Question about 2001 Pontiac Grand Prix

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Lean fuel code error on a 1999 Pontiac Grand Prix GTP,3.8L motor

The engine is supercharged and we found a vacuum valve which is connected to a bypass solenoid that is bad. The vacuum valve has two ports and when you **** on either one the plunger won't move at all. Neither Chiltons nor Hanes motor manuals identify this valve or its intended function on the engine. We did find a picture of a bypass solenoid that looks like ours in a Chiltons manual showing a break down of the supercharger and various parts. The bypass valve attaches to the vaccum valve mounting bracket and is connected via a vacuum hose. The error code that we have is P0171 Sys 2 Lean bank 1. Any suggestions would be greatly appreciated. Thanks.

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  • Anonymous Nov 18, 2008

    I have the same problem. The dealer finally identified the valve as a supercharger bypass valve and solenoid. $300 bucks for the pair. Ouch!!! Still don't know if this will fix my problem but I'm looking for a cheaper source for parts.



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  • Pontiac Master
  • 757 Answers

It is the boost bypass valve. The plunger will move with only enough throttle as it is connected to the throttle body.
The bypass solenoid you need is for the Eaton M90, the supercharger on your car. You can very likely get this part from zzperformance.

Posted on Jun 19, 2009

  • Mike LaPres
    Mike LaPres Dec 31, 2009

    Your supercharger, if it is the stock one that came with it from the factory, is an Eaton M90.

    On the bottom of the boost bypass valve you will see a linkage arm that is attached to a lever on the throttle body with a physical stopper. The valve function operates relative to throttle position, the linkage controls the travel amount of open to close of the valve plunger. The valve upon enough acceleration will close and vacuum reduces, and the intake manifold gets pressurized.
    If your bypass solenoid is bad though, it is probably leaking and causing the lean condition.



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