I was doing a vacuum test on my 99 Nissam Almera 1.6 ( GA16DE engine ) to kind of get an idea how the engine is doing after I homebrew seafoamed it, and when I blipped the throttle the vacuum reading dropped to -5 in.Hg before jumping to 23 and settling back to it usual slightly jittery reading of 20 in.Hg.
The -5 basically means higher than than atmospheric pressure, and I was wondering if anyone might have some clue what's going on? I have not been suffering any noticeable performance problems. The only thing that's stood out to me is a bit of what I assume to be valve rattle if I try to accelerate up a hill in too high a gear at less than say 2500 rpm.
One reason I'm asking this question is because I've never seen a negative reading before, and I've never read about a negative reading in any of the automotive books I've read.