What problem does a negative vacuum reading indicate when blipping the throttle?

Question

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.

EDIT

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.

Answer 1

Vacuum is the opposite of atmospheric pressure. The atmosphere at sea level holds at about ~14.7psi. This is considered "1 atmosphere" or "1 bar". The engine is an air pump. It draws air in and pushes exhaust out. More accurately, it doesn't "draw" air in, but rather, creates a vacuum and the atmosphere pushes air into the space with vacuum in an attempt to equalize the two pressures. When the throttle is closed, not much air can get through into those spaces where vacuum exists. This is considered a high vacuum state inside the engine. When you blip the throttle, the inside of the engine where the vacuum exists and the atmosphere do their dance while trying to equalize. Inside the intake the vacuum area, if enough air can come in, will become a pressurized area (greater than zero, but not quite at atmospheric pressure). If you look at the vacuum reading as a positive number and as the vacuum moves towards pressure, the number will decrease. When vacuum decreases past zero, it becomes negative vacuum or pressure. Thus the negative number when you blip the throttle. If you were measuring pressure during this time, you'd have a negative pressure reading while the intake was in a vacuum state, then move into positive pressure as you blip the throttle. But since you are measuring vacuum, this is the reading you get.

Answer 2

Engine gauge vacuum is the difference between manifold pressure and atmospheric pressure. When you snap the throttle pressure builds in the manifold. Then settles back down to whatever it was at before. If you look at a MAP pid it correlates with engine gauge vacuum.

A shakey needle on a gauge would suggest a problem with the valve train. Either they need to be adjusted or there's something else going on. If I had to guess the valve train noise and shakey needle are probably why the gauge reads -5inHg on snap throttle and pressure in the manifold isn't equal to atmospheric pressure.