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13,891 Posts
Discussion Starter · #1 ·
okay, bear with me, this is going to be a long post, that i will probably finsih up in a few days. all typed in by my stupid knowledge, and be aware of typos, but should be good to go.

just general specs, not specific to a bronco.

first the gases
"HC" is your hydro carbons. organic compound that contains carbon and hydrogen
"CO" carbon monoxide. incomplete combustion of hydrocarbon(fuel) and oxygen. Colorless, and oderless
"NOx" nitrogen oxides, nitrogen and oxygen produced at higher combustion temperatures
"CO2" carbon dioxide colorless, oderless, heavier than air
"O2" oxygen

a exhuast gas analyse or the emissions test should give you your read out for the gases at what your truck is currently producing

with good combustion
CO2 should be about 14-16%. increases with the efficency of combustion
CO and O2 shoul dbe low and equal
HC Low level is best
NOx should be just lower than max at your are fuel ratio of 14.7:1

High CO= Rich condition, CO is partially burned fuel
-High fuel pressure
-Leaky injectors
-fualty temp sensor
-thermostat stuck open
-clogged air filter
-sticking choke
-high float level
-leaking accelerator pump or power valve
-charcoal canister full, fuel to topped off, faulty purge solinoid
-Fuel in the crankcase

low CO/high O2 =Lean condition
-to little fuel
-to much air
-low fuel pressure
-plugged fuel filter
-bad fuel pump
-plugged injuectors
-vacuum leaks
-low float leve;
-plugged or wrong jets

High HC= incomplete combustion
-bad ignition, coil, plugs, wires, cap, rotor
-over advanced timing
-low compression
-stuck EGR

Low O2-Rich conidition. still burnable fuel mixture

High NOx-combustion temps over 2,000*'s
-thermstatic air cleaner stuck closed
-higher compression
-to far advanced timing
-low oil level
-cooling problems, getting to hot. thermosats, sensor, radiater......
-to low of octane
- Faulty EGR
-Vacuum leak
-low fuel pressure
-wrong heat range of spark plug
-cat converter junk

before engine warm up, when still in closed loop, should be 10-13%
after warm up, should be 14-16%
if it doesnt increase catalytic converter is not functioning properly
CO2 below 10% indiactes extremely dirty exhuast and/or cat converter isnt operating

High O2 +high CO + high HC=misfire do to over rich mixture
high O2 +low CO + high HC= lean misfire
High O2 +Normal CO +high HC= miss do to mechnical or igniton problems
High O2 + normal CO + normal HC= O2 getting into exhaust via air pump, or exhuast leak

if HC goes up, O2 should go up
If HC goes down, O2 should go down

if CO goes down, O2 should goes up
If CO goes up, O2 should go down

CO2 should be highest at the engines most efficient spot
C02 goes down if anything else moves.
part 2 come later in week. emission devices....

Charlie don't surf..
'92 Ford Bronco XLT
16,537 Posts
Nice post man, that's some very useful information.

78 Posts
THanks for this info. This is the exact pain that I am going through. I can't wait for it to be DONE :banghead . This helps me understand what I am being told by the guy that is working on my bronco helping me get it to pass DEQ (fine tuning this stuff is beyond me at this point). THis puts some of the stuff that I have been hearing into light. Good stuff. :thumbup

Super Moderator
30,909 Posts

In The Matrix
1,896 Posts
Just thought I would bump this thread for those that don't know about it, and wanted to add something.

On my last test I let it warm up for about 5 minutes, then drove it to the test site (10 min drive), idled in line for about 15 minutes, then ran the test. My carbon monoxide readings were 48.6 gpm, the limit was 20.0. I went and had the standard tests done, all looked ok, and the mechanic recommended that I run it really hard for atleast 20 minutes before going in for my next test.

This time I ran it really hard on the highway for 20 minutes before I brought it in, ran the test, and got 14.6 on the CO test, well over 30 points lower.
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