vacuum at idle?

[edh]

I've been wondering about this - since my headgasket was done i've noticed that the boost gauge fluctuates a bit at idle & now seems to sit around .4 bar when starting from cold. It will drop down a bit when it's warm

I noticed it when I picked the car up - was it at zero before? - I think so, but can't say for sure

What's normal?

 

[Fen]

I was going to say before you did that it sits at zero usually, so it sounds wrong. Something disturbed perhaps?

 

[edh]

that's what I thought

I guess it all came apart when the h/g was done- maybe it didn't all go back together OK

The challenge now is finding the leak

 

[edh]

or indeed - finding the faulty gauge

That's my first port of call - push the steering wheel up to flex the dashboard slightly, boost gauge drops to zero

I'll get another one I can plumb in directly to the intake manifold & see what's really going on.

The VDO Cockpit Vision Gauges look the most Porsche like to me

http://www.merlinmotorsport.co.uk/INSTRUMENTS-VDO-Cockpit-Vision-Gauges/c1_2_3/p6100/VISION-TURBO-BOOST-GAUGE-3-BAR/product_info.html

 

[Fen]

That's what I have albeit in psi. Of course I've had it for about 5 years and haven't fitted it yet, but we did plumb it in temporarily to check boost pressures. I'm not actually that fussed to see boost as I drive I guess.

 

[Diver944]

Ed, if you can get one that shows the vacumn as well it can be very useful if you ever have leaks and can immediately see the effect of any remedy you may try.

Autometer do one like this (you can also get a black surround) £43.35 from vwspeedshop.co.uk

 

[edh]

Paul - I'm hoping that's just the way it's marked (like the factory gauge - showing zero at vacuum)

I'll check with Merlin tomorrow (Fen - does yours work like this?)

after all - you'd be a bit worried if you had a negative air pressure!

 

[Fen]

Nope, mine is zero to 30psi. By turbo nutter MX5 had a vacuum/boost gauge, but to be honest I never really looked at it either. I do take Paul's point re: vacuum and spotting leaks, but an occasional temporary fitment would do for that.

 

[edh]

Isn't it the same thing - just air pressure? - whether it's below 1 bar or above
- some gauges set zero at 1 bar & show vacuum as negative, others set zero at vacuum

 

[Fen]

Some gauges show only positive pressure, hence zero is zero or anything below. Others show vacuum below zero so zero on them would truly be atmospheric pressure in the inlet.

 

[edh]

I've just checked with Merlin - this gauge doesn't show vacuum - thanks to Paul for pointing that out []

 

[edh]

OK - with a gauge connected to the inlet manifold, I'm reading around -8 (below atmosphere) at idle. Factory gauge is just on 0.4, so is reading OK, but does flicker. I think that's an electrical connection problem & I'm not planning to take the instruments out just to fix that.

I disconnected & blocked all the vacuum hose outlets one by one, isolating that bit of vacuum piping to see if I could see any change. It stayed pretty steady each time.