Vacuum test best practices?

[Earendil]

Why does searching this forum produce nothing helpful in this area? All I get are MOR BOOST! Threads :/

Anyway, I just got a vacuum gauge, and am wondering where the best place to attach it is for accurate readings. If multiple locations tell different stories about the engine, knowing what those are would be helpful. I started with it on the intake manifold to FPR hose. I got 16hg with rhythmic short surges to 17hg. Matches changes in the engine sound too. Video coming later tonight.

Thoughts? Should I be attaching it someplace else for better results?

Thanks!

[Earendil]

I forgot I can actually upload YouTube videos directly from my phone, durrr. No waiting until tonight for videos.

http://www.youtube.com/watch?v=nhh_4oF51mA

She holds steady at about 20 @2000 rpm. Letting off throttle causes a quick jump to around 24 before dropping smoothly back down to 16-17. All normal and expected, yes? It should be noted that I'm very happy with the way my engine runs right now. But if there is a vacuum leak, I want to snuff it out :)

[Earendil]

And here is one coming off idle.

http://www.youtube.com/watch?v=yfk54n9wSgI

I've never liked my hesitation coming off idle, but is it related to vacuum dropping to near 0? Or is that normal?

[jlevie]

A vacuum check can be a quick diagnostic in the case of a pretty sizable intake leak. But it is pretty useless in the presence of smaller leaks. Since an engine with electronic fuel injection is very sensitive to intake leaks you need a test that can find the smallest leak. That test is a smoke test of the intake.

When properly conducted, the exhaust will plugged and an adapter for introduction of smoke will replace the AFM/MAF. Smoke filled air is pumped in until the pressure is at least 2psi. That pressure is held for five or more minutes to allow the smoke to work its way to every leak. Where you see smoke, you have a leak. A smoke test will find leaks in places you'd never think to look.

Your hesitation coming off idle is suggestive of an intake leak.

[Earendil]

A vacuum check can be a quick diagnostic in the case of a pretty sizable intake leak. But it is pretty useless in the presence of smaller leaks. Since an engine with electronic fuel injection is very sensitive to intake leaks you need a test that can find the smallest leak. That test is a smoke test of the intake.

When properly conducted, the exhaust will plugged and an adapter for introduction of smoke will replace the AFM/MAF. Smoke filled air is pumped in until the pressure is at least 2psi. That pressure is held for five or more minutes to allow the smoke to work its way to every leak. Where you see smoke, you have a leak. A smoke test will find leaks in places you'd never think to look.

Your hesitation coming off idle is suggestive of an intake leak.

Yes, I know a smoke test is the best way to go about finding an intake leak, but I don't have a smoke machine, I have a vacuum gauge :)
Do you know if the intake manifold hose to FPR is a good place for it? I could try and attach it to the boot as well, if that would be more accurate.

[N2MYE30]

Not to beat a dead horse but a smoke test is really the best way. I don't have one either but from friends and connections, I was able to find someone that let me borrow one in his shop for free. Even if he were to charge me, it would have saved me from hunting and pecking. Good luck.

[jlevie]

Yes, I know a smoke test is the best way to go about finding an intake leak, but I don't have a smoke machine, I have a vacuum gauge :)
Do you know if the intake manifold hose to FPR is a good place for it? I could try and attach it to the boot as well, if that would be more accurate.

It doesn't matter where you connect a vacuum gauge, the pressure will be essentially the same throughout the intake after the throttle body.

A smoke test isn't just the best way. If you can't find the leak(s) by visual inspection it is the only way. Pay a shop to have the test run. I can't imagine it costing more than one hour's worth of labor.

[Ryann]

Your gauge readings in the videos look great. The reason the needle is dipping when you open the throttle is that the pressure on either side of the throttle plate equalizes briefly, kind of like a diesel engine (they don't have a throttle valve at all, thus no vacuum).