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| Understanding
the exhaust |
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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
CIS-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, if not all, 1980 USA
Cabriolets do not have an oxygen sensor system; instead, they use an EGR
system.
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| Smoky
& smelly
exhaust |
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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 |
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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
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Running too rich due to:
- Faulty/clogged catalytic converter
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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
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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
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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. |
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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. |
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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). |
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| Evaporative
emissions system components |
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Component |
Function |
Location |
| Charcoal canister |
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. |
Behind driver's side fender |
| Vent valve |
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. |
Up to '86: off of throttle body;
from '87: on charcoal canister |
| Expansion tank |
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. |
Behind passenger side fender |
| Gravity/vent valve |
Prevents fuel from entering the
vent system in the case of a roll-over or other accident. |
Fuel tank filler neck |
| Vented fuel cap |
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. |
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. |
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| Exhaust
system upgrades |
- Cat-back system
from Techonics 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.
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