So I just installed my 5/16th Motordyne plenum spacer and the install went great. Except, while at idle, the car makes a somewhat quiet hissing sound (enough to notice) at the lower left hand side of the plenum and when I turn it off, it hisses very loudly then disappears. Before this, I took it for about a 15-20 mile drive with the new spacer and it drove just fine. I don't have any check engine lights and the idle is right around 700-750 while warm. When I started it for the first time (cold, of course) it idled at around 1000 then dropped as it heated up. So to sum this all up, the car idles fine, I don't have any check engine lights, and the new power seems to be there. What could be making the hissing sound?

Thanks in advance.
-John

 

Most common mistake when installing a plenum spacer is having that shield around the oil filler caught between the plenum and spacer. If that's free, start looking for a vacuum line open...Gary

 

Yes there is a vacuum line on the rear of the plenum near where you are hearing the hiss. It has to be disconnected to do the install. I'd check there first to make sure it is reconnected well. As I recall it was a very snug fit over a long fitting. You might not have gotten it seated fully down to the fitting stop.

 

There isn't much on the lower left side of the plenum except for the vacuum line for the brake booster, and the PCV valve. Check the oil filler funnel gizmo around the oil filter cap as Gary suggested, and then check both of those.