The computer will cause the engine to run rich until the normal engine
operating temperature is reached which is approximately 190 degrees.
Running rich will definitely affect the emissions readings.  Also,
running the engine cooler causes more wear on metal parts and bearing
surfaces.  You should change back to the OEM thermostat for this reason
alone.  It could very well solve your emissions problems too.  Until the
thermostat is replaced I wouldn't even bother with running the codes.
If it still fails the test then run the codes.

Huh!
I sure am glad I live in Henderson county! No smog tests here, yet.
Not that it's a concern on either of ours, but the expense and hassle is
what I hated

Kate

Pinched this from a usenet post, may be helpful.
- John V

====================================
Emission (smog) test problems.
==============================

What to look for when any of CO, HC, NOx too high.

CO: rich fuel mixture
HC: incomplete combustion
NOx: hot combustion and rich fuel mixture
CO, HC, and NOx: incomplete exhaust conversion

80%-85% of vehicles pass emissions tests.
an in-tune well-maintained vehicle should pass.
allowances are made for the age of the vehicle.
for the rest some details are provded below.

note that both CO and HC rise in the absence of O2 (oxygen) in
combustion and in exhaust, and that NOx rises in the abscence of
O2 at high combustion temperature. HC also rises when there is
too much O2 in combustion.

1. CO (carbon monoxide) too high.
---------------------------------

causes:

a) not enough air (oxygen) in air/fuel mixture to make carbon
dioxide (CO2) so get carbon monoxide (CO) instead (mixture too
rich). can be too little air or too much fuel in mixture.

b) not enough air (oxygen) in exhaust system to convert CO to CO2

c) faulty catalytic converter.

look for:

a) too little air - dirty air filter, stuck choke, plugged or
sticking PCV or EGR system, plugged carbuettor or injector air
passage

b) too much fuel - carburettor or fuel injection system supplying
too much fuel

c) too little air or too much fuel - engine control system -
vacuum hoses, sensors (eg exhaust oxygen, temperature, air flow),
computer, actuators (eg fuel metering solenoid, air metering
solenoids), wires and connections (control system voltages are so
low that small resistances from dirty or loose connections make a
big difference), the computer uses the input from the sensors to
send signals to the actuators to keep the air/fuel mixture at the
optimum for power, fuel efficiency, and low emissions.

d) too little air in exhaust system - restricted outside air feed
to converter (eg air pump or pulse air system - hoses, electrical
connections, valve, pump, drive belt, sensors)

2. HC (hydrocarbons, ie unburned fuel) too high.
------------------------------------------------

causes:

a) not enough oxygen in combustion to burn all the fuel, or too
much fuel (mixture too rich), so excess fuel blows out exhaust.

b) too much oxygen in combustion or too little fuel (mixture too
lean) for uninterrupted combustion (engine misses), so unburned
fuel blows out exhaust.

c) sparkplugs not firing properly to ignite air/fuel mixture
(ignition system), so unburned fuel blows out exhaust.

d) combustion chamber too cold for complete combustion, so
unburned fuel blows out exhaust.

e) combustion chamber hot spots preventing complete combustion,
so unburned fuel blows out exhaust.

f) not enough air (oxygen) in exhaust system to convert HC to H2O
and CO2.

g) faulty catalytic converter.

note that HC in exhaust can combine with S (sulphur) from fuel to
produce rotten egg smell.

look for:
---------

a) mixture too rich - see a,b,c in CO section above. note that if
HC is too high but CO is okay then the problem is not a rich
mixture.

b) too much oxygen in combustion - leaks of air (hoses, gaskets)
and routing of hoses into intake manifold, leaks in EGR and PCV
valves

c) too little fuel - dirt in fuel/filtre/tank/lines, leaks in
fuel lines or connections, weak fuel pump, - mechnical problems
in carburettor (dirty valves or passages or linkages, worn parts
or gaskets) or fuel injection system

d) too much air or too little fuel - computer not mixing air and
fuel properly (see c in CO section above)

d) ignition system - sparkplugs fouled (type of fouling may
indicate problem), poor contact or insulation in high voltage
ignition wires or in distributor (cap, rotor), poor contact in
coil wires, low voltage in charging system (alternator,
regulator), faulty transistorized ignition control module

e) combustion chamber too cold - sticking EGR valve or sitcking
cylinder exhaust valve or bad cylinder valve timming letting
"cooler" exhaust gas into the combustion chamber.

f) combustion chamber hot spots - bits of junk (dirt, carbon) or
nicked or burned metal parts glowing red hot in combustion
chamber and igniting air/fuel mixture (engine misses) - can use
chemical cleaner in combustion chamber or take engine out on the
highway or both to burn off junk - overhaul engine to remove
nicked or burned metal parts or hardened junk

g) too little air in exhaust system - see d in CO section above.

3. NOx (oxides of nitrogen) too high:
-------------------------------------

causes:

a) too much heat in combustion chamber - air contains about 78%
N2 (nitrogen gas) which becomes chemically unstable above 2500
def F and combines with O2 (oxygen, making up about 21% of air)
to form NO which, released into the atmosphere, forms NO2 which
in the presence of sunlight combines with HC (hydrocarbons) to
form smog. note that NOx only happens when engine is under load
(car at crusing speed on road or rollers).

b) too much CO in exhaust (CO enhances the conversion of NO to
NO2).

look for:

a) too much heat in combustion chamber - the EGR system or the
valve timing is used to bring exhaust gas back into the
combustion chamber to cool it off when the engine is hot under
load at cruising speed - check the EGR system valves, solenoids,
wiring, hoses - if the car does not have an EGR system check
valve timing - (engine cooling (water) and lubricating (oil)
systems may have some effect on combustion chamber temperature?)

b) computerized engine control system - adjusts engine operation
to minimize emissions (see c in CO section above).

----------------------------------------------------------------

sources:

"Automechanics" (Prentice-Hall, 1988)
"Import Car Manual 1983-1990" (Chilton, 1989)

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