gulf_coast_228

I purchased a vacuum gauge to help diagnose a slight miss at idle and a very noticable miss at acceleration and interstate speeds (that thread is here)and I need some advice as to what I'm seeing. I'd like to test everything I can before taking it to a shop for a compression test.

I've got the pressure gauge hooked up to the top port on the vacuum canister. It's reading just below 20 and pretty steady there at idle. When I give it some throttle I see the gauge jump back then forward. If I give it a little more it does the same, just going higher.

Having never used one of these and not really knowing what I'm looking for, is this normal operation? I've searched until I'm blue in the face and still haven't found exactly what to expect when doing this.
Here's a video of what I'm seeing.

1992 Firebird GTA, 350 5.7. Any advice is appreciated, as always!

Atilla the Fun

I'm having trouble getting the video to play, but I'll try to help.

Atilla the Fun

Okay, I watched the video, but no audio. What I need is to see what you attached the gauge to. when you snap the throttle by hand, the vacuum should decrease as you open the throttle, then increase as you close it. Most helpful would be if you could put the gauge between the speedo and tach, then have someone sit behind you and film over your shoulder as you gradually accelerate up through second gear.

gulf_coast_228

That makes me feel better, that's exactly what it's doing. Here's a picture of where I have it hooked up. I'm going to need a lot more hose to put it up near the tach and speedometer, may not happen today unless I can figure a better way, and help comes as I'm the only one here at the moment.

Atilla the Fun

I'd tee directly into intake manifold vacuum, because there might be some emissions controls affecting that. And reading vacuum in neutral with quick throttle snaps isn't relevant to reading gradual part-throttle rpm increases.