Mysterious Vacuum Leak

[ej22tVermont]

So I have been trying to locate an odd Vacuum leak for a while now. My gauge is reading 10-12 in at idle, when it used to read 18 dead on. It does not fluctuate, nor is my idle affected at all. It does however cause some odd driveability issues. As in it will run perfect one moment then at steady throttle it will pulsate it's power (Steady pressure=Pull hard/pull less over and over again up the rpm's). I did the Carb spray test and nothing was found. Then I pulled Vac lines to check for loss of vacuum. All seem to function, but the ones going to the boost control solenoid. Question is , What now? Are those Vac lines normally pulling Vac during idle (common sense tells me it should). Any input would be great. The only codes are for bad VSS (been bad for 7 years though!), and number 35 purge control. I replaced the solenoid about 2 years ago, and don't think that is my issue. But I would love to hear some input as I would like to solve this issue before the snow begins to pound our region. Thanks!!!!!!!!

Oh yeah, while it reads low vac at idle, but does hit full boost at mid 2000's and peaks around 9-10 psi. When the pulsating power problem happens more than a little bit though, it will go in and out of limp mode (boost cut to 5 psi) all while driving my 25 miles to work a day. Thanks in advance, you all are awesome!

[Legacy777]

Silly question, but have you verified your boost gauge is accurate and hasn't changed? Using another gauge or tester to verify vacuum would be my first step.

I know you posted about the vacuum, but I have a thing for not solving other "perceived" problems before addressing known problems, i.e. VSS and canister purge solenoid.

Without a VSS the ECU will run the fans all the time and do some odd things. The canister purge solenoid shouldn't cause any issues, but when I had mine fail I did occasionally get some weird idle stumbles. I can't really explain why, but my hunch is that it may had been open at idle, which it's not supposed to.

Do you know how to use a multimeter? I would check the solenoid's resistance and the resistance of the wiring from the solenoid to the ECU.

Do you have a MT or AT? Have you done any troubleshooting on the speedo and why it doesn't work?

[macipusy]

One location which is not intuitive to check for vacuum leaks is around the o-rings between the fuel rail and manifold.

One thing you can do, is plug the turbo inlet (right at the turbo) with a can (brake clean or something similar diameter as the silicone inlet) and clamp it. Then blow compressed air into the BPV to manifold hose....disconnected on the BPV side. Then listen for air leaks. This worked well for me in the past to locate inconspicuous leaks.