>>Beg to differ. Had terrible vacuum leak where passenger valve cover overlapped timing chain cover (mating surfaces not quite flush). Steady pin point stream of solvent would kill engine. If that isn't a vacuum leak don't know what is.<< Now wait, we need to distinguish between what's noraml and what not. In normal operation there should be no vacuum under the valve covers. I'm not saying you can't get some there if something's amiss, but if you're sealing the covers your covering up another problem. >>Still have bad leak around timing cover crank seal. Most likely way that air is entering engine is via valve covers (vs block seal).<< The timing cover crank seal is a pressure seal, not a vacuum seal. Try this: With engine running pull off the hose at your air cleaner from your oil filler breather. That goes into the valve cover right? Any vacuum there? Now cover the end of the hose with your finger and wait. Tell us what happens. I'm serious, try it. If there is a vacuum nothing will happen right? You'd only be plugging a leak wouldn't you? Come to think of it, why would a path to the induction system be ported into the air cleaner in the first place? Please, tell us what happens when you plug this "vacuum" source. >Rich A just plugged vacuum leaks in his cam access plates, again under valve covers.< Uh, is that the guy in MD? No comment... >Have you ever heard of anyone running PRV with valve covers removed?< Why yes, I have...no problem. Does your engine stumble at idle when you remove your oil filler? If that's a path into the induction system, that would be a pretty big leak right? Or put a rubber glove over the oil filler port, what does it do? Again, there is opinion and there is data. Port a vac gage into your valve cover and measure it...or maybe the guy whose the expert on idle speed control can expalin it to us. ;)