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1988 Spider Quadrifoglio
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The clear tube leads to the manifold vacuum sensor, which is located behind carpet, behind the passenger seat. The rubber hose should connect to a nipple on the intake plenum. Look at the rear off plenum under where the idle bypass and auxillary air hoses connect.
 

· Premium Member
1988 Spider Quadrifoglio
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576 Posts
The vacuum sensor is part of the ignition system. Without it, you don't get ignition advance - your car will be slightly slower and you will have worse fuel mileage.

The altitude sensor is completely different and part of the fuel injection system. L-Jetronic system does not control ignition. The ignition controller is a separate box.

You should hook up the vacuum sensor if it doesn't leak. If it's busted, cap off that hose, and the nipple on the plenum. Leaving the nipple open will leave you with a big vacuum leak and false air. Everything downstream of the air flow meter must be nicely sealed.
 
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