Thanks much, Doc. I'm trying to chase down a vac leak (I assume), which is causing heat to the side vents. I checked out all the usual suspects -- connections by the fuse box, vac switchover block, line to heat valve, lines to vac reservoir and reservoir itself. Is the entire vac system interconnected? If so, any ideas on where else I can look? The problem is not major because heat only begins to be discharged from the side vents while engine is under load -- like up long grades -- then after leveling out the heat stops. Could this be symptomatic of a bad heat valve?