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 Exhaust & Emissions


 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.


 Smoky & smelly exhaust

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.

 Failed emissions test: Why it happened & how to make it pass
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).

 Exhaust system upgrades
The most widely-used upgrade is a cat-back system from Techtonics Tuning (TT), high-flow catalytic converter and a Borla muffler.  However, an exhaust system, in whole or part, can be bought from any of these aftermarket manufacturers:
  • Techtonics Tuning
  • Autotech
  • Neuspeed
  • Eurosport
  • Borla
  • Magnaflow
  • Flowmaster
  • Jetex
  • Dynomax (general opinion of the Dynomax: louder than the others & fails sooner than others)

Another common upgrade is switching to the Golf III VR6 exhaust system. 

What sound do they all make?  That all depends on the brand you buy and what components they all are paired with.  The general result of any aftermarket upgrade exhaust is that they are louder than the stock system.  Some are just slightly louder, while others might anger your neighbors.  From owner feedback, the widely-used TT/Borla system provides a louder, more deep/throaty sound than stock, but is not loud not enough to disturb your neighbors.

Interested in a stainless steel exhaust system?  They are available from many of the above manufacturers, but for a hefty price, as it is with most stainless products on the market.  Many of the stores listed on the parts page supply the above aftermarket exhaust systems, including stainless versions.

All 1995-2002 Cabrios use the same exhaust system, and the same as the 1995-1999 2.0L ABA Golf III's, depending on the manufacturer's part number (some cross-fit, others are listed as "except convertible" for some reason).  Despite this, be sure the aftermarket parts fit for your car model and year if you desire a direct-fit replacement.

 Factory exhaust system
If you're looking to replace the OEM exhaust system, Ansa, Bosal, AP, Walker & Leistritz are the main manufacturers of stock replacement exhaust components.

All 1995-2002 Cabrios use the same exhaust system, and the same as the 1995-1999 2.0L ABA Golf III's, depending on the manufacturer's part number (some cross-fit, others are listed as "except convertible" for some reason).  Despite this, be sure to order parts for your car model and year.


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