Odd Exhaust Temps And Failed Smog Test


The Grumpy Grease Monkey mechanical engineer.
Staff member
Grumpy I have a 1985 vette that recently failed emission testing,
It failed smog check a couple of weeks ago.
Hyrocarbons were out around 870. CO was 5.4.
Changed over to new iridium platinum spark plugs.
Re-tested, the Numbers went down dramatically. HC 202, CO 0.83. which just barely passed
Im thinking oxy sensors?
exhaust temps are about 60 degrees higher than they were last time I tested, at the header flange
Engine near-stock 350

adjust your valves , do a compression check,
changing plugs reducing the Hyrocarbons that much indicates,
you were not getting consistent combustion,before the plug change
your cats may be damaged from the prolonged overly rich mixture.
your exhaust temp changes more than likely due to more complete / better combustion
pull trouble codes, yes if the oxygen sensors are more than a few months old they may be fouled
and need replacing, verify the fuel pressure regulator and injectors are not leaking,
fuel pressure in the fuel rail should be about 38-40 psi and hold,
for at least 5 minutes after keys turned off,
if it drops faster you have a leaking fp reg ,and/or injector,and/or fuel pump,
check ALL THE engine heat sensors
sounds like a vacuum leak ,
worn cam lobes ,and/or leaking exhaust,
or bad tune up
check ignition timing, should be about 6-8 at idle and reach full advance at about 38 at 3000 rpm

yes a decent infrared temp gun is a helpful diagnostic tool
keep in mind , even if everything running well,
the front two cylinders tend to run a small amount cooler (20 degrees) simply because the increased cooler air flow over the header tubes
the actual exhaust gas and header temp will vary with the combo specs , your looking for consistency not a specific temperature at the exhaust ports

spark plug gap should be near .043 and
test for loose vacuum hoses,
do a tune-up, verify ignition firing order
use a multi meter to test ignition wire OHMS resistance they should not exceed about 150 ohms per foot
high hc is basically unburnt fuel

read the linked threads and sub links












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