hissing noise on idle

vw754

vorsprung Durch Technik
Joined
Nov 4, 2006
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i had a turbo go on me last week,had it replaced,took exhaust off a little oil came out of it,luckily no oil went in airfilter or pipes as was only doing 30-35 mph when turbo gave way,once all back together im experiancing a problem :

above the battery is a black box with about6 pipes going to it........

when i rev the car it makes a loud hissing noise which is noticeabl on idle too,ive unplugged the unit then the hissing noise dissappears,soon as i plug it in again the hissing starts,the turbo feels like there is lag just before it kicks in,is this something do with the unit?

it was ok prior to the turbo dying,mechanic said maybe the cat has got blocked and its causing the unit to hiss,but im not sure cuz when i unplug it 1 pipe has suction on it,if it were hissing due to the cat being blocked it would be trying to breath rather than suck.........any ideas?
Car stll smoke a little as ive only just got the job done today,what would symptoms be of a blocked cat?
my turbo kicks in and car go's like a rocket,just get that turbo lag that i never had before.
 
serious though.. i had a hiss and it turned out to be a damaged pipe to the charcoal cannister... worth considering