Updated: 25-Aug-19

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Exhaust & Emissions

The evaporative emissions system equipment is tied into the engine vacuum system; for diagrams of the evap system and OBD I diagnostics, please go to the Engine page.

Understanding the Exhaust

HC = Hydrocarbon = unburned fuel

CO = Carbon-monoxide = burned fuel

NOx = Nitrogen oxides = reactive gasses produced during the combustion process

When the above three are well-balanced, the car should be running at its best.  When one or all of the above is out of balance, failing the emissions test is likely to result.

If your early Cabriolet/Rabbit Convertible has K-Jetronic lambda, but does not have an oxygen sensor in the exhaust manifold, the oxygen sensor may be mounted in the catalytic converter.  This catalytic converter's part number is 175131701.  Most, but not all, 1980 USA Cabriolets do not have an oxygen sensor system; instead, they use an EGR system.

Exhaust Odors and Smoke

Blue smoke: Oil is continuously entering the combustion chamber due to internal seal/gasket failure (it's seeping past the piston rings); this also fouls the spark plugs leading to misfire (running rough).

Black smoke: Excess fuel is entering the combustion chamber due to a faulty fuel pump, leaking injectors, vacuum leak, etc.; this also causes a running rich condition that increases fuel consumption.

White smoke: Coolant is leaking into the combustion chamber due to head gasket failure; this also causes the oil to turn to "chocolate milk" and can ultimately lead to a blown engine.

Rotten egg smell: Sulfur is being emitted from the exhaust due to a faulty catalytic converter.  This is usually caused from running rich for too long.  If this condition is allowed to continue, eventually the fume build-up will cause the engine to shut down.  Catalytic converters usually last for 50,000 miles; if this condition occurs prior to this mileage interval, check the fuel system.

Exhaust Upgrades

Cat-back system from Techtonics Tuning (get a resonator with it to make the car quiet as stock while being free-flowing).

Borla muffler (Dynomax is, according to those who've used it, loud and will fail quickly).

Swap the single-downpipe (aka "toilet bowl") exhaust manifold to a dual-downpipe from an older car (Corrado, Scirocco 16V, Rabbit, etc.). A header is another option, but it's also louder and more prone to cracking.

Failed Emissions Test

High NOx

High CO, Low O2

High HC

High CO

Running too lean (high combustion temps) due to:

  • Vacuum leak(s)
  • High idle due to vacuum leak
  • Faulty/clogged catalytic converter
  • Faulty oxygen sensor
  • Timing is advanced too far
  • Incorrect air-fuel mixture
  • Dirty air filter
  • Faulty ignition components

Running too rich due to:

  • Faulty/clogged catalytic converter

Running too rich due to:

  • Timing is advanced too far
  • Vacuum leak(s)
  • Misfiring (too much fuel in the air-fuel mixture)
  • Fouled spark plugs
  • Poor fuel injector spray (clean or replace fuel injector(s)
  • Faulty/clogged catalytic converter
  • Faulty O2 sensor
  • Filthy engine oil

Is OK at low speed but fails at higher speeds:

  • O2 sensor is bad
  • Vacuum leaks
  • Faulty/clogged catalytic converter

Running too rich due to:

  • Faulty warm-up regulator
  • Faulty fuel distributor
  • Incorrect air-fuel mixture

If the CO reading is adequate and the NOx reading is not excessively high, simply richening up the air-fuel mixture a bit may be enough to pass the emissions test.

These two usually go hand-in-hand: If there isn't enough O2 in the mixture, CO doesn't convert to CO2. Also, with a rich mixture there isn't enough O2 to burn all the HC so your HC reading goes up.

How to Make the Car Pass

  • Conduct a complete tune-up
  • Fix any vacuum leaks
  • Check and adjust the timing
  • If the car idles below 900rpm, increase the idle speed to 900-1000rpm
  • Replace the air filter (if using a K&N, use a paper filter for the test)
  • Replace the engine oil
  • Replace oxygen sensor and/or catalytic converter

You can use products such as "Guaranteed To Pass" or even 90% rubbing alcohol, retard the ignition timing, and/or mess with the air-fuel mixture, but it's best to figure out why the car is failing emissions in the first place and to fix the problem(s).

Evaporative Emissions Components

Charcoal Canister

Function: Stores unburned fuel vapors (hydrocarbons) until they can be purged back into the combustion chamber via a vacuum-actuated vent valve.  During the purge, the vacuum valve also allows fresh air to enter the canister in order to oxygenate stored hydrocarbons.  The canister can become saturated with fuel if you overfill ("top-off") the tank; if this occurs, the charcoal in the canister will need to be replaced. Also called a carbon canister.

Location: Behind driver's side fender.

Vent Valve

Function: Vacuum-actuated vent valve.  Up to 1989 the vent valve is a canister bypass valve; the canister is open to be purged by intake vacuum.  From 1990 onward, the valve is opened via vacuum for the canister to be purged.

Location: Up to '86: off of throttle body; from '87: on charcoal canister.

Expansion Tank

Function: 1. Accommodates the expansion and contraction of fuel in the tank due to temperature and pressure changes.  2. Traps excess fuel due to overfilling and natural thermal expansion, and vents fuel tank vapors to the charcoal canister.  3. Prevents the fuel tank from collapsing when fresh air is not drawn into the fuel tank as the fuel level drops.

Location: Behind passenger side fender.

Gravity/Vent Valve

Function: Prevents fuel from entering the vent system in the case of a roll-over or other accident.

Location: Fuel tank filler neck.

Vented Fuel Cap

Function: Allows air to enter the fuel tank during periods of low pressure (vacuum), while preventing the release of air containing hydrocarbons during periods of high pressure.

Location: Rear, passenger-side quarter panel.

If any component malfunctions (saturated charcoal canister, blockage, faulty valve, collapsing vent hoses, etc.), the system will not vent properly and may cause emissions test failures and poor running conditions.  Please refer to Bentley Service Manual page 5-15 for system diagrams.