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Boost gauge only reading 4Inhg at idle


Lynx

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Hi,

I have a GC8 STi V5,

recently i have noticed the boost gauge is only showing 4Inhg vacuum at idle and only boosting 4psi max

(Autometer Gauge)

Now when i first got the car it was showing 21 Inhg vacuum at idle if i remember correctly and boosting 14.7psi

I have tried a different gauge when is brandnew and still shows the same readings and also swapped out the vacuum hose to the gauge

It idles fine and drives fine...

I have checked over all the vacuum hoses and they seem to be fine.

What may cause this?

Thanks

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 Lynx said:

Oh forgot to mention i also changed the vacuum hose that goes to the boost gauge

Something is leaking. If your car was running that little boost and that little vacuum your brakes wouldn't work, your car wouldn't idle or accelerate properly, and you'd be using a LOT of gas.

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 Lynx said:

thanks, Would a cracked or something in the intake pipe cause this?

A cracked intake would cause much worse problems. Your issue is somewhere between the manifold and the MAP sensor (and your boost gauge)

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does it still feel like its boosting 15psi ?

to show very little vac on gauge but still idle properly and have breaks that work - either the line is kinked - the line is leaking or the line its tee'd into is kinked / leaking

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 Swindog said:

Whats your boost gauge line hooked onto on the manifold or is it tee'd with something else?

Its tee'd into the this sensor/solenoid under the IACV, its got a green nipple with a plug.

It was running fine before and my previous GC8 was also tee'd off there with no problems

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 Lynx']

[quote name='Swindog said:

Whats your boost gauge line hooked onto on the manifold or is it tee'd with something else?

/quote]

Its tee'd into the this sensor/solenoid under the IACV, its got a green nipple with a plug.

It was running fine before and my previous GC8 was also tee'd off there with no problems

That's the Canister Purge Control Valve... you don't want to use that, you need to use the line that feeds the MAP sensor. Otherwise you'll get funny readings while the CPCS is operating

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hmm - the purge solenoid does get its line off the manifold tho - so if its teed pre solenoid its just a normal vac feed isnt it ?

tee it into the bov cant go wrong

and most people would say DONT tee into the map sensor line - the map sensor signal is rather important - its quite easily for the long line to the gauge to get a leak - which will probably result in alot of damage if map line has a leak on it - map will be seeing a lower value than whats actually happening

only my opinion however - i would never ever tee into the map sensor

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 WRXONP said:

hmm - the purge solenoid does get its line off the manifold tho - so if its teed pre solenoid its just a normal vac feed isnt it ?

tee it into the bov cant go wrong

and most people would say DONT tee into the map sensor line - the map sensor signal is rather important - its quite easily for the long line to the gauge to get a leak - which will probably result in alot of damage if map line has a leak on it - map will be seeing a lower value than whats actually happening

only my opinion however - i would never ever tee into the map sensor

Based on our earlier discussion that Subaru ECUs are not MAP-sensor driven, the only thing the MAP sensor is used for is boost limiting. The MAP sensor line is the absolute best place to plug it in. That or the FPR feed (for cars like mine where the MAP sensor is manifold-mounted).

The BOV line is also not a good idea because during transitive (pressure increasing/decreasing) periods it will not reflect actual MAP as the volume of air moving through the hose to drive the BOV is very high and so there will be significant lag in the line.

The CPCS line will fluctuate rapidly when the solenoid is operating, and if the solenoid fails open may read completely incorrectly.

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I would never tee off anything important - including map or fpr - why risk it

I've never had any problems teeing off the bov - as the vacuum / boost In the bov only

Moves the piston / valve - on boost the hose is purely pressurised - even torque performance tee into bov lines for running the dyno

If you remove carbon canister completely - you can just hook age straight into manifold

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So theres two vacuum on the manifold just behind the alternator.

One goes to the BOV

and the other goes to the Canister Purge Control Valve,

Now my question is, does it go only and directly to the purge control valve or does it tee off and goes to another nipple?

Thanks

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  • 3 weeks later...

Ok so got it to read 17-18InHg at idle now, and when i give it a quick rev it will drop ti 21InHg and go back to 17Inhg at idle.

When I give it full boost it reads 17PSI then it will drop to about 13-14PSI.

Another Leak???

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boost will normally drop off after its initial peak as the factory boost control is pretty average.

an electronic boost controler running an after market boost solenoid will help hold boost level and not drop off.

Also an actuator with a stronger spring can help as the boost controller doesn't need to work as hard.

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not sure, never paid much attention to my vacuum, but if your worried find someone with a smoke tester, they pull your airfilter off and pressurise the whole system with coloured smoke, if there's a leak you'll see it.

Im about to get it done on my car before I get it dynoed

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