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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.
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| Smoky
exhaust |
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Blue
smoke: Oil is entering the combustion chamber due to internal seal/gasket
failure; this also fouls the spark plugs leading to misfire.
Black smoke: Excess fuel is entering the combustion chamber due to a faulty
fuel pump, leaking injectors, 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. |
| 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
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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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| 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.
A header is another option, but it's also louder and more prone to cracking.
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