could a tight valve cause low idle vacuum, rough idle, and a rich condition ( rich condition probably caused by low vacuum and MAP and fuel press. reg). By saying tight i mean hanging open, not closing all the way. But when i rev the car, the roughness goes away, and the car drives fine. Please Help! My vacuum, according to the MAP sensor using a scan tool, is around 16 in.

 

90Form,

If you suspect a tight valve, check the intake vacuum with an analog gauge instead of the scanner. If the gauge needle bounces, you may have a valve sealing problem. Don't operate the engine any more than necessary if that's the case, or you could be replacing that valve soon.

You hypothesis sounds valid. The pulsations and overall lower vacuum could affect the MAP and ECM enough to cause a rich mixture. And since the mixture control on an S/D scheme (as I understand it) assumes that at a given TPS and engine RPM, vacuum should be at a given point. If the vacuum is not, it assumes that the load must be greater, so it adds some fuel.

It would be pretty easy to pull the rocker covers and check the push rods for free movement. A tight one should be readily apparent.