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.

  • was the gauge sitting at 0 before you started?
    – rpmerf
    Jun 16 '16 at 10:52
  • @rpmerf yes it was. Jun 16 '16 at 13:30
  • 1
    How long was it at -5? And did the needle drop really quickly? Here's my thought. The needle on the vacuum gauge has momentum. If it dropped really quickly and stayed a very short (sub 1 second) time, this could be the needle over shooting and correcting itself.
    – cdunn
    Jun 16 '16 at 16:39
  • @cdunn While that sounds like a reasonable possibility, I've never seen it do that on any other car I've ever checked. Jun 17 '16 at 10:58

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.

  • Fine, but my point is that from everything I've read, getting positive pressure buildup when blipping the throttle isn't something I should see, i.e. the vacuum would normally drop to say the 0-5 in.Hg of vacuum, but positve pressure is abnormal. I was thinking maybe it could symbolize excess back pressure or something... Jun 16 '16 at 14:10
  • 1
    I wouldn't dismiss this. Air has mass and it's moving quickly which means it has momentum. You could easily build up a low, short lived, and non-harmful but negative vacuum because of this effect.
    – cdunn
    Jun 16 '16 at 16:44
  • @cdunn - you could compensate for this by restricting the vacuum going to the gauge (somewhat collapsing the tube the vacuum is travelling through). This would make the vacuum pulse much less prominent to the gauge, slowing down the needle. Jun 16 '16 at 17:22
  • Where are you checking this vacuum, and what size orifice? It's easy to think of the pressure (or lack of) always instantly equal everywhere, but it's not that simple... especially after an intake tube that was designed to create entrainment of an airstream to improve cylinder filling.
    – SteveRacer
    Jun 16 '16 at 17:22
  • @SteveRacer good point you want to make sure you're on manifold vacuum somewhere like the brake booster hose.
    – Ben
    Jun 16 '16 at 17:49

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.

  • Could you explain a little more in-depth why you think that. Jun 17 '16 at 5:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.