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[valliant72]

hi, i can hear the hissing of the plenum slightly in the car, not usually noticed as have had the window open alot and music playing, is this normal. Took the plenum off on wednesday changed all the seals, torque upto right lbs, check all pipes for any leaks. Changed plugs for bosch super 4's. Engine runs smooth idles perfectly, no misfires or anythng, oh i also changed the lamda sensor aswell as the eml light came up, but still get hissing is this normal, i imagine some hissing  as air is going through, if i put my hand on the passenger side of the plenum the hissing dies down as im just blocking the sound out. So its coming more from that side of the plenum. :-?

[Entwood]

Sounds like a sealing ring has either slipped or been missed

Best thing is to take the plenum back off and make sure all the seals are intact, seated properly, and they don't move on refitting the plenum.

Should only take 15 minutes or so ..

[valliant72]

i should of put that i changed the seals cause of the hissing before, cause the last time the plenum was off was when VX did the cam cover gasket and wasnt sure if they had put it back properly, so i changed the seals myself, everything went on ok, all seated properly as i double checked everthing before i put it back on. I even checked the plenum for cracks but found nothing.

[Entwood]

My thoughts are that if the hiss diminishes when pressed slightly by your hand .. whch is how I read your first post .. then the plenum is not sitting quite "right" ...

What you could try is lightly spraying carb cleaner around the suspect area, if the engine note changes you are spraying into the leak .. might help pinpoint the problem .. ??

[MarkG]

Common source of hissing in that area is the brake servo vac pipe. About an inch along from the point it exits the plenum, the plastic pipe rubs against part of the AC system creating a hidden hole. Check it out and wrap some duct tape round the pipe at this point anyway. If it is allowed to get worse, you will find the brakes losing power

[valliant72]

Well, ive checked everything, all pipes are sound, plenum is sitting ok on new seals, but still i get a hiss, im wondering if it because when i cleaned the breathers i never replaced the breather to plenum seal or the 2 small o ring seals?

[MutantCav]

Breather box to plenum...small o ring with a horse shoe looking bit across it...hisses if its split...if you changed it recently then may well have split...did you grease it before putting it back on???

Good news is its only about £3 from Vaux...