGR or CCP solenoid might be compatible with what you have as long as the plugs are the same.