Hissing Noise Coming From The Plenum

[TuLaLiT]

Hey guys,

I got my stock plenum polished up and when I put it back on again ( new gaskets ) I got this hissing noise coming from around the plenum when the throttle is open up.

Its definetely not a vacuum leak because the car is idling perfectly normal and its boosting fine

Any help would be greatly appreciated

Thanks

[GTRNUR]

It can still be a vacuum leak, just a small one. You can try pressure test your intake manifold by blocking the pipe where it comes from the turbo's and then pumping up the intake manifold. If there is a leak it wlll be audiable at 2-3psi.

If everything is good you should be able to pump it up to 15psi and it should hold that pressure. Just be sure your plug doesnt blow out of the intake plumbing or it could launch quite a distance and cause some damage or injury.

[naut]

running gauges? could be the boost hose to your gauges

[WYTSKY]

what about the auxilary air control? was that taken off when polished? not sure how 34's go... but in 33's it is at the back of plenum (towards the firewall) and it lets in a small amount of air when idle (so the car doesnt stall when the throttle is closed) via a screw.... maybe you can just hear that because its clean now? or whats the condition of the gasket of it?

[TuLaLiT]

Its kinda hard to explain the problem so I will describe it the best I can

The hissing noise just appears when the throttles just open up , so if I hold the throttles open in any rpm it will NOT make the hissing noise

It sounds like "pssst" then goes away after that

The car is idling fine and boosting fine.

naut : The car is an R34 GTR its only running the factory boost sensor on the screen , you reckon I should check the vacuum hose ?

WYTSKY : I did taken out the AAC valve and put it back on with NEW gasket and I did NOT fiddle with the screw that controls the idle

Any more ideas / input ?? Im confused now

Thanks boys

[GTRNUR]

By having the noise go away when you open the throttles it is confirming that it is a vacuum side leak. Meaning the leak is between the throttles and the cylinder head.

3 possabilities...

1. The gasket you relaced under the AAC valve.

2. Throttle shaft seals

3. throttle gaskets

Pressure test it and it will tell you which one it is.

[TuLaLiT]

VSPEC R33 : I have replaced all the six individual throttle gaskets , I did MAKE my own gasket for the AAC from gasket paper I have lying around

and which one is the Throttle Shaft seals ?

Thanks

[wolverine]

PCV is probably causing the hissing.

situation normal i'd say.

[STATUS]

spray aerostart around all the gaskets and hoses if there is a leak the idle will pick up.

[TuLaLiT]

Okay so I took the car to 2 garages and all of them sprayed brake kleen on the plenum + AAC and they couldnt seem to find any leak at all

ALTHOUGH the car is idling higher than before , its just above 1k rpm on idle

Anymore thoughts guys ??

[GTRNUR]

Pressure test. Trust me you wont regret it. Youd be amased at how you can find the smallest leaks so easily, and as a reward your boost response will be better than you ever thought possible when its fixed.

[meet07]

I agree with Wolverine......mine does the same thing. ended up being the pvc valve. Pinch it under idle and see if the hissing changes.

[EFI Performance]

tulalit where are you located mate???

QLD????

If so we have a span on gas machine that we can check of air leak's, vacuum leak's and boost, takes two Min's to hook up, or go to a shop that has one in your area.

[TuLaLiT]

Hey guys thanks for the replies

EFI Performance : I live in New Zealand , so I will take the car to ST Hitec on friday and see what he says

Meanwhile I will replace the AAC gasket again see if it changes anything

Any feedbacks will be much appreciated

[GTRNUR]

Since some of you mentioned the PCV valve, it occurred to me to mention the one way valve that is on the clutch vacuum booster. Ive seen them fail and then once the diaphram fails in the clutch booster it will actuall suck air from the clutch booster leak which will increase the idle speed.

Usually you can hear the leak inside the car if you just operate the clutch pedal when the car is idling. Its pitch will change as the diaphram moves.

[TuLaLiT]

Well I took the car to ST Hitec and he said I shouldnt worry about it and just leave it and he couldnt find any leak whatsoever

I might rip the plenum off one more time to replace it with metal gaskets this time

Thanks boys