Plenum drains blocked - a helpfull guide.

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techie
B5.5
Posts: 585
Joined: Sun Dec 23, 2007 7:12 pm
Location: Newport Pagnell

Plenum drains blocked - a helpfull guide.

Post by techie »

As this seems to come up daily on the forum I thought i'd put together a little fact sheet if you like on the causes and symptoms of this very common B5 + B5.5 fault (does seem to happen more on 5.5's though I must say). This is an age old Audi fault that unforunately VW inherited when they nicked the platform for this Passat.

How the system should work -

Rain water runs down the windsceen and onto the scuttle, from here is in the case of the B5 collects in the plenum area (where the battery etc sits, a secondary bulkhead if you like) on the B5.5 it will run accross the plastic cover and down the sides. Either way water collects in the plenum and drains out of two small bungs/drains under the battery/servo (also above the steering rack) and out onto the floor.

How the system usually works -

Rain water runs down the windscreen and onto the scuttle, then runs down into the plenum and cant go anywhere because the drains are full of leaves and mulch. Water levels builds up and swamps the seals on the pollen filter housing and ECU box and leaks into the car. However the pollen filter seals are very poor and have been known to leak anyhow if ther eis a collection of leaves at the edge of the cover on the 5.5.

Common symptoms -

Alarm issues (going off randomly)
Bonnet open light not working (B5.5, maybe B5)
Central door locking not working
Indicators staying on (maybe just one side)
Brakes sticking on
Wet carpets
Condensation in car
Constant smell of old socks in car.

This can be a very expenive fault to put right. I have known cars to collect enough water in the plenum that they flood the brake servo with water which corrodes the back of the brake master cylinder, also freezes up in cold weather. A bit less extreme but usually the most common complaint is the alarm going off all the time or the oil level warning light will not clear when the level is topped up. The cause of this is water ingress through the pollen filter housing. Water runs down inside the car to the connectors at the A pillar. The back one which is a grey plug houses the wiring for the bonnet pin switch, this corrodes out and creates issues. Also the water runs down into the CDL ECU which sits under the carpet on the passenger side. Issues arise on the drivers side as water passed the ECU box or the bonnet cable grommet, usually a failure of the throttle pedal is the first sign of trouble.

These are all very common faults but they all can be avoided if the pollen filter housing is sealed properly and the rain guide hasnt been lost on top of the pollen filter and the plenum drains and plenum area are kept clean and clear of debris. To seal the pollen filter housing properly I use sealing cord (available from VW). Its like Dum Dum (for those in the know) on a roll, does the job perfectly but can be taken apart again if needs be. Silicone sealer however wont help in this matter.

There is a very handy how to here on this subject -

[color=#FF8000][u] [size=150]CLEAR PLENUM DRAINS[/size] [/u][/color]

However should anyone be suffering some of these symptoms and needs some help I could repair for you and do a more indepth 'how to' for a very modest fee (compared to dealers anyhow).
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