Downpipe and O2 sensor removal on VW Passat 4 cylinder engine

difficulty: 2/5
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Introduction

This article shows how to remove the downpipe on a VW Passat TDI (2004 or 2005) with 2.0L BHW engine.  

Other similar generation 4 cylinder Volkswagen Passat engines should be similar except that their intake manifolds are on the other side.

The downpipe is pictured below.  It's the first section of exhaust pipe which contains the first catalytic converter.  It's normally called the downpipe because it goes down.  See 1000q: turbocharging FAQ for more pictures of performance downpipes.  It's not possible to remove the downpipe without removing the O2 sensor and lifting the engine or lowering the subframe to get more clearance.

The oxygen sensor (O2 sensor) reads the amount of unburned oxygen in the exhaust stream.  Unlike a gasoline car, the VW Passat TDI doesn't directly use the O2 sensor for fuel metering at all.  The O2 sensor is used to meter the exhaust gas recirculation valve (EGR).  See 1000q: EGR FAQ for more details.  The O2 sensor wire goes behind the engine and the plug is located under the coolant reservoir.

O2 sensor removal procedure

First remove the intake air box to get access.  See 1000q: engine air filter housing for details.

Remove the O2 sensor: unplug its plug below the coolant reservoir.  There is a small tab which locks it.  Unscrew the O2 sensor using a 7/8 wrench or o2 sensor wrench.  It will probably be stuck so give it a good squirt of PB Blaster or other penetrating lubricant.  Also spray the other exhaust nuts/bolts and let them soak.

First loosen all the bolts/nuts before removing any of them.  This will hold it steady so that you can loosen the other pieces of hardware.  

Remove the 3x 12mm nuts at the turbo.  The innermost 12mm nut on the turbo is difficult to reach because the oil line is in the way.  You should be able to reach it from behind with a joint and some extensions from underneath the car (shown in the next picture).  If not, remove the EGR metal hose to get clearance (2x 6mm allen head bolts each side and some nuts).

This is the EGR pipe that's in the way.  I don't think it's possible to remove the downpipe with it in the way, at least on the TDI engine, but you could try.  To reach the upside down 6mm allen bolts, use a long extension from underneath the car.

Remove the 13mm bolt/12mm nut at the exhaust pipe end (the downstream end).  You can also remove the clamp on the end of the exhaust pipe (2x 16mm nuts) to get more play to maneuver it out.  Loosen but do not remove the passenger side transmission and engine mounts (1 nut each) and support/lift the engine/transmission to tilt it and get more clearance.  You can also loosen the 13mm/18mm bolts holding the subframe to the body.  

CAUTION: Do not remove the bolts completely, just loosen them 1", one at a time or else the entire engine/transmission/subframe will fall out and down.  If it falls on you it could result in serious injury or death!  The 18mm bolts are also single use stretch bolts so if they are loosened, you should replace them.  See 1000q: oil pump replacement for more tips on slightly lowering the subframe. 

Are there any broken pictures or links?  If you have any questions on replacing the exhaust downpipe on your Volkswagen Passat TDI, please ask in the TDI help forum, thanks!