Exhaust & Emissions

General Info expand_more

Understanding the Exhaust Exhaust System Upgrades Evaporative Emissions Components

Troubleshooting expand_more

Odors & Smoke Failed Emissions Test Faulty Catalytic Converter Removing Exhaust C-clamps

DIY Guides expand_more

Oxygen Sensor Guide Refurbing Charcoal Canister

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Updated: 29-Oct-2025

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

While the OBD system (check engine light) is emissions-related, it and the fault codes list can be found on the Gauges 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, or cylinder crack; 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.

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 (new ones are readily available). Also called a carbon canister.
• Location: Inside left fender.

Vent/Purge Valve

• Function: Vacuum-actuated vent valve. While the engine is running, vacuum opens the valve drawing fresh air into the charcoal canister and pulling stored fuel vapor into the intake manifold to be burned.
• Location: Off of throttle body (to 1986) or on charcoal canister (from 1987).

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: Inside right 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.

Failed Emissions Test

High NOx	High CO, Low O2
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
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.
High HC	High CO
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 injectors) Faulty/clogged catalytic converter (see next section) 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
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. Tap here for emissions testing information.
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).

Faulty Catalytic Converter

Symptoms of Clogged Converter:

• Performance and/or drivability issues
  • Reduced engine power, especially up hills
  • Poor acceleration
  • Difficulty starting, or failure to start
  • Engine hesitation or sputtering
  • Reduced fuel efficiency
  • Engine overheating, but cooling system is working fine
  • Failed emissions test
• Sensory-related issues
  • Rotten egg or sulfur smell
  • Dark or excessive exhaust smoke
  • Rattling noise at or near catalytic converter
  • Excessive heat in the engine bay and/or under the car; may be felt in the interior

Notes:

• The car may actually have no symptoms other than failing an emissions test.
• If the faulty catalytic converter is not that old, it may have failed due to poor manufacturing, or due to the car's engine burning oil and/or coolant, having a misfire, etc.

C-clamp Removal

Official C-clamp tool:

• 
• Tighten the single nut to open the clamp enough for removal and installtion.

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Exhaust U-bolt clamp:

• 
• Size: 2½ inches
• Alternate turning the two nuts outward until the clamp is sufficiently open enough to remove; alternate turning the two nuts inward after installation.
• Tip: Buy two, one for each clamp.