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re: '87 5000s fails emissions test
I assume you are referring to the EPA's new Inspection and Maintenance
(IM240) 240 second test which is done on a dyno and simulates a real
driving cycle, (ie. acceleration, braking, cruise etc).
As was mentioned, an excessively lean mixture will increase the NOx
emissions. Do you know at what point in the 240 second driving cycle that
the excessive NOx emissions occurred? Here is some additional info on NOx
that I pulled from my portable 4 gas exhaust analyzer handbook.
"The Oxides of Nitrogen (NOx) are formed in the combustion chamber if
combustion temperatures exceed approximately 2500 degrees F. The air
entering the engine is composed of approximately 78% Nitrogen and 21%
Oxygen. A chemical reaction takes place between the Nitrogen and Oxygen to
form NOx emissions when combustion temperatures get too high. NOx
emissions are controlled by compression ratio, camshaft timing, and
Exhaust Gas Recirculation (EGR) systems and (TWC) catalytic convertors."
The Three Way Catalysts (TWC) used today use Platinum and Palladium as a
catalyst to oxidize (adding oxygen) which reduce Hydrocarbon (HC) and
Carbon Monoxide (CO) emissions but also contain Rhodium which allow a
reduction (taking away oxygen) to lower NOx emissions. The graph of
emissions (HC, CO and NOx after passing through the catalyst) versus
air/fuel ratio shows that even though HC and CO emissions continue to go
down with slightly leaner mixtures (past the ideal 14.7 to 1
Stoichiometric ratio), NOX emissions go ballistic (much higher) with
air/fuel ratios leaner than 14.7 to 1.
You might want to reset the ignition timing back to the stock setting,
adjust the open loop idle mixture (older frequency valve systems) without
the O2 connected back to 0.8% to 1.0% CO upstream of the Catalyst. If you
have no access to an exhaust analyzer you can check the frequency valve
duty cycle reading with the O2 connected (45-55% duty cycle) if your car
has the older system for mixture control. The newer cars (late 86 I think)
use a different system (CIS-IIIE with mixture control by varying the
current through a differential pressure regulator). You will need to put
an amp meter (milliamps) in series with the pressure regulator and check
that the current is 10mA + or - 1mA and fluctuating (Note: some of the
vent hoses need to be plugged during this adjustment) . If you have access
to an oscilloscope you can look at the O2 sensor and watch how it cycles
between the lean 0.1V and the rich 0.9 V reading at a steady cruise. This
cycling between the upper and lower voltage reading amounts to only a 1-5
percent change in the air/fuel ratio. If the O2 reading is pegged at
the bottom (0.1V) at steady cruise you are too lean and outside the
control limits of the system. Hope this helps.
| My '87 5000s failed the new, more stringent Arizona emission
| test yesterday
| and I need some advice as to what might need "fixing"
| otherwise I will have
| to take it to a shop and [gasp!] pay someone else to fix it
| for me...
| Both the HC (.82gm/mile) and CO (5.6gm/mile) were well within
| spec; only the
| NOx reading was too high (6.8gm/mile v. a limit of 3gm/mile).
| In the past 4
| months, I've replaced the plugs, plug wires, cap & rotor, O2
| sensor, cat and
| air & fuel filters; I also readjusted the FI mixture and
| advanced the timing
| by 6 degrees. The car runs better than it has in 2 years and
| gets better gas
| mileage to boot but unless I can get it to pass emissions...
| Other than resetting the timing to factory specs, I don't
| know what else to
| try. I know how to deal with HC and CO but have no
| experience dealing with
| NOx and since I only have one free retest (after that,
| they're $16.75 per!),
| I want to "round up the usual suspects" before trying again.
| /| |_| |> | - |> |_| |> |
| aka Jeff Goggin
| Scottsdale, AZ