OK, I've done a bit of searching and can't find a decent answer.

does anyone know what sort of vacuum I should be seeing at idle if everything's sealed and not leaking?

I've been chasing down rough idle, mis-fire and apparent lack of boost problems and thought it might be caused by a leak somewhere. my vac/boost gauge reads about 14-15 inHg at idle and goes up to about 21 inHg when decelerating.

I read somewhere that it should be around 20 at idle, but then elsewhere mentioned around 15, so I'm confused.

cheers,
.wook

EDIT: for a 6G74.

http://autospeed.com/cms/title_Using-a-Vacuum-Gauge-for-Engine-Diagnostics/A_2393/article.html

looks like my vacuum isn't quite where it should be, but seeing as I'm at about 700m above sea level that might be about right (17-22 inHg, - ~2 inHg for 2300 ft above sea level = 15-20 inHg).

from the workshop manual...


MANIFOLD VACUUM CHECK
1. Before inspection, set the vehicle to the following condition.

Engine coolant temperature: 80–95C
Lights and all accessories: OFF
Transmission: Neutral P range

2. Connect a tachometer or connect the MUT–II to the data link connector.

3. Connect a three-way joint to the vacuum hose between the intake manifold plenum and the fuel-pressure regulator, and then connect a vacuum gauge.

4. Start the engine and check that the idle speed is within the standard value range. Take a reading of the vacuum gauge.

Limit: min. 60 kPa

60 kPa is about 18 inHg. so I reckon I've still got a leak.

cheers,
.wook

Mine used to be at about 20 at idle:
http://www.youtube.com/watch?v=YA8WZri-B0g

found the culprit finally (thanks Dan!!)

it was the vacuum actuated bypass valve. it had a tear about 20mm long in the diaphragm which was obviously robbing my vacuum and (more importantly) my boost.

got a new one on order and should be here tomorrow. for the record, it's a RH Eaton S/C Actuator, also known as a Magnuson Bypass Valve. CAPA sold me one separately, but most other places won't separate the butterfly/valve assembly from the actuator.

should be at full boost by this time tomorrow... can't wait!

cheers,
.wook

love the sound of your car dude, was nice.

Is the same component that Magna00's sprintex ingested?? Seems a worry, but glad to see you're getting a quality replacement. I love happy endings :D

love the sound of your car dude, was nice.
sounds even better now with a 3" exhaust!! :D


Is the same component that Magna00's sprintex ingested?? Seems a worry, but glad to see you're getting a quality replacement. I love happy endings http://www.aussiemagna.com/forums/images/smilies/abiggrin.gif
I doubt it. this is after the blower, on the right hand side of the manifold.

looks like this...
http://www.yellaterra.com.au/_resource/image/part/542476B6-1803-6D49-863CD2B9D37E9B14.jpg

I think you'd be hard pressed to ingest anything from this into the blower.

cheers,
.wook