vacuum not really vacuuming like it used to

so this started on my way back from Quebec last week. My boost gauge used to bury the needle at 30 IN-Hg. Now in park it reads 20. In gear at a stop sign its around 10. Under low rpm, if I let off the petal, instead of plunging to 30 like it used to it "sticks" for a second at 10, and then fires to 20. Below is a vid showing first at a stop sign in gear, then some light driving , and then after the pause, a few revs in neutral. ( i deliberatly did not get into POS boost for the vid) Each time the needle plunges, I've already been off the gas for a few seconds:



39-00:30(stopped at stop sign)
00:30-00:26(driving a bit and letting off)
00:12-00:15(little bit more driving, "sticky" needle is more visible here)

00:15 - 00:00 (3 small revs to end of video)

The revs at the end probally shows what I mean the best.
kinda shaky, i know, but i was alone driving and trying to film it to.

so my first thought was vacuum leak. Sure, old original lines. I look around and find a split in the short little one going to the boost acutator. Ok, more looking, no other leaks, but the lines are in shitty shape. No problem, I changed ALL the vacuum lines. Start it up. Idles much smoother, steps into the boost much nicer, so fixing the split did something. But the gauge is STILL doing it. That vid is from AFTER new vacuum lines.

Any ideas? could it be a problem with the line going to the boost gauge?(dont really want to have to take the dash/kick panel apart to check the routing) The car runs fine. Much better now with the brand new vacuum lines, but this gauge is bugging me. if I had another gauge I'd try that...

could you please post about how much/what type lines to buy to redo all the vac lines - and whether you prefer rubber or silicone? and about how long it took to do the job?

Albertj

I wanted slicone, but normal vacuum lines are all that were avaible on short notice. I just got 3' each of 3/16, 1/4, and 1/8(all inside DIA). The job prob took half an hour, depends on how fast or slow you go. I prefer to replace one run of line at a time, to avoid confusion.

--And to bump my first post, no ideas? anyone? does this mearly look like a roblem with the line runing to the boost gauge?

This could have nothing to do with it, but when you replaced the lines, did you use a larger diameter tubing? If so, maybe the vacuum is slower to build?

AA wrote:

This could have nothing to do with it, but when you replaced the lines, did you use a larger diameter tubing? If so, maybe the vacuum is slower to build?

That would have no effect. (or such a miniscule effect it wouldn't really show on a gauge like that)

I'm puzzled too though... Maybe your brake booster is leaking?

it also did this before replacing the lines. I replaced the lines because I figured it was a vacuum leak...

I'll take a look at the brake booster...

could it be the gauge itself is misreading? do you have an actual scanner to compare readings to?

I wondered about the gauge... I have a scantool, but does anyone know any way for it to show the boost? being a series 1, the scan tool only reports 0 or 100% boost...

You watch the MAP sensor. If you are reading MAP KPa you'll see if it's vacuum or boost.